AMPHIBIOUS HABIT 83 



had to be substituted. This was done by the production of the sporo- 

 phyte from the zygote : once fertilised a zygote might in these plants 

 divide up into a number of portions (carpospores), each of which would 

 then serve as a starting-point for a new individual ; and dry circumstances, 

 under which they would be powdery, would favour their dispersion, as 

 in the lower Liverworts. In proportion as these plants spread to higher 

 and drier levels (in accordance with the advantage which they gained by 

 escape from competition, and more free exposure to light for assimilation) 

 the chance of a frequent recurrence of the circumstances necessary for 

 sexual reproduction would be diminished, and the dependence upon 

 carpospores for propagation would increase ; consequently the number of 

 spores produced by each sexually formed sporophyte must be larger, if 

 the race is to survive, and be in a position to compete. Any increase 

 in the number of spores entails greater supply of nourishment during 

 their formation : this in the phylum of the Bryophytes , is chiefly supplied 

 from the gametophyte, which shows distinct adaptation to sub-aerial habit, 

 while the means of nutrition on the part of the sporophyte itself are in 

 these plants very limited, and the external morphological complexity of 

 it very slight. In other phyla, however, such as the Filicales, Lycopodiales, 

 and Equisetales, the sporophyte itself assumed the function of nutrition : 

 a higher morphological differentiation of the parts followed, and a more 

 clear distinction between the organs which were to supply the nutriment 

 (stem, leaves, and roots), and the parts devoted to the formation of 

 spores (sporangia) : this for the first time stamped the sporophyte with a 

 character of independence and permanence, while the number of spores 

 produced might now be practically unlimited : in these respects the 

 Pteridophytes are immeasurably superior to the Bryophytes. One strange 

 point in the whole story is, however, the tenacity with which these plants 

 (under the obvious disadvantages which it entails when their habit is 

 sub-aerial) retain their aquatic type of fertilisation : it is only when we 

 reach the Phanerogams, where the sporophyte attains its climax while 

 the gametophyte is almost abortive, that we see the sexual process 

 accommodated to that sub-aerial life which had led to the dominant 

 position of the sporophyte ; for in them the fertilisation is siphonogamic, 

 being carried on by the pollen-tube : these plants are therefore independent 

 of external fluid water for their fertilisation, and this fact has doubtless 

 contributed largely to their present ascendency. When, as in the preceding 

 sketch, we consider what the results of the migration from water to land 

 must have been, the permanence and constancy of the antithetic alter- 

 nation explains itself. The permanence or morphological fixity of a 

 phenomenon in any phylum is in a sense proportional to its importance 

 in the well-being of the organisms : given a conservatism in the mode of 

 fertilisation (which it is difficult to explain), the rise and progress of 

 the sporophyte in the Archegoniate series appears to be a natural 

 outcome of the migration from water to land. 



