196 



THE INDIVIDUAL ORGANISM 



purely superficial; the holdfast and "stem" are merely supporting struc- 

 tures, and the "blades" are not comparable to the blades of leaves, since 

 they consist of semi-independent cells, as in other algae. 



In all the algae maintenance is accomplished principally through the 

 independent functioning of the individual cells. The ability of the algae 

 to get along without a high degree of organization is the result of their 

 mode of life. They do not need an epidermis specialized to resist evapora- 

 tion, for they live in water or in very moist situations; strong mechanical 

 tissues are not required because of the prevailingly small size of the body 

 and the fact that the larger algae are all aquatic and are buoyed up by 



Fig. 13.13. Ricciocarpus natans, a floating liverwort, showing the dichotomous branching of 

 the thallus. The small oval plants between the liverworts are Lemna, one of the smallest of 

 flowering plants. (Photo by Prof . W. C. Steere, courtesy Cranbrook Institute of Science.) 



water; a transporting system would be superfluous, since each cell has 

 access, immediately or at a few removes, to the surrounding water with 

 its contained gases and minerals. 



Intermediate multicellular green plants. The liverworts and mosses 

 (Bryophyta) are the simplest land plants. They are doubtless derived 

 from some group of algae, but they differ from all the thallophytes in 

 their higher degree of structural organization, related to the requirements 

 of life upon land. The most important of these requirements is protec- 

 tion against excessive loss of water by evaporation. In most bryophytes, 

 the body has become several cell layers thick, so that relatively less 

 surface is exposed. The outermost layer has become specialized into 

 an epidermis, composed of cells that secrete a thin layer of cutin, the 

 cuticle — a structure also present in the higher plants, as we have seen. 

 The cuticle forms a relatively impervious coating over the exposed parts 

 of the plant and prevents excessive water loss, but at the same time 



