Grafts et al. 



88 



Water in Plants 



In cap plasmolysis (Figure 23) the cytoplasm swells markedly at the 

 ends of the cell, where there has been separation from the wall. This effect 

 can be induced by using a plasmolyzing solution containing salts of K, Na 



or Li (HoFLER, 1939). Calcium salts antagonize 

 the action. In tonoplast plasmolysis (Figure 24) 

 two or more distinct vacuolar membranes are formed. 

 Still other types, limited to certain kinds of cells, 

 have been described. Strugger (1935) is a source 

 of literature on plasmolysis types. 



Several types of response are included under the 

 term "stimulative" plasmolysis. Separation of the 

 protoplast from the wall may result from a number 

 of kinds of stimuli : mechanical, chemical, photo, and 

 thermal. Plasmolysis is usually observed at a time 

 when the cells are immersed in water and hypotonic 

 solutions. The reaction is often reversible. Several 

 explanations of this phenomenon have been oflfered : 

 release of imbibitional water by the protoplasm, an 

 active contraction of the protoplast, and an increase 

 in protoplasmic permeability, permitting solutes to 

 leak out. 



Under certain conditions, some cells exhibit vacu- 

 olar contraction, a marked reduction in size of the 

 vacuole due to swelling of the cytoplasm. There is 

 no separation from the wall as in plasmolysis. Ex- 

 amples of vacuolar contraction are illustrated in Figures 25, 26, and 27. 

 Of this, and of stimulative plasmolysis, more will be said in Chapter VIII. 



Fig. 23.- — Onion epi- 

 dermal cell showing cap 

 plasmolysis. Redrawn 

 from Strugger (1935). 



Fig. 24. — Tonoplast plasmolysis of onion epidermis. 

 K, the hyaline, strongly swollen nucleus ; P, the cyto- 

 plasm, and T, the tonoplast. (Strugger, 1935). 



It seems obvious from this and the previous discussion of plasmolysis 

 that the protoplasm cannot be regarded as merely a thin membrane per- 

 mitting passive diffusion of water to and from the vacuole. In all experi- 

 mental work with plasmolytic methods or vital staining, utmost care must 

 be taken against unconscious injury of the cells by use of unanalysed "tap" 

 and "distilled" water. Where cells are to endure long exposure, salt solu- 



