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 
