Fig. 4, a and b: Feulgen squash preparations of Allium triquetrum chromosomes, a, diploid complement (2n = 18, two of each type of chromosome) from a root tip meristem cell in mitosis. The arrows point to one pair of homologous chromosomes. The constrictions in the chromosomes in median, sub-median, and sub-terminal positions are the centromeres, made conspicuous through the use of colchicine during preparation. b, Haploid complement (n = 9, one of each type of chromosome) from a maturation division of a pollen grain produced from a previous meiosis. The arrow points to the single chromosome, equivalent to the two in a. The chromosomes have been squashed from the pollen cell wall (right).