356 OUTLINES OF EVOLUTIONARY BIOLOGY 



bee, Bombus terrestris, thus affording an example of imperfect 

 adaptation. 



\ We may now consider a few definite examples of the manner 

 in which flowers may be specially adapted in structure so as to 

 secure the advantages of cross-fertilization by insect agency. 



If a number of plants of the common primrose, the oxlip, the cow- 

 Blip or the polyanthus (species of the genus Primula) be examined 

 carefully, it will be seen that in each case two quite different 

 forms of flower occur. In other words the flower is dimorphic. 



The two forms are known to 

 gardeners as " pin-eyed " and 

 " thrum-eyed ' ' respectively. In the 

 pin-eyed flowers (Fig. 180, A) the 

 style (g) is comparatively long and 

 the stigma (n) appears as a round 

 knob, like the head of a pin, in the 

 centre of the flower, at the entrance 

 to the long tube formed by the 

 lower part of the corolla. The 

 anthers (a) lie much lower down in 

 the tube, so that they are invisible 

 until the flower is cut open. In the 

 thrum-eyed flowers (B)the positions 

 of the stigma and anthers are 

 reversed ; the style being much 

 shorter the stigma lies only half 

 way up the tube, while the anthers 

 appear in the centre of the flower, 



in the mouth of the tube. The two kinds of flower are 

 always found on separate plants, and the long-styled and short- 

 styled plants are said to occur in about equal numbers under 

 natural conditions. The pollen grains also differ in the two 

 kinds of flower, those of the thrum-eyed being larger than those 

 of the pin-eyed and of somewhat different shape. This particular 

 kind of dimorphism, which is sometimes known as heterostylism, 

 is extremely characteristic of the genus Primula, and it has 

 recently been shown that the distinguishing features of the two 

 forms are inherited in a Mendelian fashion. 



The flowers of the primulas are fertilized by the agency of 

 insects such as humble bees, and Darwin found that if insects 

 v>ere carefully excluded by covering the flowers with a net, little 



A 



\ 



B 



FIG. 180. -Heterostyled Flowers 

 of the Oxlip (Primula elatior] 

 in longitudinal section. 

 (From Vines' " Botany.") 



A, long-styled; B, short-styled flower, 

 a, anthers; c, corolla; f, ovary; 

 g, style ; k, calyx ; n, stigma. 



