SOME OTHER TYPES OF INDIVIDUAL ORGANIZATION 197 



it hinders free interchange of oxygen and carbon dioxide between the 

 plant tissues and the air. This difficulty is met by the development of 

 stomata, of a very simple type but functioning as do those of the higher 

 plants already described. The development of a cuticle and stomata, 

 structures first found in the bryophytes and characteristic of all the higher 

 plants, has made it possible for plants to live in air instead of water. 



Another feature of the land environment that requires adaptation 

 by the plant is the fact that only a part of the body is in contact with a 

 medium (the soil) from which water and essential salts can be obtained. 

 This makes necessary the existence of special absorptive structures. 

 No such structures are needed or found among the thallophytes, the 

 rootlike holdfasts found in some of the algae playing no part in absorp- 

 tion. Here the bryophytes show another advance in organization; the 

 parts of the plant that touch the soil develop numerous hairlike processes 

 called rhizoids, which in part act as holdfasts but also absorb soil water. 

 They are not true roots and compared with roots are relatively inefficient 

 as absorbing organs. 



Although the bryophytes are true land plants, they are in some respects 

 quite imperfectly adapted to land life. 1 They are most numerous and 

 successful in moist or wet environments, and although some of them live 

 in situations that at times become very dry, they do so only by suspending 

 all activity during the period of drought. At such times they wither and 

 apparently die, and begin to function again only with the return of ade- 

 quate moisture. None of the bryophytes attains large size, and most are 

 quite small. Among the factors responsible for this are the following: their 

 mechanism for absorbing soil water is not very efficient and could not 

 supply the needs of a large plant with extensive evaporating surface ; they 

 have no well-developed mechanical tissues to support increased weight ; 

 and, lastly, they have no vascular system to distribute water and food 

 to the parts of a large body. 



The most highly organized green plants. At the highest level of 

 individual organization, we find the members of the last two plant divi- 

 sions, the Pteridophyta (ferns and fern allies) and Spermatophyta (seed 

 plants). These two groups of plants are fully adapted to life upon land. In 

 spite of a great amount of diversity in form and details of structure, they 

 are, in general, built on the same basic pattern as the dicotyledons, that 

 we have already studied in detail. The body is divided into root, stem, 

 and leaf ; it is covered with a protecting epidermis and cuticle or has other 

 evaporation-resisting coverings that replace the cuticle; it contains 

 mechanical tissues that support the body and permit the development of 



1 One important factor that prevents the bryophytes from colonizing the drier 

 parts of the land is their dependence upon water for the accomplishment of fertiliza- 

 tion, as will be explained in the section dealing with reproduction. 



