115 



pollen, so that, on visiting any other flower, it must inevitable 

 rub off some of this pollen on to the edge of the stigmatic disc 

 and may thus effect cross-fertilisation. The flowers of this 

 species are often visited by small ants and beetles which 

 consume both the pollen and the nectar. Such insects only 

 occasionally spring a stamen and then, as they do not leave the 

 flower at once, but continue to crawl about in search of food, 

 they must at all events occasionally bring about the pollination 

 of the stigma with the flower's own pollen. Moreover, as the 

 pollen- covered valves of a sprung stamen are practically on the 

 same level as the stigmatic surface, it seems certain that some 

 of the flower's own pollen must, sometimes at least, reach its 

 stigma. Thus while self -fertilisation is not prevented, more 

 or less frequent cross-fertilisation is insured. 



103. It must always be re- 

 membered that, although cross-fertilisation may be, and 

 apparently is as a rule, preferable to self -fertilisation, it is 

 easy for a plant to, as it were, over-reach itself, if in endeavour- 

 ing to secure the former it makes the latter absolutely 

 impossible ; for in the event of cross-fertilisation not being 

 effected, which is in many cases a possible contingency, 

 no seeds at all would be formed and sexual reproduction 

 would absolutely fail. Thus many plants appear to find it 

 safest to at all events insure self -fertilisation in the event of the 

 failure of their efforts to secure cross-fertilisation. This is 

 perhaps most clearly seen in plants which, like many violets, 

 possess two kinds of flowers, (1) large conspicuous flowers 

 adapted for cross-fertilisation, and (2) small, inconspicuous 

 closed flowers adapted solely for self-fertilisation ( cieisto- 

 gamie flowers], the latter insuring a supply of seed in the 

 event of the ordinary flowers not being fertilised. 



On the other hand there are certain species which trust en- 

 tirely to cross-fertilisation. The pollen of some of these plants 

 not only fails to effect the fertilisation of its own flower but 

 acts as a poison, and if applied to the stigmas results in the 

 death of the flower. Finally it must be noted that if a stigma 

 has been pollinated with pollen from its own flower, self -fertilisa- 

 tion may be prevented and cross-fertilisation effected, even if 

 the stigma is pollinated after a considerable interval with 

 pollen formed in another flower of the same species, owing to 

 the latter pollen being prepotent, i.e. able to effect fertilisation 

 quicker than the flower's own pollen. 



104. To insure the wide distribu- insemina- 

 tion of seeds various devices are employed by plants. In tiou of Seeds - 



i % 



