3i6 



The Living Plant 



pollen and stigmas (figure 117); — a plan of structure called 

 dimorphism. When the suitable insect visits in succession several 

 flowers of the different kinds, it receives pollen on its body from 

 the upper stamens in a position to leave it on the tall stigmas, and 



the same for the shorter kinds; 

 while any accidental pollination 

 of a stigma from the same or a 

 similar flower produces no effect, 

 because of the differences in pollen 

 and stigmas aforementioned. 



But although such elaborate ar- 

 rangements exist in adaptation to 

 the prevention of close pollination, 

 in other kinds of flowers there are 

 features which as obviously secure 

 it. Thus, in a great many of the 

 simpler and regular kinds of flow- 

 ers, the pollen falls normally on 



Fig. 116.— An Iris flower in partial sec- -i afitrmfl'^ of thp qnmp flnwpr 

 tion. Above the stamen is a potaloid ^'^^ StlgmaS 01 lUe Same HOWer, 



style ending in a projecting shelf of ^nd produces close fertilization in 



which only the upper surface, shielded 



from the stamen, is stigmatic. (Copied Case UO CrOSS polleU is reCeivcd, 



from Gray's Structural Botany.) , it 1 1 • i • ^ 



thougn II cross pollination does 

 occur, then cross fertilization is effected instead. But a much 

 more extreme case is found in those flowers which never open 

 at all, and in which the pollen-tubes grow out from the 

 anthers to the immediately contiguous stigmas, and thence 

 effect fertilization in the usual way (figure 118). Such flowers, 

 called cleistogamous, lie close to the ground, and are well known 

 in Violets and some kinds of Oxalis; but this fact is conspicuous 

 about them, that the same plants in all cases possess also the 

 ordinary showy kinds of blossoms cross pollinated by insects. 

 Obviously, therefore, cleistogamous blossoms, like the cases of 

 close pollination earlier mentioned, represent a method of en- 

 suring close fertilization in case a cross should happen to fail, 



