326 CHAPTERS ON EVOLUTION. 



separation of stamens and pistil. Other conditions, more or less uniting 

 these dispositions of the stamens and pistil, may be found in flowers. 

 In a daisy which is a collection of flowers we find the outer or white 

 florets to possess pistils but no stamens, and the yellow and central 

 florets to possess both stamens and pistil. We can readily discern 

 that all such arrangements secure pollen of essentially foreign kind for 

 fertilisation. Self-fertilisation is, in fact, impossible in such cases as 

 those just described ; and some very curious facts are found in botanical 

 archives concerning the difficulties experienced in obtaining " seeds " 

 where one of the necessary elements usually the pollen for fertilisa- 

 tion was absent. The variegated laurel presents a case in point. The 

 first specimen of this species introduced from Japan was a pistillate 

 or female plant, and could produce ovules from its flowers, but no 

 "seeds" ; inasmuch as, no pollen from another and staminate plant 

 was forthcoming. The plant was largely reproduced from slips alone 

 until within comparatively recent years, when staminate plants being 

 imported, pollen was then forthcoming for the production of seed. 

 The Egyptians have long been in the habit of bringing palm-branches 

 bearing stamens from the desert, in order to fertilise the domesticated 

 pistillate or fruit-bearing palms grown at home. This necessary pro- 

 cess was frustrated in 1808, when the French occupied Egypt, and 

 when the stamen-laden branches could not, in consequence of foreign 

 invasion, be procured. In the well-known Vallisneria spiralis, a 

 water-plant of Southern Europe, which, like the willow and palm, has 

 stamens and pistils on separate plants, the pistillate flowers are borne 

 to the surface at the proper period by the relaxing of a spirally coiled 

 stalk on which they are supported. The stamen-bearing flowers, on 

 the contrary, are borne on short stalks, and, becoming detached 

 therefrom, float to the surface of the water. There they scatter their 

 pollen, which reaches the pistillate flowers, and the latter being fer- 

 tilised, are drawn by their stalks once more beneath the water, where 

 the seeds mature and the fruit in due course ripens. 



The present is perhaps a fitting stage of our inquiries to remark 

 that the tendency towards cross-fertilisation in nature is nowhere 

 more strongly marked than in cases where a plant is utterly infertile 

 with its own pollen, but perfectly fertile when impregnated with 

 pollen from another plant of the same species, or, in some not- 

 able instances, from ah entirely different species of plant. Species 

 of passion-flowers have been found sterile with their own pollen, 

 although " slight changes in their conditions, such as being grafted 

 on another stock, or a change of temperature, rendered them self- 

 fertile." More extraordinary still, however, is the knowledge of the 

 fact that the pollen of some orchids actually acts like a poison if 

 placed in what one would have deemed the most natural position for 

 it, namely, on their own stigmas. Such facts as these entirely alter 



