ZYGOMORPHY AND DIMORPHISM 323 



flowers, however, the stamens are few, and the pistils 

 few or only one. Here often the flowers become one- 

 sided (zygomorphic), of such a structure that access to 

 the nectary can be obtained only at such a point that 

 pollination is rendered all the more certain. In this 

 connection adaptation of flowers to certain insects is 

 very apparent. Thus certain 

 orchids are of such a structure 

 that only certain butterflies or bees 

 can reach the nectary, and in so 

 doing pollinate the flowers. Other If \/_ ^ 

 insects either cannot reach it at 

 all, or in so doing fail to remove 



' 3 FIG. 202. Proterogynous 



the pollen Or transfer it tO the (Plantago) and proteran- 



drous (Claytoma) flowers. 



stigma. 



589. In connection with entomophily it was early ob- 

 served that many flowers were of such structure that self- 

 fertilization (i.e. pollination with pollen of the same 

 flower) is impossible. Thus in the majority of such 

 flowers the pollen is all shed before the stigma is recep- 

 tive (proterandrous) , or much less frequently the stigma 

 passes the receptive stage before the pollen is set free 

 (proterogynous). In some plants the 

 flowers are "dimorphic," i.e. on certain 

 individuals the stamens are at one level 

 and the stigmas at a different level in the 



riG. 203. Di- M . -i i* * i i 



morphic flower same flower, while in other individuals of 



(Primula). '. 



the same species they occupy the reverse 

 positions. An insect visiting the flowers of the first 

 plant, becomes pollinated at a definite part of its body 

 which does not come into contact with the stigma at 

 all in that same type of flower. When, however, it 

 visits the other type of flower, the stigma is at the 

 level of the stamens of the first type, and it comes in 



