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 + NS, 
that only certain butterflies or bees 3 
can reach the nectary, and in so 
doing pollinate the flowers. Other 
insects either cannot reach it at 
all, or in so doing fail to FEMOVE -y,,. 903 Proterogynous 
the pollen or transfer it to the Fianteno) yennd, prey 
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 
Fie. 203.—Di- es - oe Aaa" 6 
er flower same flower, while in other individuals of 
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 
