HETEROSTYLY 



Heterostyly takes various forms. The simplest and best known 

 is that found in most of the diploid species of Primula. Here there 

 are two types of plant distinguished by the shape of their flowers: 

 the thrum with a short style has the neck of the corolla filled with 

 a thrum of anthers; the pin with a long style has the anthers halfway 

 down the corolla (Fig. 62). In pin and thrum types opposite organs 

 are in corresponding positions, so that a pollinating insect naturally 

 transfers the pollen of one to the style of the other. 



This simple mechanical means of encouraging cross-pollination 

 is not, however, all. The pin flower has a rougher stigma and smaller 

 pollen grains than the thrum. Thus there is evidently some physiolo- 

 gical difference beneath the morphological one. This difference was 

 revealed by Darwin's experiment of comparing the amount of seed 

 set when pin and thrum plants are crossed and when each is bred 

 with its own kind. The one, the "legitimate" mating gives high 

 fertility; the other, the "illegitimate" mating gives lower fertility 

 (Table 21). 



TABLE 21 



SEED SETTING IN THE PRIMROSE (DARWIN, 1877) 



We now know that this effect is due to a difference of growth 

 rate of pollen tubes, as with ordinary incompatibility. The illegiti- 

 mate is slower and, if the two are mixed together, the legitimate 

 gets there first. But there is one striking difference from ordinary 

 incompatibility: all the pollen of one plant behaves in the same way. 

 Yet the pollen (and eggs) of one of the types must be of two kinds 

 in the heredity it carries, because crossing pins and thrums gives 

 equal numbers of pins and thrums. Intercrossing pins gives only pins. 

 Intercrossing thrums, as found in nature, gives 3 thrums to i pin. 

 Thus thrum, in nature, is always heterozygous, and the thrum 



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