IV. RELATIONS OF PLANTS TO WATER 



100. Water as a Factor in Living Matter. Water is the most 

 abundant constituent of living matter, serving as an organizing 

 fluid for the colloidal matter, and as a solvent for the crystalloids, 

 making a medium of exchange between the different organs of 

 the protoplast, and serving most important uses in all metabolic 

 processes. It is also to be considered as a nutritive substance, 

 yielding its constituent elements for the construction of essential 

 compounds of living matter. It serves as a medium for the in- 

 troduction of food-material into the protoplasts, and their con- 

 duction from one part of the body to another. In addition the 

 water imbibed by cell-walls determines their ductility, elasticity, 

 and flexibility, while the rigidity of the entire body of the plant 

 is more or less dependent upon the amount of water absorbed 

 and held inside of the membranes of the protoplasts. The ab- 

 sorption, transpiration, guttation and conduction of water by the 

 plant will receive special attention in the sections devoted to 

 these functions. The proportion of water in protoplasts may 

 be as much as 98 per cent, of their gross weight, and living 

 matter may exist under certain conditions in seeds, spores, etc., 

 with only five or six per cent, of this liquid. Reduction below 

 the last-named figure may result in disorganization. The fatal 

 proportion below which death ensues varies greatly in different 

 structures however. It is probable that leaves may not live 

 with less than 35 per cent, of their weight of water. The opti- 

 mum point is far above this, while the maximum can hardly be 

 distinguished, since it is rarely possible to induce a protoplast to 

 acquire too much water. The minimum, or point at which the 

 ordinary activities of the cell cease and rigor sets in is not so well 

 defined in the relations of protoplasm to this agency as to others. 

 At a certain point, which might be termed the minimum, most of 

 the activities cease and a partial rigor sets in, but respiration and 

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