POLLINATION 71 



flower to another. This is known as cross-pollination. In those 

 plants in which the results of cross- and self-pollination have been 

 compared, cross-pollination has usually been found to yield the 

 more satisfactory results. 



We might even without experiments infer this from the variety 

 and complexity of the arrangements in flowers which on the one 

 hand prevent self-pollination or render it difficult, and on the other 

 lead to cross-pollination, often by very complicated relations 

 between the plant and other living beings. The two great agencies 

 by means of which pollen is conveyed from one flower to another 

 are the wind and insects. We shall thus have to consider the 

 general arrangements in relation to cross-pollination in wind- 

 pollinated and insect-pollinated flowers. Some few plants have 

 the pollen carried to the stigmas by means of water, and in others 

 the transfer is effected by animals of other kinds, such as bats, 

 humming-birds, or snails. These exceptional methods of pollina- 

 tion hardly come into consideration in studying British plants, 

 and need not be dealt with further. 



Whether a plant is wind- or insect-pollinated, we may find 

 on studying it that self-pollination is rendered difficult or im- 

 possible. It is obvious, for instance, that when the staminate 

 and pistillate flowers are borne on distinct male and female 

 individuals, as in the Willow (Vol. IV. p. 45) and the Red Campion 

 (Vol. III. p. 190), self-pollination is an impossibility. The existence 

 of staminate and pistillate flowers on the same individual, as on 

 the Pine (Vol. IV. p. 62), also makes self -fertilisation more difficult 

 and cross-pollination more likely, and the same may be said of 

 such cases as the Ash (Vol. IV. p. 55) and the Cow-Parsnip (Vol. IV. 

 p. i), where flowers of different kinds occur on the same plant. 

 Even when all the flowers contain both stamens and pistil, self- 

 pollination may be prevented, or at least rendered unlikely. 

 Sometimes self-pollination can occur, but no result follows from 

 it ; the flower is self-sterile. In such a flower as the Orchid 

 (Vol. III. p. 200, Fig. 92) the positions of the anther and stigma 

 are such that pollen will not get upon the latter if the flower is 

 left undisturbed. More commonly self-pollination is prevented 

 in flowers with both stamens and pistil by these maturing at 

 different times, so that the flower is at one time in a pollen- 



