64 OBGANOGRAPHY. 



148, II), they will touch and adhere to its head, and be 

 dragged from their jolaces when the insect departs (Fig. 

 148, III). The pedicels dry quickly, and curve downwards 

 (Fig. 148, IV) ; when, therefore, the insect approaches 

 another flower of the same kind, the pollen masses, or polli- 

 nia, as they are called, strike against its viscid stigma, and a 

 portion of the pollen is retained. The polliuia of this flower 

 are in the same manner transferred to the next visited, and 

 so on. When the access of insects is prevented, no seeds 

 are produced, showing that self-fertilization is impossible. 



8i. Many tropical plants cultivated in the conserva- 

 tories invariably fail to produce seed. The cause of this 

 is to be found in the fact that the tropical insects which 

 alone can efiect their pollination are not present. It is not 

 at all seldom that only a certain species, or, at most, only a 

 few species, of insects can fertilize a particular kind of 

 flower, as in the case of bumble-bees and Eed Clover. 

 Many of the adaptations for cross-fertilization, it should 

 be remembered, do not absolutely prevent self-fertilization, 

 so that if insects fail to visit the flowers a few seeds may, 

 nevertheless, be produced. When the flowers are evidently 

 arranged to favor self-fertilization, and prevent cross- 

 fertilization, they are said to be cleistogamous (Gr. 

 kleidos, closed). But no known species is altogether 

 cleistogamous. 



82. Examples of cleistogamy are furnished by one set of 

 flowers of Viola, Oxalis, some Grasses, etc. " Their petals 

 are rudimentary, or quite aborted ; their stamens are often 

 reduced in number with anthers of very small size, contain- 

 ing very few pollen grains, which have remarkably thin 

 transparent coats, and generally emit their tubes while still 

 inclosed within the anther-cells ; and, lastly, the pistil is 



