To begin at the beginning, we must first look at chromosomes. If we take the tip of a root or stem that is actively dividing, and look at single cells, suitably stained, under a high enough magnification, we can see the chromosomes—a constant number of dark, sausage-shaped bodies in each nucleus (5.2). In most cacti, for example, there are 22, 11 derived from one parent and 11 from the other. Each set of 11 chromosomes contains the genes in linear sequence, and by their interaction with the cytoplasm they control the expression of every heritable character of the plant. In the simplest case, one gene controls one character—flower colour, for example. Genes are liable to mutate (that is, to change permanently), and to these mutations we owe the diversity of plants as seen today.
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