552 BOTANY OF THE LIVING PLANT 



itself thoroughly to sub-aerial conditions. The climax of the game- 

 tophyte on land is attained in the hornosporous Mosses and the leafy 

 Liverworts. It appears of independent, though limited growth in 

 the hornosporous Ferns. But with heterospory it fades into insignifi- 

 cance, and in the Higher Seed-Plants it survives as a mere relic. 



Rise of the Sporophyte. 

 Although Brown Algae such as the Laminariales afford evidence of 

 the rise of the sporophyte generation it is in the progressive series of Land- 

 living plants that this phenomenon is most strikingly disclosed. By 

 adaptation of form and structure it has met, in its highest terms 

 successfully, the requirements for mechanical strength, for protection 

 under drought, for exposure of a large surface for photosynthesis, and 

 for ventilation of extensive nutritional tissues. In all these respects it 

 is the superior of the gametophyte, and perhaps the structural feature 

 that has contributed most to its supremacy is the ventilation-system 

 of intercellular spaces, controlled by stomata at their exits to the open 

 air. This differentiates the sporophyte from the gametophyte more 

 clearly than any other structural character, and stamps it as adapted 

 to sub-aerial life. The end of its vegetative existence is the formation of 

 spores (tetraspores). The more numerous these are (other things being 

 equal) the better the chance of survival of the species, and of its spread. 

 The vegetative development may be regarded as a means to that end, 

 and in hornosporous forms its nutritive capacity imposes a natural 

 limit on spore-numbers. The dispersal of the spores is dependent in 

 primitive land forms upon a dry atmosphere. This is in strong 

 antithesis to the necessity for external liquid water for fertilisation, 

 which is the end of their gametophyte. 



The sporogonium of the Bryophyta is usually held to represent 

 the most primitive type of sporophyte among Land-living Plants, 

 and recent discoveries of very early fossils tend to support that 

 opinion. Its limited plan of construction, its ephemeral character, 

 the absence of appendages, and its dependence throughout life upon 

 the gametophyte, are all indications pointing in the same direction. 

 Moreover, the fact that the spore-production in each sporogonium 

 arises from one undivided sporogenous tissue, not from a number of 

 distinct sporangia, also points to the same conclusion. Within the 

 Bryophyta the various forms may be seriated so as to give probable 

 indications of progressive advance in various important characters. 

 These suggest that in very primitive types apex and base were 

 defined early. A sterile stalk and central columella were acquired, 



