100 PLANT-LIFE ON LAND [ch. 



secondly, the adoption of a fixed position by the 

 plants of the ordinary land-vegetation for purposes 

 of nutrition, have introduced certain disabilities into 

 the life of the plant, and that these have had far 

 reaching effect upon the course of further develop- 

 ment. The early encystment, while it prevented the 

 free movement of the whole body of those aquatic 

 organisms which adopted it, affected their general 

 biology in a comparatively minor degree. For it Avas 

 still open to them to extrude individual protoplasts, 

 for purposes of propagation, into the water in which 

 they lived. And many of the lower forms do so 

 habitually, thus securing by their motility both 

 spread of the germs and opportunity for fertilisation. 

 But with the spread of vegetation to the land, where 

 the medium of water no longer gives free opportunity 

 at all times for the movement of germs, the need for 

 other means of spread became urgent. The results 

 are seen in the multifarious methods of distribution 

 of spores and seeds. The adoption of the seed-habit 

 necessitated also some mechanical means of transfer 

 of the pollen. This, together with the advantages 

 derived from intercrossing, have led to the wonderful 

 and varied devices for securing pollination. The 

 prime cause of the difficulties was the encystment of 

 the protoplasm, and the consequent immobility of the 

 organism as a whole. Fixity of position has in fact 

 dominated the whole course of evolution of plants. 



