226 LIFE HISTORIES AND EVOLUTION OF THE 



arose which were more complex than the thallus, since they 

 showed three regions, stems, leaf-like blades, and holdfasts. 

 We shall see later that the stems, blades, and holdfasts of the 

 highest algae do not correspond to the stems, leaves, and roots of 

 fern and seed plant, which developed very much later through 

 a complicated history. But this differentiation of the plant body 

 in the thallophytes is, at least, a response to the same sort 

 of influences as guided the development of the higher plants. 

 These influences were in part the evident advantages to a plant 

 of being fastened to a suitable attachment, from which it can 

 grow and present as much surface as possible to the sunlight. 



In conclusion, one should think of the algae as comprising a 

 large number of divergent lines, whose relationships are some- 

 times so distant that one cannot make even a good guess as to 

 the evolutionary history. The stoneworts (Char ales} constitute 

 perhaps the best illustration of such an isolated group. Very 

 few of the algae now living are near the theoretical main line 

 of ascent to the liverworts and mosses. The algae should be 

 thought of as spreading out in many directions, each group 

 developing in its own particular line of evolution, adjusting 

 itself as best it may to its particular sort of life. Some pos- 

 sible relationships have been suggested in the accounts of the 

 various groups, but the subject is too complex to be given 

 detailed consideration here. 



