THE PLANT KINGDOM 249 



proportion of the activities of plants seems to be devoted to the 

 successful accomplishment of the sexual process. 



4. Alternation of Generations. — The three steps in plant 

 evolution which we have mentioned — the appearance of the 

 multicellular individual, of functional and structural differentia- 

 tion, and of sexual reproduction — occurred also among animals. 

 The fourth belongs exclusively to the plant kingdom. It is the 

 evolution of that remarkable double life-cycle which we know as 

 the "alternation of generations" (Fig. 138), a process simple 

 enough in its underlying principle and in its expression among 

 the lower plants, but which in the higher groups leads to such 

 complexities than an understanding of the process of repro- 

 duction is more difficult in the vegetable kingdom than among 

 animals. 



A. In Thallophytes. — In most members of that great and primi- 

 tive group the Thallophytes, which include the algae and the 

 fungi and which is the lowest of the four main divisions of 

 the plant kingdom, a fertilized egg gives rise directly to a single 

 new individual, just as is the case among animals. Beginning 

 rather obscurely among some of the higher Thallophytes and 

 reaching its full expression first in the liverworts and mosses, we 

 find a modification of this simple and direct method. The 

 fertihzed egg, instead of producing a new plant like the parent, 

 divides into a group of cells which separate and are liberated 

 as non-sexual spores. Each spore may now give rise to a 

 new plant. In this way a single sexual union produces a 

 whole group of new plants instead of one; and it is this ad- 

 vantage, seized upon and perfected by early members of the 

 plant kingdom, which probably led to the characteristic "alter- 

 nating" systems of reproduction in all plants above the Thallo- 

 phytes — a fertilized egg producing a group of spores, each of 

 which grows into a sexual plant, in which fertilization again 

 takes place. 



B. In Bryophytes. — Such a simple condition as this, however, 

 was evidently soon outgrown and now occurs among only a few 

 lowly forms. The rapidly enlarging group of cells which had 

 their origin from the fertilized egg soon began to produce some- 

 thing more than a mass of spores. The outermost cells became 

 differentiated into a protecting wall, and the spore-case or 



