266 EVOLUTION OF PLANTS 



terrestrial environment have become greatly modified. 

 Thus there are developed various provisions against 

 injury from loss of water, either by the plants as a 

 whole acquiring the power of becoming completely 

 dried up without being killed, or the outer tissues of 

 the plant becoming more or less impervious to water ; or 

 the more delicate portions may be protected in various 

 ways from the injurious effects of drouth. The tissues 

 are always firmer than those of water plants, as the 

 plant no longer is supported by the medium in which 

 it is growing, but must depend upon the rigidity of its 

 own tissues. 



The spores in the mosses and all the higher plants 

 have lost the power of locomotion possessed by the 

 zoospores of the aquatic algae, and this loss of motion, 

 as well as the thick walls with which they are fur- 

 nished, are adaptations to the changed environment, 

 where the spores depend for their distribution, not 

 upon water, but upon air currents. It is interesting to 

 recall that even in these terrestrial plants there is a 

 reversion to the primitive aquatic condition when fer- 

 tilization is effected. 



The abandonment of the aquatic habit in the higher 

 plants is associated with marked increase in the impor- 

 tance of the sporophyte, or non-sexual spore-bearing 

 generation. This first results in the very marked 

 alternation of generations in the Archegoniates, 

 mosses and ferns, and finally has produced the seed 

 plants, where the gametophyte is greatly reduced and 

 is never capable of independent existence. The inde- 

 pendence of the sporophyte, first found in the ferns, is 

 associated with the development of special organs, stem, 



