2 INTRODUCTION 



of the great groups of Bryophytes, Pteridophytes, and Seed-plants may 

 be found in close juxtaposition, and sharing the same external conditions. 

 On the sea-littoral it is otherwise : there Algae are found associated 

 together almost to the exclusion of other plants. Nevertheless, occasional 

 Phanerogams do invade the belt between tide-marks, and thus even this 

 limit between the Vascular Flora of the land and the Algal Flora of the 

 sea-littoral is apt to be blurred. 



It is plain, then, from such simple examples as these, which might 

 be indefinitely varied and extended, that the problem of the origin of 

 a Land-Flora is not to be solved by any mere reading of the facts of 

 distribution into terms of the evolution of the characteristic plants of 

 the land. Some other basis than that of distribution at the present day 

 must be found for the solution of the problem. It is to be sought for 

 in their comparison as regards structure and function, and that not only 

 in the most complete condition of full development, but also in the 

 successive phases of the individual life-cycle. 



The study of the form and structure of plants, as well as of their 

 physiology, directs attention naturally to the water- relation : this more 

 than any other single factor dominates the construction of land-living 

 plants, while comparison with kindred aquatics shows how profoundly 

 land-living plants are influenced by the necessity of adequate water-supply. 

 But not only is this dependence of land-plants on water a general 

 feature of the whole life-cycle: in certain' large groups of plants it is 

 found that leading events in the individual cycle are directly dependent 

 upon the presence of external fluid water. The importance of such 

 matters in relation to the present problem of the Origin of a Land-Flora 

 will be gauged by their prevalence and constancy in large groups of 

 organisms. Now in the whole series of Archegoniate Plants (Mosses and 

 Ferns), and in some Gymnosperms the act of fertilisation can only be 

 carried out in presence of fluid water, outside the actual tissue of the 

 organism : their spermatozoids are for a time independently motile in 

 external water, and it is a mere detail that in the higher and more 

 specialised forms, the distance to be traversed is only short from the 

 point of origin of the spermatozoid to the ovum which it is to fertilise. 

 The importance of fertilisation need not be insisted on here : everyone 

 will admit it to be a crisis, perhaps the most grave crisis, in the life-cycle 

 of the plant. When this critical incident in the life is found, in so large a 

 series of allied plants as the Archegoniatae, to be absolutely dependent on 

 the presence of external fluid water for its realisation, that fact at once 

 takes a premier place in any discussion of the relation of plants to water. 



A comparison of the Seed-Plants with the Archegoniatae leads without 

 any doubt to the conclusion that their method of fertilisation by means 

 of a pollen-tube is a substitution for that by means of the motile 

 spermatozoid. The Seed-Plant by adopting this siphonogamic mode of 

 fertilisation becomes thereby independent of the presence of external 



