POLLINATION 111 



effected while it is still enclosed within the pollen grain and at 

 the expense of some external agency, for it is not until the pollen 

 grain has alighted upon the stigma and the pollen tube is put 

 forth that the " generative cell " begins to exercise its own feeble 

 powers of locomotion. 



The transference of the pollen to the stigma is effected usually 

 in one of two ways, either by the action of the wind or by the 

 agency of insects. Flowers which are pollinated in the first of 

 these two ways are said to be anemophilous, and they are usually 

 small and inconspicuous, as in the grasses and plantains, and many 

 forest trees. It is a very extravagant method of pollination, 

 involving the production of enormous quantities of pollen, most 

 of which is wasted, for only a very minute percentage of the 

 pollen grains will ever chance to alight upon stigmas. We 

 realize this when we see the enormous quantities of yellow 

 pollen dust which are blown off the pine trees in spring time. 

 The entomophilous or insect-pollinated flowers, on the other hand, 

 have hit upon a much more economical way of doing the 

 business. The development of nectar or honey as a bait for 

 their insect visitors, and of gaily coloured petals or sepals and 

 sweet scents as a means of attracting them, and in many cases 

 of elaborate mechanical contrivances to secure that the insect 

 shall not obtain its reward without doing the work of pollination, 

 all co-operate in bringing about the desired end, and the study 

 of these various adaptations forms one of the most interesting 

 chapters in biological science. We shall refer to it again when 

 we come to deal with adaptation and natural selection. 



In many cases a complete sexual differentiation is manifested 

 by the entire flowers themselves, some having stamens without 

 carpels and others carpels without stamens, and being spoken of 

 as "male" or "female" flowers accordingly. We see this, for 

 example, in the case of the vegetable marrow, where both kinds 

 of flower are borne on the same plant, while in other cases, such 

 as the weeping willow and the Japanese Aucuba, the whole plant 

 may be either " male " or " female," producing flowers of the one 

 kind only. Thus the terminology which strictly speaking is 

 applicable only to the sexual gametophyte, has, as a matter of 

 convenience, been extended to the asexual sporophyte in order to 

 describe the secondary sexual characters which have been trans- 

 ferred to it in consequence of the suppression of the former. 



The telescoping of successive generations one within the other 



