42 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 5 



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appear in the cytoplasm, and these rifts enlarge and run 

 together until they form a single great central sap-filled 

 cavity ; and thus the cytoplasm is left as a thin lining inside 

 the wall, against which it is held tightly pressed by the pres- 

 sure of the sap. Obviously the arrangement is one which 

 gives a maximal spread of surface with the minimal amount 

 of protoplasm ; but spread of much surface is an obvious 

 functional need of an organism which has a mode of nutrition 



requiring extensive expos- 

 al ure to light, and a wide 

 range in the air and the 

 soil. Within the sap cav- 

 ity occur also various cell- 

 contents, — food grains, 

 special secretions, crystals, 

 and others, — according to 

 the respective functions of 

 the cells. 



The details of cell struc- 

 ture, especially the shape, 

 size, thickness, and compo- 

 :== 7~^ ) ) U V sition of the wall and the 



character of the contents, 

 are most diverse in dif- 

 ferent tissues, though ex- 

 hibiting usually an obvious 

 relation to the particular 

 functions of the respective 

 parts (Fig. 17). This rela- 

 tion between structure and function becomes even clearer 

 when the study is extended to animal cells, which also are 

 protoplasmic ; for here the cell construction is dominated by 

 the very different habits of animals, which are freely and 

 actively locomotive instead of sedentary and passive. The 

 protoplasm of animals and plants is, however, the same in 

 all essentials, and the organisms are so different only because 



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Fig. 17. — Generalized outlines of the 

 principal shapes of cell walls of plants. 

 They are all derivable, by more rapid 

 growth in particular parts of the wall, 

 from the small spherical form in the 

 center. With these shapes occur all 

 degrees of thickening of the walls. (Re- 

 duced from Ganong, The Living Plant.) 



