ECOLOGICAL 187 



carpels as female organs, they are sporophyls specialised to produce 

 microspores and macrospores; but the flower is more clearly than 

 ever the reproductive part of the plant. 



Ecologically regarded, the flower is that part of the seed-plant 

 that makes the reproduction more secure by establishing certain 

 environmental inter-relations. Thus it is plainly an ecological fact 

 that the majority of flowers attract insect visitors which make sure 

 work of cross-pollination ; and it is another ecological fact that many 

 fruits are attractive to birds which scatter the undigested seeds. 

 Let us begin with pollination. 



POLLINATION. — In ancient times it was known, though very 

 vaguely understood, that a male floral spike of the Date Palm should 

 be hung over a female floral spike if the fruit was to form aright; 

 but it was not till near the end of the seventeenth century that 

 Camerarius demonstrated that in ordinary flowering plants pollina- 

 tion is necessary if fertile seeds are to be formed. Pollination is the 

 actual deposition of the pollen-grain on the stigma of the pistil, and 

 it leads to the downgrowth of a pollen-tube into the style, towards 

 the ovule in the ovary. A male nucleus in the pollen-tube enters into 

 intimate union with the female nucleus (or egg-cell) inside the 

 embryo-sac of the ovule ; and this is fertilisation — the beginning of 

 a new individual life. As the result of fertilisation an embryo is 

 formed in the seed. 



As the great majority of flowers have stamens producing pollen, 

 and carpels producing egg-cells, and as these are in close proximity, 

 nothing seems more natural than self-pollination or autogamy. In 

 the strict sense this means that the pollen of a stamen may serve to 

 pollinate the stigma of the same flower; and this eventually amounts 

 to what is called self-fertilisation in animals like the liver-fluke and 

 the tapeworm. But, as a matter of fact, this autogamy i^ very rare; 

 for in many cases the stamens and the carpels of one and the same 

 flower are not ripe at the same time, and in other cases there are 

 mechanical reasons hindering the autogamy. But while self-pollina- 

 tion is relatively rare, its occurrence has been observed in some 

 common flowers, such as Geranium, Willow-herb, and Enchanter's 

 Nightshade, and it is notably frequent in flowers living in Alpine or 

 Arctic habitats where insects are scarce. 



In some flowering plants there is a strange arrest of development 

 in the flowers, which fail to open and often remain underground. 

 They necessarily show self-pollination. Such flowers, called cleisto- 

 gamous, often occur very early or very late in the year; and they 

 have the advantage of being well-protected from rain and from 

 intruders, and in having their seeds self-sown. Cleistogamy occurs 

 constantly in the subterranean flowers of the fragrant violet and the 

 milkwort; it occurs occasionally in Dead Nettles, Wood Sorrels, 



