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A TEXTBOOK OF BOTANY [Ch. IV, 10 



donment of the leaves and the assumption of photosynthesis 

 by the compact, rotund, water-storing, ribbed stems, which 

 possess many structural features connected with restriction 

 of transpiration (Fig. 141). The difference in aspect and 

 structure between forest plants, desert plants, and water 

 plants shows how profoundly plant form is affected by water 

 supply. In accordance, indeed, with this relation to water, 

 most plants fall under three well-recognized groups, the 

 desert plants being called xerophytes, the water plants 



Fig. 133. — The Giant Kelp, Mac- 

 rocystis pyrifera, which grows up- 

 wards of 200 feet long. (From 

 Le Maout and Decaisne.) 



hydrophytes, and the intermedi- 

 ate or ordinary plants mesophytes. 

 The mesophytic is of course the 

 best condition for plant life, and reaches its highest perfec- 

 tion in the rank growths of the tropical forests and jungles, 

 though it is nearly as well attained in the deciduous forests 

 of temperate regions. 



The primitive water plants, the Algae, in their highest 

 development are distinguished by a thallus, familiar in 

 the fronds of brown Rockweeds (Fig. 132) and the red Sea- 

 mosses. The thallus is neither leaf nor stem, but rather a 

 more primitive structure from which leaf and stem have not 

 yet differentiated. Some of the greater Algae, as for ex- 

 ample the giant Kelp of the Pacific (Fig. 133), have de- 

 veloped a distinct leaf and stem structure, though it by no 

 means represents the evolutionary ancestor of the shoot of 

 the higher plants. 



The term shoot is used in connection with the flowering 

 plants to designate stem and leaves collectively. 



