548 BOTANY OF THE LIVING PLANT 



The Land Habit. 

 No circumstance of life has more profoundly affected Organic 

 Evolution than the progression from water to land. The further the 

 comparative study of the simpler living beings is carried, the more 

 the conclusion is confirmed that the birth-place of Animals and Plants 

 was in the water. The comparative study of the Higher Animals and 

 Plants demonstrates fully that it is in subaerial conditions that both 

 have reached their highest development. Accordingly it becomes a 

 question of supreme interest what are the effects impressed upon the 

 organism by a Land Habit in place of its original aquatic surroundings. 

 Certain factors stand out clearly. First, the mechanical requirement 

 for support and maintenance of form. The body, whether of Animal 

 or Plant, is nearly of the specific gravity of water. When immersed it 

 is buoyed up, and provided the water be not itself in violent motion 

 there is little demand on the organism for mechanical resistance. It 

 is different, however, with subaerial organisms, which require not only 

 to support their own weight in the lighter medium of air, but must also 

 maintain their form under the impact of winds and other stresses in- 

 cident to life on land. The larger the organisms the more insistent will 

 be their demand. Such questions of mechanical strength have 

 been taken up for Plants in Chapter X. and need not be discussed 

 again here. A second factor is the need for protection of the proto- 

 plast against loss of water by evaporation. The evidence of its 

 importance is seen in the very general presence of a cuticularised 

 wall covering all the exposed surfaces, as well as in the simple fact 

 that no Land-living Plant sheds its ova from the parent, as so many 

 Algae do. A third factor is the need for internal aeration of the 

 tissues wherever they assume large bulk. Another which has contri- 

 buted to the moulding of Plant Organisms living on land is the require- 

 ment of a large surface of exposure to light and air for photosynthesis. 

 Such factors as these must be considered in their effect on the Evolu- 

 tion of Organisms showing sexuality and alternation, as they adapted 

 themselves to a Land Habit. 



Among the various plants that show the Hofmeisterian alternation 

 the search has naturally been for types of sporophyte which may 

 be held as primitive. The smallest were those first selected as possible 

 links for comparison : and in particular, Riccia. But in view of new 

 detailed knowledge and of the fossil evidence, the trend of opinion 

 is now towards Bryophytes in which both generations bore photo- 

 synthetic equipment fitted for subaerial life, as seen in the Antho- 

 cerotales (p. 475), the Sphagnales, and many Bryales (p. 47^). The 



