354 THE BIOLOGY OF FLOWERING PLANTS 



a haustorium, and that the transport of passive male cells 

 to the embryo sac is secondary ; in the course of evolution 

 the organ has taken on a new function. Branched pollen 

 tubes are also found in some angiosperms. 



We have already noted one ancient character in the 

 microsporangium in the mechanism of dehiscence. We 

 may here refer to two ancient spore characters retained by 

 the pollen grain in many cases, though lost in the embryo 

 sac. The spore is a reproductive body capable of rest and 

 of dispersal. Both features are shown by the pollen grain. 

 The period of rest is, however, generally short, and has 

 significance only in that it permits pollination to take place 

 for a period after the anthers have dehisced. Dispersal no 

 longer means a scattering of the new generation ; it is 

 specialised and provides for the transference of the pollen 

 to the stigma and the bringing together of male and female 

 cells. 



§ II. Pollination — Agencies 



The study of the ways in which pollination takes place 

 may be said to date from the publication in 1793 of Christian 

 Konrad Sprengel's book Das entdeckte Geheimniss der 

 Natur im Bau und in der Befruchtung der Blumen, a 

 pioneer work of the first importance, and a model of exact 

 investigation. Since then a colossal literature has arisen 

 round the subject, in which the names of Darwin and 

 Hermann Miiller are conspicuous. In Knuth's Handbook 

 of Floral Biology we have a standard compendium of 

 knowledge. Pollination is dealt with in all text-books of 

 botany, and many of the more striking cases are familiar, 

 so that we may here confine ourselves to a statement of 

 principles with a few illustrative examples. 



Pollination may be carried out by a number of agencies. 

 The most important are insects {entomophily), and the wind 

 (anemophily). Much less common is pollination by birds 

 (ornithophily), or by snails (malacophily), or by water 

 {hydrophily) . 



