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 
zodspores of the aquatic alge, 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, 
