CHAPTER III. 



THE LIVING CELL. 



In order to understand that " continuous living system " of which 

 the plant body consists, it will be necessary to consider in more detail 

 the structure and physiological properties of the cell-units which 

 compose it. As stated in the previous chapter, the mature plant does 

 not consist entirely of living cells, but it is in them that the physiolo- 

 gical activities of the plant are concentrated. 



Fig. 17. 



Optical longitudinal section of a single cell of a hair of the Cucumber. Externally 

 is the cell-wall (C.W.). Its inner surface is lined by a layer of cytoplasm (cyt.), 

 surrounding a large vacuole (V.). In this the nucleus (N.) is suspended centrally 

 by numerous cytoplasmic threads. Movements may be seen in these during vitality, 

 which convey ' chloroplasts (chl.), and even crystals (cry.), thus showing active 

 circulation in the living cell. (After Sachs.) 



The visible structure of typical living cells has already been described, 

 but the main features will be revised at this point, with the help of 

 Fig. 17, which shows a single cell from one of the coarse hairs that 

 roughen the surface of the shoot of the Cucumber. Externally the 

 cell is limited by a definite cell-wall of almost uniform thickness. Its 



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