Ill 



probable that, as is the case with sexual and asexual methods, 

 according to the conditions under which plants exist, sometimes 

 self- fertilisation, and sometimes cross-fertilisation, may be most 

 advantageous. That cross-fertilisation, however, is, on the 

 whole, most advantageous in the majority of cases, is indicated 

 by the fact that a very large number of contrivances exist which 

 appear to aim at on the one hand the prevention, or at least the 

 postponement, of self -fertilisation, and on the other hand at 

 facilitating cross -fertilisation. 



99. For the transference of 



pollen from the stamens to the stigmas the principal agencies pilous, Omi 

 employed by plants are wind, insects, and birds. Plants which thophii 

 are pollinated by the wind are called anemophilous and usually 

 have small inconspicuous flowers, with no brightly-coloured 

 perianth, and with no sweet nectar or attractive odour. A 

 the pollen is distributed in all directions by the wind, it is 

 usually produced in very large quantities to insure some of it 

 reaching the female organs, as is the case for instance in Pines 

 and Firs. In Grasses wind -pollination is aided by the fact that 

 the large anthers are versatile and swing freely in the wind, 

 while the feathered stigmas offer a large catchment surface for 

 the pollen. Anemophilous plants are also frequently gregarious, 

 such as are Pines and many Grasses. 



Plants which are pollinated by insects are termed entomo- 

 philous and* those pollinated by birds are ornithophilous. These 

 as a rule characterised by the possession of conspicuously- 

 coloured floral envelopes with often also nectar and attractive 

 scents. The insects, or birds, attracted to the flowers obtain 

 nectar, or pollen, or both, as food and in their visits carry out 

 for the plant the desired transference of some of the pollen 

 to the stigmas. 



100. The chief contrivances by 



means of which plants endeavour to bring about cross-fertili- ing Cross 

 sation are: 



(1) The separation of the sexes. This is effectually secured 

 in dioecious and monoecious plants and to a certain 

 extent also in polygamous plants. This is also 

 effected by dichogamy, i.e. although the stamens 

 and pistil occur in the same flower they mature at 

 different times. Flowers in which anthers mature 

 fost are protandrous, those in which stigmas mature 

 first and become ready for the pollen before the 

 stamens dehisce are protogynous. 



