320 THE BIOLOGY OF FLOWERING PLANTS 



Now it requires but little consideration to see that this 

 type of reproduction is ill suited to secure the dispersal of 

 the offspring. For its success it depends not on dispersal 

 of the gametes, but on their concentration in a limited 

 neighbourhood. The chances of meeting of these minute 

 cells, scattered by water currents, must in any case be small. 

 In Chlamydomonas and other free-swimming organisms 

 the necessity of linking dispersal with reproduction does not 

 exist ; and this is true of most animals which are cha- 

 racteristically motile. The plant at an early stage of its 

 evolution, however, became sedentary ; its lack of mobility 

 is one of its most striking attributes. In all sedentary 

 plants dispersal can take place only when reproductive 

 bodies are cut loose from the parent. Even when the egg 

 cell is set free it is unsuited to dispersal ; when it remains 

 attached, dispersal by it is impossible. This is the condition 

 in the land flora. Some other means of dispersal is there- 

 fore imperative, and there is no doubt that this, along with 

 the possibility of more rapid multiplication given by cells 

 which do not require to pair, is the fundamental function 

 of asexual reproduction. In the algae it is carried out by 

 zoospores, or by non-motile spores which drift in water 

 currents. In the mosses and ferns the spores are never 

 motile. Power of motion, however, ceases to be important 

 when the varying, and often violent, air currents of the 

 land surface are available. Spores are minute bodies and 

 are carried, literally like dust, over great distances. With 

 loss of motility the spore has evolved a positive character 

 of the utmost importance, the double wall which gives 

 the possibility of resistance to desiccation, and of lying 

 dormant for long periods. The spore is thus rendered 

 independent of an immediate supply of water or of a supply 

 at any particular time. The reproductive body of the land 

 plant must be capable of resisting drought and of air 

 dispersal, and the spore of the moss or fern fulfils these 

 conditions. 



Significance o£ Sex. — We have thus a plausible explana- 

 tion of the importance of asexual reproduction, which, 



