158 PLANT-LIFE ON LAND [CH. 



for the most part relatively dry. A fixed habit is 

 obligatory for them in order to secure their supply 

 of water and soluble salts from the soil. The 

 development of special tissues for conduction and 

 for mechanical strength followed, and these are seen 

 in a simpler type in the Mosses, but in quite an 

 advanced condition in the Ferns and Horsetails. 

 It was, however, on the propagative methods that 

 the change of habitat from water to land brought 

 special disabilities. The absence of external fluid 

 water, except at intervals, put a check upon pro- 

 pagation by self-motile germs. Fertilisation by 

 spermatozoids motile in water could only take 

 place at times of rain or copious dew. As a set 

 off against this the formation of numerous detached 

 spores, dry and dusty when ripe, and thus readily 

 transferred by the wind, became a marked feature 

 of this primitive vegetation of the Land. It is seen 

 carried to high perfection in Mosses and Ferns 

 (Chapter III). By this means, while sexual repro- 

 duction in such plants was necessarily an uncommon 

 event, a vast number of individuals might be produced 

 as a consequence of a single fertilising act. This 

 arrangement provided adequately for the maintenance 

 of the race, and for the spread of its individuals. 

 The provision for intercrossing was, however, deficient. 

 It is true that there is an attraction of the motile 

 sperm to the archegonium ; but the sphere of such 



