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Journal of the Biological Society
Victoria University College
Wellington New Zealand
The return of spring is each year heralded by the appearance in our gardens of snow-drops, daffodils and crocuses. These flowers last for only a few weeks, and as summer approaches other kinds of flowers take their place, only to be replaced themselves by the autumn-flowering species. This unfailing sequence of spring, summer and autumn flowers is too familiar to cause much surprise, but it is a very extraordinary phenomenon, and only in the past 30 years have botanists obtained an understanding of the factors which control flowering.
It has of course been known for a very long time that most plants will flower only at certain times of the year, and that in many species, and especially in cultivated plants, ‘early-’ and ‘late-flowering’ varieties occur. Nevertheless all attempts to relate the different times of flowering to seasonal differences of temperature or light intensity had met with failure. In 1920, however, two Americans, Garner and Allard, suggested a much more effective way to control flowering. They found that a variety of tobacco called Maryland Mammoth would not flower during the summer when grown in Washington, D.C., although other varities of tobacco flowered freely under the same conditions. However, when Maryland Mammoth plants were grown in a warm glasshouse during the winter they flowered profusely, suggesting that some seasonal factor was effective in controlling the formation of flowers. Garner and Allard suspected that this factor was the length of day.
By the simple experiment of artificially altering the day-length they were able to cause many species of plants to flower at unusual times of the year. Thus Maryland Mammoth tobacco which would normally not flower during the long days of summer could be made to flower at this time by covering the plants with light-tight boxes at 4 p.m. each day and removing the boxes at 9 a.m. each morning, thus subjecting the plants to days of only seven hours. In the same way a variety of soybean called Biloxi which normally flowers very late in the summer could be made to flower in midsummer if it received only seven hours light each day. If Biloxi soybeans are grown in the winter in a warm glasshouse they will flower profusely, but if the length of winter days is extended by leaving an electric light on near the plants from about 5 p.m. to midnight they will remain vegetative. Conversely, species which normally produce their flowers in mid-summer can be made to flower even in mid-winter by extending the hours of daylight with artificial lighting.
Garner and Allard were able to classify a large number of species as either short-day plants which would flower only if the length of day was less than about 12 hours, or long-day plants which flowered only when the day-length exceeded about 12 hours. The response of plants to the relative length of day and night they called photo-periodism. While many plants remain vegetative indefinitely if grown in days of the wrong length, some others will flower irrespective of the length of day. These are called day-neutral plants.
Although it was possible to classify plants as long-day or short-day species it soon became evident that no particular length of day could be chosen to separate the two groups. Some short-day species will flower only when the length of day is less than 12 hours, others will flower so long as the days are not longer than 16 hours. Thus for every species there is a critical day-length. Short-day plants will flower only in days shorter than the critical day-length for the species, long-day plants will flower only when the length of day exceeds this value. The important distinction seems to be that, whereas short-day plants require a certain minimum length of darkness after each light period to flower, long-day plants will flower even when growing under continuous illumination. In fact, darkness seems to inhibit flowering in long-day plants.
In their native habitats plants seem to have adapted their flowering response to the day-lengths which they normally encounter. Thus species which are native to the tropics, where day-lengths tend to be constant at about 12 hours throughout the year, are generally short-day plants, those from more temperate regions, which have longer summer days, are often long-day plants. This explains the frequent failure of plants moved to new geographical regions to flower despite the favourable temperatures in which they are grown.
Before discussing the research which followed the first demonstration of photoperiodism, we can briefly indicate the changes which occur as a plant
The first visible indication of flowering is a change in the shape of the apical meristem of the shoot, but this can be seen only by careful examination under a microscope. During vegetative growth the meristem, from which new leaves and stem tissues arise, is usually conical in shape and quite small, often only 0.1 mm. in diameter, but with the onset of flowering each meristem becomes flattened into a broad disc which may give rise to a single flower, e.g. poppy, or to many flowers, e.g. capitulum of daisy.
During the later stages of flower development some of the younger internodes of the stem elongate, raising the flowers above the vegetative body of the plant, and the leaves in this part of the stem are usually smaller and of simpler shape than those farther down the shoot.
The importance of Garner and Allard's discovery was soon recognised and a large number of botanists, especially in America and Russia, turned their attention to elucidating the various steps involved in the photoperiodic response.
It was soon found that it was not necessary to maintain a plant in a favourable length of day for its whole life in order to cause flowering, but that a relatively brief exposure to the correct day-length was sufficient, after which the plant would flower even if it were subsequently placed under unfavourable day-length conditions. The stimulation of flowering caused by days of the correct length is called photoperiodic induction. Different species were found to differ considerably in the number of photoperiods needed to induce flowering. Chrysanthemum requires up to 30 successive short days while Biloxi soybean will flower after being exposed to only two short days. The American cocklebur, Xanthium pennsylvanicum, is remarkably sensitive to photoperiods of the correct length. It is a short-day plant and remains vegetative indefinitely in days of more than 16.5 hours illumination. However, a single short day is sufficient to induce flowering even if the plant is subsequently placed in long days. Once flowering is induced a plant may continue to flower for a very long time; a Xanthium plant after being induced by only seven short days, flowered continuously for the following 18 months even though it was kept in long days for all of this time.
Within certain limits photoperiodic induction produces a quantitative flowering response. For although two short days are sufficient to initiate flowering in Biloxi soybeans, heavier flowering follows further short-day
In long-day plants that require several photoperiodic cycles for floral induction the treatment may be given on consecutive days or may be broken into parts separated by several short days. Plantago lanceolata requires 25 long days for maximum flowering response. But by breaking the long-day treatment into two periods of 10 and 15 days separated by 20 short days, maximum response is still obtained. Evidently under the correct length of day a permanent physiological change occurs in the plant. This change persists during unfavourable day-length conditions, and is augmented when the plant is again subjected to the correct photoperiod. Since the initiation of flowers at the shoot apex is an ‘all-or-none’ process, the changes caused by the inductive day-length must first build up to a threshold level before any flowers are initiated.
However, some varieties of wheat, some oats and rye, carrots, cabbages and some other plants do not flower in their first season despite the favourable day-lengths which they encounter at this time. These plants, which normally flower in their second season and are therefore biennials, require a period of several weeks of cold weather before they will flower. This is usually received during the winter, but by subjecting these plants to a period of cold in the seedling stages, or even during germination, they can be made to flower in their first season provided they encounter the correct length of day. This explains the ‘bolting’ or precocious flowering of some biennial crops when they are planted too early in the spring. The cold treatment of germinating cereal seeds, called vernalisation, so that they will flower and fruit in their first season, forms much of the basis of recent agricultural practice in Russia.
The time at which ornamental plants flower is of considerable importance to florists as out-of-season flowers may fetch high prices, and the practice of regulating the daily duration of illumination so as to control the time of flowering of their crop is now being quite extensively employed by commercial flower-growers in America and Europe. By covering such short-day plants as Chrysanthemum with heavy black cloth early each afternoon in summer they can be induced to flower several weeks before the normal time. By leaving lights switched on in the glasshouses until midnight each night during autumn, flowering of Chrysanthemum and orchids can be prevented. Plants can then be made to flower in winter when flowers are in short supply. Sugar cane, another autumn-flowering species, is in some regions prevented from flowering by artificially extending the length of autumn days by floodlighting the fields, thus lengthening the period of vegetative growth and increasing the yield of sugar.
In early experiments the whole plant had been exposed to the favourable photoperiod, but by a series of ingenious experiments in which different
Fig. 1 a-f illustrate experiments which demonstrated the localisation of photoperiodic perception in Xanthium pennsylvanicum. In long days Xanthium remains vegetative (a), but flowers in short days (b). If the immature leaves and apical bud are placed in a box which can be darkened early each afternoon so that this part of the plant receives short days while the mature leaves are exposed to long days the plant does not flower (c), but if the treatment is reversed so that only the mature parts of the shoot receive short days, flowering is induced (d). Subjecting a single mature leaf to short days is sufficient to induce flowering (e), but plants from which all the mature leaves have been removed will not flower even in short days (f).
These experiments suggest that under short-day conditions a flowering stimulus is formed in the mature leaves of Xanthium and is transported to the growing apices of the shoot where it causes flowers to be initiated. The flowering stimulus, which resembles plant and animal hormones by being formed in one part of the organism and utilised, after translocation, in another part, has been called florigen but so far all attempts to extract florigen or any chemical substance which induces flowering when applied to another plant have failed. However, experiments can be carried out which allow us to determine some of the characteristics of this hypothetical substance florigen.
Subjecting a single mature leaf of a Xanthium plant to short days is sufficient to induce flowering, but many other species were found to remain vegetative after this treatment (g). Perilla ocymoides, another short-day plant, would flower after a single leaf had been photoperiodically induced only if all other mature leaves of the plant had been removed (h) or placed in a dark box so that they received no light at all (i). It therefore seems probable that florigen in some species at least is destroyed or inactivated in the presence of leaves which are not producing it. This was demonstrated conclusively by a Russian botanist called Cajlachjan, using plants of Perilla on which only one mature leaf was left. These plants flowered if the single remaining leaf was in short days (j) but not if it was exposed to long days (k). Cajlachjan then subjected each half of the leaf to a different length of day by wrapping black paper each afternoon over the part which was to receive short days. The plants flowered even when only the proximal half of the leaf was in short days, the distal half being in long days (1), but they failed to flower when the distal half of the leaf was in short days and the proximal half in long days (m). If longitudinal halves of the leaf were exposed to long and short days simultaneously the plants failed to flower (n). However, plants flowered in all those treatments where one
Evidently in short-day plants florigen is formed only in leaves which are exposed to short days. It travels through leaf tissue receiving short days or continuous darkness but will either not pass through leaves exposed to long days or is inactivated in them. The inhibition exerted by parts of a leaf receiving long days is only apparent when the long-day part of the leaf is between the short-day part and the terminal bud of the shoot.
We have seen that florigen moves from a mature leaf to the apex of the same shoot but we may ask whether florigen also travels from one shoot to another. To test this, two-branch Xanthium plants were used. One branch (the donor branch) was maintained under short days which induce flowering, the other (the receptor branch) was kept in long days which prevent flower formation in this species. However, both donor and receptor branches of these plants flowered (r), indicating that florigen can be translocated over considerable distance in the plant.
The translocation of florigen has also been demonstrated by grafting experiments in which a photoperiodically induced Xanthium plant (the donor) was grafted to a receptor plant kept in long days. In successful grafts the florigen was translocated across the graft union into the receptor plant which subsequently flowered even though it had never been exposed to short days (s).
A striking experiment which indicates that the florigen of long-day and short-day species is identical was carried out by grafting long-day donor plants to short-day receptors or vice versa. In each case the donor species was maintained under the appropriate inductive day-length, which was of course unfavourable for flowering of the receptor. But in each case the receptor plant flowered despite the adverse day-length conditions, as a result of the florigen it had received from the induced donor plant.
A natural case of grafting and florigen transfer is that of dodder, a parasitic plant which attaches itself to a host plant by means of haustoria which penetrate the tissues of the host. If the host is a short-day species and becomes photoperiodically induced the attached dodder plant will flower. If, however, the host is a long-day species the parasitic dodder plant will not flower in short days but only after the host begins to flower in long days.
Before florigen will cross a graft it is necessary that the two cut surfaces should heal together in a graft union which suggests that the stimulus is able to travel only through contiguous living cells. In the leaf florigen can be made to travel through mesophyll cells by severing the veins of the leaf blade, but in the petiole and stem it probably travels in the phloem with the stream of photosynthetic products. Thus if the petiole tissue is killed by steaming but left intact, florigen cannot pass through the dead cells, even though the transpiration stream travelling in the xylem continues.
From these experiments what general conclusions can we arrive at concerning the mechanism of flowering? It was suggested that florigen of long-day and short-day plants is identical, but it seems that the steps in its synthesis in the two groups must differ. It will therefore be best to look at each separately.
Before short-day plants will flower they must be subjected to a day-length which does not exceed a certain critical value and each short day must be followed by a night of relatively long duration. Short-day plants will not normally flower if kept in continuous darkness and it seems to be the regular rhythm of short days and long nights which causes them to flower. The necessity for light suggests that photosynthesis or one of the intermediate reactions of photosynthesis may be involved in flowering (reaction I). The relationship between photosynthesis and flowering is also suggested by the following facts. Both processes require carbon dioxide to be present in the air and both proceed only in light of high intensity. Furthermore, some plants such as potato which have large photosynthetic reserves are able to flower even when kept in darkness, and in some other short-day plants injected sugars can substitute for the light period.
For flowering to occur each favourable light period must be followed by a long period of uninterrupted darkness. From this we may infer that the substance formed in the light is utilised during the darkness. This dark reaction (reaction II) becomes effective in causing flowering only when the dark period exceeds the critical night-length for the species. Evidently the substance formed in the dark reaction is synthesised only slowly in the leaves each night, and therefore a fairly prolonged period of darkness is necessary. We know this because lengthening the days beyond the critical day-length by means of supplementary artificial illumination even of low intensity prevents flowering. The dark reaction is photosensitive, only occurring in the absence of light. Thus short-day plants will not flower even in short days if the night is interrupted by a brief period of light (reaction X). This light break need last only about one minute and be of low intensity illumination to destroy the beneficial effects of a long night. Furthermore, a light break is most effective in preventing flowering when it is given at about the middle of the dark period. Earlier or later interruption of the dark period is less effective, and if, as previously
The substance formed in the dark reaction is translocated from the leaves, where it is synthesised, to the apices where it causes the initiation of flowers. During this time a further reaction must occur to render the flowering hormone light-stable (reaction III), otherwise plants requiring more than one photoperiodic cycle for induction would always fail to flower, for the hormone would be destroyed during each light period. It is thought that the hormone is light-sensitive only in the leaves and that in the stem it is converted to a light-stable form. Once the hormone reaches the apex it evidently begins to be synthesised in meristematic tissue, and its further synthesis there is independent of length of day, for once flowering has commenced it will continue under all conditions of day-length and after all photoperiodically induced leaves have been removed from the plant.
The sequence of photoperiodic induction in short-day plants can be written as follows:—
Horizontal arrows indicate reactions favourable to flowering, vertical arrows those which inhibit flowering.
Long-day plants flower with least delay in continuous light of high intensity, periods of darkness of all durations decreasing the flowering response, until when in excess of a certain critical value darkness prevents flowering entirely. Thus darkness does not seem to play a positive role in the formation of a flowering hormone in long-day plants. As in short-day plants a high intensity light reaction is a necessary preliminary to flowering (reaction I). The flowering hormone is synthesised directly from the product of this reaction in periods of light (reaction III), and is inactivated during periods of darkness (reaction II). Thus in long-day plants darkness is inhibitory to flowering whereas in short-day plants long periods of uninterrupted darkness are necessary for flowering to occur. Long-day plants will not flower in short days for two reasons. First, in short days insufficient hormone would be synthesised to reach the threshold level, and secondly, this hormone would all be inactivated in the ensuing long dark period.
As in short-day plants the effect of darkness can be negatived by light breaks of low intensity and short duration given at the middle of the dark period (reaction X). However, the effect of the light break is opposite in the two groups because in short-day plants a light break prevents flowering while in long-day plants a light break promotes flowering. Thus again light appears to have two distinct actions, high intensity light promoting flowering, low intensity light breaks reversing the effect of darkness.
The sequence of photoperiodic induction in long-day plants can be written as follows:—
Horizontal arrows indicate reactions favourable to flowering, vertical arrows those which inhibit flowering.
These schemes for the sequence of floral induction in long- and short-day plants are as yet hypothetical explanations of observed facts. The reacting substances formed under the different conditions have not yet been identified, but recognition of the several steps involved in the synthesis of a flowering hormone is a necessary preliminary to a search for the
One of the difficulties faced by workers on photoperiodism is that an experimental alteration in the length of day results in a concomitant alteration in the night-length, the combined length of the day-night cycle still being 24 hours. However, to test general hypotheses it would clearly be desirable to alter the length of one period without affecting the other, or to vary the length of both in the same direction simultaneously. In recent years several experiments in which all of the light is supplied from artificial sources have been carried out, using light-dark cycles varying from one to 72 hours. These experiments give added support to some of the present ideas, but indicate that others may have to be further modified or should be accepted only with caution. However, until more species have been subjected to this type of experiment it is not possible to assess the general validity of the results.
So far we have considered the role of day-length only as it affects the initiation of flower primordia, but although this is the most spectacular response may other activities of the plant are sensitive to the effects of altered day-length.
The vegetative growth of both long-day and short-day species is usually more prolific under long days. In short days the root growth of many species is strikingly decreased, onions may not ‘bulb’, and potatoes may fail to form tubers. Leaf size, shape, texture and pubescence have been shown to vary greatly in many species under different lengths of day.
The onset of dormancy, development of frost resistance, and shedding of leave in deciduous species are, in part, responses to the shorter day-lengths encountered in autumn, and can be delayed by extending the daily period of illumination. Dormant plants can often be made to shoot prematurely by increasing the length of day.
The initiation of flower primordia may not always lead to the completion of the reproductive cycle, because the development of flowers and fruit may require different photoperiodic conditions from those which stimulate floral initiation. Some varieties of soybean are day-neutral and initiate flower primordia even in continuous light. However, the primordia do not develop further under these conditions. Flowers do develop but fail to open when the length of day is 16-18 hours. In day-lengths of 13-16 hours flowers open but no fruit are set, fruit developing to maturity only when the day-length is less than 13 hours. In Xanthium continuation of short days beyond the number necessary to induce flowering results in a high percentage of abortive pollen grains and the development of few fruits, many of which are empty having developed without fertilisation.
The length of day may also determine the type of flower which develops. In Viola cleistogamous flowers, which never open and are self-pollinated,
The length of day is clearly a very potent factor controlling as it does many of the vegetative and reproductive phases of development. Knowledge of the way in which light affects flowering is already being put to use in agriculture and horticulture, but before we can make general use of photoperiodic principles to control more exactly the development of crop plants, we will require much more detailed information concerning the separate stages of development of each species and the chemical control of these several stages.
It is customary for most people to think of sharks only in the terms used by many writers of adventure fiction. In other words, all sharks are sleek ravening monsters of the sea, differing little from each other in appearance, but characteristically with gaping maws and hideous teeth ever ready to seize a human victim should the opportunity present itself. A concept such as this does poor justice to sharks as a whole, for although they do not number many species — only some 250 are known throughout the world — they show a diversity of form, habitat, diet and development equal to that of many other and better known animal groups. Within the confines of their marine environment they invade many niches, and exhibit a variation in structure enabling them to pursue many more ways of life than the fiction writer would have us believe.
In discussing the diversity of a group of animals, it is perhaps useful to begin by giving an account of the sizes reached by the smallest and largest members, for in this way comparisons can be made with other known animals or objects which will then serve as standards. The size variation of sharks is striking. In terms of overall length, the largest species known, the Whale Shark, is at least thirty times longer than the smallest, and reaches certainly to 45 feet and possibly to 60 feet. In contrast the smallest species of which there are several amongst the Cat Sharks and some of the Smooth Dogfishes, mature at lengths of 1 foot 6 inches, and never grow much beyond that. The range between these two extremes is well represented by other species, not only at the small end of the scale, but also by several species of large sharks. Thus the Basking Shark which is known to reach 40 feet and possibly more, and the White Pointer of which the largest specimen measured was 36½ feet long, indicate that the Whale Shark is not an isolated ‘giant’ species but simply the largest known living member of the well graduated series of fishes which we know as sharks. Some fossil sharks apparently grew to an even larger size, for fossil teeth up to six inches long, and from a species similar to if not identical with the White Pointer, have been found in various parts of the world. The longest teeth in the 36½ feet White Pointer mentioned above were only about two inches long, so that it has been assumed that the fossil White Pointers grew to 80 or 90 feet.
The maximum weight attained by the larger sharks is not inconsiderable even though it far from parallels the weight of the other large marine vertebrates, the whales. Unfortunately there are no records of the weight of the largest sharks known, but a specimen of Whale Shark 38 feet long
Diversity in the body form of various sharks is on a comparable scale to that of their size, and the differences are sufficiently marked to separate them into seven major groups each of which has its own distinctive combination of characters. Three of the groups, the Six- and Seven-Gilled Sharks, the Galeoid Sharks and the Spiny Dogfishes, are superficially rather similar, with a ‘typical’ shark-like appearance, but differ from each other and from the other groups in such fundamental features as the number of gill-openings, the presence or absence of various fins, fin-spines and so on. As these features do not greatly alter the overall appearance, they do not call for attention here. An illustration of a Galeoid Shark is given (Fig. 1) for comparison with members of the other groups mentioned below.
The four remaining groups of sharks show a remarkable diversity of form. The Port Jackson or Bullhead Sharks (Fig. 2) which have a massive head, with prominent ridges above the eyes, an almost terminal mouth, and such a characteristic profile as to result in the common name ‘Bullhead Shark’. are the least modified when compared with the ‘typical’ sharks, though there is no doubt about their distinction.
The Frill-Gilled Sharks (Fig. 3) include only one species which is so far known only from deep water off Japan and the eastern North Atlantic, and are eel-like sharks with long, thin bodies and a terminal mouth. The single dorsal fin is set well back along the body, as are the pelvic fins, thereby enhancing the eel-like appearance.
The Saw Sharks (Fig. 4) are rather small sharks, of tropical and sub-tropical waters. They show a remarkable development of the snout which extends forward as a long blade-like process armed with a row of tooth-like structures on each side, and with a pair of barbels on its under-surface about halfway along its length. It is worthy of note that this feature is paralleled in the Skates and Rays, which are closely related to the Sharks, by the Saw-Fishes which grow to a large size — 20 feet or more — and have similar blade-like snouts armed with heavier teeth, but lacking the barbels.
The Angel Sharks (Fig. 5) are small sharks which superficially have a very strong resemblance to skates and rays and particularly to the Fiddler Rays. In other words their heads and bodies are greatly flattened, the paired fins are extended in size, the dorsal fins are placed well back and near to the tail, and the hinder part of the body is sharply marked off from the flattened front part. The resemblances shared in these features by the Angel Sharks and the Fiddler Rays are so great as to necessitate the distinction of the Angel Sharks only by such characters as the lack of fusion of the front margins of the pectoral fins along the sides of the head, and the at least partly lateral position of the gill-openings.
The diversity of form that is seen in the above brief examination of the main groups of sharks by no means exhausts all that occurs in sharks as a whole. Within some of the main groups an almost equal diversity is known,
The Thresher Shark (Fig. 6) differs from all others in that the tail is long and sickle-shaped, its length equal to that of the body. If we add to this the fact that the body is very streamlined, and that in one kind of Thresher Shark the eyes are extremely large, we get altogether a very unusual-looking fish. The Thresher Shark feeds chiefly on schooling fishes, and is reputed to occasionally use the peculiar extended tail as a flail to stun its prey. Other accounts repute that the Thresher will swim around a school of fish while thrashing its tail in the water, thus frightening the fish and keeping them together until it can get amongst them.
The Hammerhead Shark (Fig. 7) when seen in side view has nothing extraordinary about its appearance. The same cannot be said when it is viewed from above or below, for the sides of the head in the region of the eyes are so much expanded that the head resembles a double-ended hammer. The eyes are placed at the outer ends of these ‘hammer’ processes, and hence are literally out on stalks.
Other differences in the shape and appearance of sharks may also be seen in the Galeoids. The Mako Sharks (Fig. 8) which are a well-known big-game fish in New Zealand as well as in other parts of the world, are so streamlined as to be truly torpedo-shaped. The head of a Mako is sharply pointed and thus contrasts strongly with that of the Tiger Shark (Fig. 9) which is almost square-cut. One of the Smooth Dogfishes recently discovered in New Zealand waters has such an attenuate body as to be almost eel-like, though its head is flat and shovel-shaped. Colour differences are also prominent in many Galeoids. The majority of sharks are dark coloured above and lighter below, the dark colour usually being grey, blue or brown, and the lighter colour white or cream. As an example of this the Blue Shark is a brilliant indigo blue above and snow white below. However, many of the Cat Sharks and Carpet Sharks have a pattern superimposed on their backs and sides, and in young specimens of these species the resultant effect is very attractive. The patterning may take the form of spots or blotches, checker-board squares, or chain-like markings. The Tiger Shark is similarly patterned with rather irregular transverse stripes, though these disappear in older animals. It may be added here that in a few sharks, notably some of the deep-sea Spiny Dogfishes, there are luminescent organs in the skin.
Although most sharks are marine, at least a few species are found far upstream in large fresh-water rivers such as the Ganges, while one species is landlocked in Lake Nicaragua. More sharks occur in sub-tropical and tropical waters than elsewhere, and so far only a few species of the genus Somniosus, the Greenland Shark, are known from polar waters. One specimen of Somniosus was washed ashore at Macquarie Is. early this century and is our only record of the genus in southern waters. However,
Oxynotus (Fig. 10) which has high, enlarged dorsal fins, and several other species with flattened, extended, shovel-like snouts. The scales or dermal denticles on these deep-water Spiny Dogfishes are usually very large, and range in shape from bristle-like structures to three-pronged or rounded blades which are variously sculptured on their outer surface. A similar variation in denticle shape and structure occurs in other sharks also, though it is less obvious because of the commonly minute size of the denticles. The greatest depth in which any shark is known to have been caught is 1,500 fathoms; conversely many Carpet Sharks occur on weedy or rocky shores in water only a few feet deep. Most sharks are not markedly gregarious, though a few species such as the Basking Shark seem to form into definite schools' but with rather limited numbers, while others, including the common Spiny Dogfishes which live in depths down to about 100 fathoms or more, may be caught in such large numbers as to suggest schools of a very considerable size.
Three modes of development occur in sharks. By far the greatest number of species are ovoviviparous, the embryos developing within the uterus of the mother, and nourished by the contents of the yolk-sac together with nutritive secretions from the uterus which are absorbed chiefly by the yolk sac. Such embryos are at first enclosed in a thin, horny capsule, though this does not persist throughout the developmental period. In a few species, including the Bullhead Sharks, the Cat Sharks, and some of the Carpet Sharks, development is oviparous, and the eggs are each surrounded by a thicker, horny capsule which hardens when the eggs are laid and protects them while development is proceeding. Often these egg capsules are provided with horny tendrils which serve to attach them to seaweed, and prevent them from being carried away by tides or currents. A few sharks are viviparous, as, for example, the Tiger Shark, and in these species the yolk sac becomes intimately connected to the wall of the uterus
All sharks are carnivorous, with the possible exception of the Whale Shark which may include seaweed as part of its regular diet. The larger predaceous sharks feed chiefly on fish, squid, and other animals not always of small size, as for example seals, sea-birds and turtles. A Greenland Shark was even found to have swallowed a reindeer. A lack of discrimination is shown in the diet of some sharks, for carrion of any sort, and rubbish of all kinds including such unlikely articles as a sack of coal, a kerosene tin, and bottles both empty and full have at times been found to be acceptable to large hungry sharks. The largest sharks, however, the Whale Shark and the Basking Shark, feed only on small planktonic organisms such as shrimps and small schooling fishes, in the same manner as whalebone whales. Like the whales, they have a sieving apparatus for straining this minute food from the water, and consisting of bristle-like gill-rakers in the Basking Shark, and a sponge-like tissue in the Whale Shark. In both cases the teeth are minute. The teeth of most sharks are directed slightly backward, and hence are suited mainly for seizing and retaining food rather than for cutting or crushing it. Even in those sharks with very broad, triangular, blade-like teeth, such as the White Pointer, the gaps between adjacent teeth are so considerable as to prevent a complete cutting action. The upper and lower teeth may also be totally different in appearance, especially in some of the deep-water Spiny Dogfishes, in which the lower teeth are reclined blades while the uppers are awl-shaped or even needle-like. The Smooth Dogfishes and the Bullhead Sharks have flat, pavement-like teeth, which are used to crush the shell-fish, crabs and prawns which form a major part of their diet. In contrast to these hard-shelled feeders it can be cited that some omnivorous sharks will even utilise gelatinous medusae or jellyfish as food.
The above few examples illustrate a little of the variety seen in the food-habits of sharks. It is to be hoped that they will indicate, together with the other features of form, habitat and development mentioned before, something of the diversity and hence the interest of the group as a whole. Such interest is worthy of greater appreciation than the attention which is commonly focussed on sharks due to the sinister reputations of only a few of the species.
Insects popularly known in New Zealand by their Maori name of ‘weta’ belong to the Order Orthoptera and in classification fall into the two families Henicidae Karny, 1937, and Rhaphidophoridae Kirby, 1883. The former family includes all those forms generally referred to as tree or ground wetas which, according to Maori legend, are really the ‘true’ wetas. These are the ‘Taipos’ of the West Coast of the South Island: ‘taipo’, which comes from the Maori, means ‘the devil who comes by night’, and is really a highly descriptive term for the bush-weta. The Rhaphidophoridae includes those insects generally referred to as ‘cave wetas’ though many of their species are also free-living in the forests and are often found on trees.
Many of the species of Henicidae in New Zealand are very large formidable-looking insects; they are, in fact, among the largest insects in the world. In spite of their rather terrifying appearance they are, however, retiring insects which largely hide by day and come out to feed at night. They are seldom aggressive and will only bite with their mandibles if seriously molested, and though the bite from a medium-size weta may be painful only the larger specimens can puncture the skin and draw blood. Normally, when disturbed, these insects tend to run away, seldom showing fight. They run somewhat clumsily but fairly rapidly, seeking out new shelter. Their normal means of defence consists in raising the hind legs high above the abdomen, then drawing them suddenly downwards and backwards so that the large sharp spines on the tibiae can be brought to bear against the attacker. This vigorous raising and lowering of the hind legs produces at the same time a shrill rasping sound from the sounding organ. The overall effect can be quite frightening and is generally sufficient to deter another animal, such as a cat, dog or bird, from an immediate attack upon the insect, which uses this opportunity to make a rapid get-away. A scratch inflicted by the tibial spines of a weta can be quite dangerous due to infection from decaying material amongst which the insect may have been moving.
In handling wetas, to avoid as
The large wetas belonging to the genera Deinacrida and Hemideina seldom jump, but the smaller species of Hemiandrus and Zealandosandrus jump vigorously and will often travel considerable distances in one leap.
The species of Hemideina inhabit foliage and burrow in decaying trunks and branches of trees or in fallen logs, while those of Deinacrida occur amongst foliage or amongst stones, often on high mountains. Hemiandrus species are usually found inhabiting small burrows in soft banks and occasionally fallen logs, whereas Zealandosandrus species are nearly always found in association with fallen trees and logs or in burrows in the earth under stones on mountain-sides.
The technique described in this note is essentially the same as the Avena straight-growth method developed by Bentley and Housley (1954); modifications have been introduced to make the technique suitable for class experiments. Advantages of the method are:—
No special apparatus is required.
In contrast to the coleoptile curvature test, satisfactory results can be obtained consistently without previous experience in the technique.
The linear growth responses are more simply measured than the curvatures in the coleoptile curvature test or the pea test (van Overbeek and Went, 1937).
Oat grains are soaked overnight (the variety Onwards, kindly supplied by Crop Research Division, D.S.I.R., has been found satisfactory). These are then sown fairly thickly on the surface of moistened ‘vermiculite’ in a seed box and are covered thinly with a mixture of approximately equal volumes of vermiculite and sand. The substitution of vermiculite for sand (as used by Bentley and Housley) has two advantages. Firstly, the amount of water used in the initial wetting of the substrate is far less critical since the margin between providing sufficient water for growth on the one hand, and avoiding water-logging with consequent loss of aeration on the other, is greatly increased. Secondly it disposes of the necessity for subsequent waterings during the growth period.
In order to maintain the high humidity necessary for the growth of the test seedlings, the seed box is supported above free water contained in a deep tin, which substitutes for the constant humidity room used by Bentley and Housley. Blotting paper dipping into the water, lines the sides of the tin and a sheet of glass is used for a cover (Fig. 1a). The tin is placed in the dark in an incubator at a temperature of about 25° C.
When the coleoptiles emerge through the surface a red photographic safety lamp supported above the glass plate is switched on. This prevents excessive growth of the first internode. When the coleoptiles are 1.4-1.5 cm. long (about 80 hours after sowing the grain under the conditions described) the seed box is removed and coleoptile sections are prepared in a dark room using a red light which is phototropically inactive.
For the preparation of the sections the following are required:—
A cutter made from two safety razor blades screwed to either side of a piece of wood, 1 cm. in thickness. (Fig. 1b).
A microscope slide on the underside of which is stuck a rectangular piece of white paper so that one of its edges demarks a line 3 mm. from one of the longer edges of the slide.
Coleoptiles are first broken off between finger and thumb at ground level, those of an unsuitable size being discarded. Ten coleoptiles are then laid side by side on the moistened surface of the slide. These are arranged
so that their tips are at the edge of the slide subtended by the 3 mm. line (Fig. 1c). The slide is then upturned and the coleoptiles lowered on to the cutting edges so that one of the latter corresponds in position to the 3 mm. line on the slide. The 3 mm. tips are discarded and the 10 mm. sections lifted from between the razor blades with a pair of forceps. The difficulty of gauging accurately the position of the cuts makes this method of preparing the sections relatively slow. Use of a cutter of the type described by Bentley and Housley would overcome this but
The sections are floated on the solutions to be assayed which are contained in petri dishes. These are incubated in the dark at a temperature of 25° C. for 24 hours. At the end of this period the length of the coleoptile sections is measured to the nearest mm. using a flexible celluloid rule.
The results shown in Fig. 2 are based on an experiment in which five sections were floated on 15 ml. of solution in a 9 cm. petri dish. Three dishes were used in each treatment. Comparison with results reported by Bentley and Housley (1952) suggest that simplifications and modifications of method have not impaired the sensitivity of the assay.
The cyperaceae or sedge family is one of the largest families of flowering plants, variously estimated at up to 80 genera and 3,000-4,000 species. Of universal distribution, it is found in all latitudes and climates from sub-polar regions to the equator and from seashore to alpine meadow. Far from being a family of swamp-dwellers as is usually supposed, it is represented in an astonishing range of habitats, in fact, almost any in which flowering plants will grow. Although the sedges reach their greatest luxuriance in swamps or river-flats and can always be relied upon to occur in such localities, they must not be regarded as confined to such areas. Coastal sand-dunes and beaches, fields, dense forest, scrub and fellfield each have their representatives, many of which are endemic to the particular habitat.
It will be seen then that there is a necessity, in almost any ecological or vegetational study, for a knowledge of the group.
Many elements of the New Zealand sedge flora are of interest from the geographical viewpoint and have a strong bearing on the question of the origin of the flora. Although this is not the place to discuss this question it will be of interest to note that there is a strong affinity with Australia, many species apparently common to the two areas, another with Polynesia and the Indo-Malaysian area, mainly at the generic and subgeneric level, and other relationships, less strong, with South America and South Africa. While acknowledging the influence of these areas, the incidence of endemism must be borne in mind. Many of the genera, and certainly those with more than four New Zealand representatives, have at least some species confined to New Zealand especially in Gahnia, Uncinia and Carex. It appears possible, for instance, that more than half of the species of Uncinia, a little-understood genus, are confined to New Zealand. In Carex a very large proportion of the species are endemic, whole sections of this cosmopolitan genus having apparently evolved within the boundaries of these shores.
Economically, the family is comparatively unimportant except where certain species occur as weeds of cultivated land. In New Zealand this is generally on poorly drained acid soils and a correction of these conditions is usually all that is necessary to gain control. A few species had importance in the culture of certain peoples, e.g. papyrus (Cyperus papyrus Linn.) in ancient Egypt. Tiger nuts or Zulu nuts, the tubers of Cyperus esculentus
The flower is usually regarded as being derived from a typically trimerous form. Each flower is in the axil of a glume and may or may not be surrounded by a perianth. When present the perianth may be of 3-many parts which are scale-like (Oreobolus) (Fig. 34), filamentous (Scirpus (Fig. 26), Eleocharis (Fig. 22)), plumose (Carpha) (Fig. 37) or thick and fleshy (Lepidosperma) (Fig. 24). Other forms are present in overseas species and genera. The stamens are usually (1-)3-6 in number. Some species exhibit a peculiar habit, especially Gahnia and Cladium, in which the glume-margins curl and become involuted near the apex, firmly clasping the filaments just below the anthers; the filaments elongate, sometimes to many times the length of the flower, while remaining attached to the base of the fruit so that the latter in falling from the glume is held suspended from the spikelet (Fig. 45). The fruit is a one-seeded nut (as distinct from the caryopsis of grasses) and may be 2-angled (lenticular), 3-angled (trigonous) or many-angled depending on whether the ovary has 2, 3 or more style-branches respectively.
In the sub-families Cyperoideae and Scirpoideae, the spikelet is regarded as a simple racemose form with sessile flowers arranged in a spiral or distichous manner on the rachis. This appears to be the basic type for the family but the sub-families Rhynchosporoideae and Caricoideae exhibit important and characteristic modifications. In the former, the flowers are formed sympodially, i.e. each flower is terminal on the shoot and surrounded by its subtending glume; the rachilla is axillary to this glume and is produced between the glume and its flower; the rachilla then produces another terminal flower and the process is repeated. The effect of this is to make the flower appear not axillary to its glume but opposite it, with the rachilla between the two. As a result the flower usually appears, on casual examination, to be associated with what is morphologically the node below it, particularly if the rachilla is short and the glumes and flowers crowded. The condition is best seen in Schoenus pauciflorus where the rachilla is long and the flowers separated (Fig. 39b).
In Mariscus ustulatus, the base of each primary branch of the inflorescence is surrounded by a ‘sleeve’-like structure known as the prophyll. If each branch was reduced to a single flower and the rachis so reduced that it did not carry the fertile parts beyond the prophyll then a condition would be reached very much resembling the female flowers of the sub-family Caricoideae. The ‘utricle’ in Carex and Uncinia is considered to be a prophyll at the base of a one-flowered spikelet.
Uncinia fruit (Figs. 19, 20) offers substantiation. The ‘hook’ is a rachilla arising from below the flower and sharply reflexed at the first node. Occasionally aberrant flowers are found in which the rachilla, instead of forming a hook, produces one or more glumes with or without male flowers. On this interpretation, it is now customary to refer to the inflorescence of Carex and Uncinia as being composed of one-flowered female spikelets with one or more many-flowered male spikelets. In these two genera, the structure which Cheeseman, using the old nomenclature, describes as a ‘female spikelet’ must now be referred to as a ‘female spike’. The glumes of Uncinia and Carex are therefore morphologically equivalent to the foliar bracts of, say, Mariscus. (Mariscus is cited merely as a readily-observed example and does not carry the implication that this genus or group should be considered the ancestral form of the Caricoideae. Authorities generally look to the Rhynchosporoideae for the progenitors of the sub-family.)
The illustrations (Figs. A-H) and explanations may be useful in understanding the structure of the inflorescence and flower and the nomenclature used in describing these features.
The first key below is intended to distinguish the families most likely to be confused with the Cyperaceae while the second key, including both native and alien elements of Cyperaceae, is arranged in sub-families so that the grouping is as nearly as possible ‘natural’.
The following explanatory notes are intended as a guide to those wishing to proceed to the species level. Most points of variance with Cheeseman's Manual (1925) are nomenclatural, i.e. the species are transferred to a different genus, and descriptions of them will be found under their former genus. Most introduced species are given except in Carex which will be found in Allan's Handbook of the Naturalised Flora of New Zealand, Government Printer, 1940.