PLANTS 37 



algae, reproduction does not occur; in others, such as the desmids and 

 diatoms, it has not advanced beyond the fusion of two single cells. 

 Both plant body and reproductive organs undergo some advance in 

 the thallophytes, but this is relatively slight until liverworts and 

 mosses are reached, the mosses exhibiting a threefold differentiation 

 into protonema, leafy gametophyte, and semi-independent sporophyte, 

 The specialization of the shoot in many ferns ajiproximates that of 

 the flowering plants, though the gametophyte is still regularly autono- 

 mous and the disseminule is a spore rather than a seed. 



AVith the advent of the flowering plants, the shoot began to 

 vuldcrgo a marked differentiation entirely independent of the phyletic 

 form or family, though this process was little felt by the gymno- 

 sperms. In consequence, it is only in the angiosperms that the 

 specialization of life forms becomes a characteristic feature and is 

 accompanied by even greater modification of flower and fruit. Since 

 the flowering plants comprise practically all the dominants of terres- 

 trial vegetation today, it will suffice here to consider their life histories 

 alone. These are treated under the following main heads: germina- 

 tion, growth, movement, propagation, and reproduction, together with 

 the expression of these in the community functions, reaction, compe- 

 tion, and coaction. 



Germination, Seeds and fruits differ in a number of respects that 

 have a bearing upon that portion of the life history represented by 

 germination. Protection in the form of a hard or bony coat operates 

 against destruction, through digestive fluids, of most fleshy fruits or 

 seeds, though it also may be concerned with rendering them unpala- 

 table. For a few, germination is made possible, or at least is im- 

 proved, by passage through the digestive tract of some animal, and 

 this is likewise true of some fungus spores. Finally, in a few grasses 

 and geranials, the awns of the fruit serve not merely for dissemina- 

 tion, but also for forcing the seed end into the soil, as is notably the 

 case in Stipa and Erodium. 



Growth. The development of a representative flowering plant ex- 

 hibits three major phases, seedling, juvenile, and adult, though without 

 definite limits between them. Two related processes, propagation 

 and reproduction, are likewise of major significance, but their posi- 

 tion in the sequence of development is often less definite. Flowering 

 and fruiting may mark the last phase of the life cycle, as in the 

 annual, or the final phase for the year, but they may also occur at 

 the outset of the season or during its middle course, to be followed 

 by the development of leafy shoot or underground stem. A consider- 

 able number of herbs do not flower until the second year, and some 



