WATER AND LAND PLANTS 



173 



Coming on shore has perhaps had a more profound effect 

 on evohition than any other factor. It is only when land 

 was reached that the higher plants and animals arose. But 

 a change from an aqueous to a terrestrial existence had to be 

 met by many modifications. Animals and plants are, as a 

 rule, of about the same specific gravity as water. Hence they 

 are uplifted, buoyed up, and do not require to make great efforts 

 to move about in that medium. Cer- 

 tain animals move, like birds and 

 bats and many insects in the air, 

 in space of three dimensions, 

 whereas animals that live 



a.c.- 



on the earth are confined 

 to one plane. Land plants, 

 on the other hand, must 

 develop certain organs for 

 fixation. Their tissues must 

 be strengthened so that 

 they can support their own 

 weioht and resist the winds 

 and storms which harass the 

 surface of the earth. Then 

 again, their watery proto- 

 plasm must be protected 

 by such substances as cork 

 from undue evaporation 

 and their most external ^^^ 



62. Longitudinal section through a 



cells must develop a cuticle nucellus; s.e., central nucleus of the em 

 or they would dry up. But bryo-sac; o, egg; « and a.c, other cells of 



, "^ , , , t • 1 the embryo-sac. After Darwm. 



perhaps the most essential 



feature of terrestrial vegetation is the fact that the o^gg is 



protected within the parent plant. 



As we have seen, the sporophyte in the land plant is the 



dominant partner. So long as the gametophyte is small and 



insignificant, as it is in the fern, it is in danger; but when 



gradually, step by step, the gametophyte became embedded 



and retained within the body of the sporopliyte (as seems to 



have Iiappened in the evolutionary history of the flowering 



plants) greater security was achieved. In those cases where 



