CLEISTOGAMY 97 



accorded to it by whicli it would he enabled to first reach and fertilize 

 the ovules. 



Coercion of Insect by Special Forms. — The assuming of a form con- 

 venient for the visiting insect, to which reference has been made, is 

 very frequently interfered Math for the purpose ' of forcing the insect 

 into such a position as shall favor or compel the removal of the pollen, 

 a labor which is by no means agreeable to it and which it not rarely 

 seeks to avoid, as in the case of the bee, which cuts a hole at the base 

 of some corollas, through which its food may be extracted. 



No better illustration of such coercion of the insect by special form 

 could be selected than that of the Asclepias (Fig. 27G). The nectary is 

 at a, in the bottom of a large slipper-shaped pouch. Into this pouch 

 the insect would naturally thrust its proboscis in the direction of the 



276. 



Fig. 275. Highly magnified papillose stigmatic surface. 276. Vertical section of flower of Asclepias: 

 a, nectary; b, blind pouch; c, horn; d, pollinium; e, glutinous corpuscle of same; /, stigma. 



point b, thus avoiding contact with the pollen. The appendage c, 

 however, cuts off this line of approach, sei)arating the blind pocket b 

 from the nectary a. In order to reach the latter, the insect is now 

 forced to seek an entrance at the ])oint d, his head being thus forced 

 into contact with the pollen at e, which adheres and is carried away to 

 be applied to the stigma/ of the next flower visited. 



In spite of the possibility of thus effecting a rough classification of 

 some of the methods of securing cross-pollination, it is yet true that the 

 great majority of instances are not subject to classification and must be 

 denominated special, or else that they combine some si)e(ial arrange- 

 ments with such general methods as have been described. 



Cleistogamy. — Flowers which are self-fertilized before exi)ansion are 

 Clei.stogamous. Occasionally, fertilization takes i)laee without the 

 removal of the pollen from the anther. 

 7 



