192 PHYSIOLOGY OF NUTRITION 



§12. The Importance of Water in Plants. v — Physiological processes can- 

 not go on without water in the cells. 1 About 80 or 90 per cent, (by weight) of 

 the active plant cell is water, and the water content is small only in so-called 

 resting tissues, such as those of dry seeds. When such relatively inactive tissues 

 become active, the acceleration of the physiological processes is preceded or 

 accompanied by pronounced absorption of water. At the same time much of 

 the insoluble material of the inactive cells (starch, oil, etc.) is modified so as to 

 become soluble in water, and this dissolves with the advance of renewed activity. 

 Also, with the entrance of water many colloidal substances in the cell (which 

 are not, or do not become, truly soluble in water) absorb this liquid to a marked 

 degree and swell to a corresponding extent, even becoming so completely dis- 

 persed in the water that the hydrosol thus formed becomes liquid and assumes 

 many of the properties of a true solution. In nearly dry cells these cell colloids 

 are largely in the hydrogel phase and are virtually solid. 



As the cell colloids pass into the sol phase and the crystalloids dissolve, 

 those of the latter that are electrolytes become increasingly dissociated, so 

 that they become much more active chemically. The water also is dissociated 

 and a cell well supplied with water thus contains many different kinds of kations 

 and anions, the concentrations of which determine the rates and directions 

 of many chemical changes. Especially are the hydrogen ion (kation) and 

 hydroxyl ion (anion) concentrations important in this way. 2 



Aside from being the medium of solution and dispersion of the non-aqueous 

 cell substances, and aside from its influence on ionization and chemical action, 

 water is also an essential material in the synthesis of organic compounds. The 

 hydrogen and oxygen of the plant body are to be considered as derived from 

 water (see Part I, Chapter I). Water is also a necessary material for the 

 hydrolysis of many complex carbohydrates, proteins, fats, etc., into simpler 

 compounds (e.g., starch and cellulose into sugar, cane sugar into glucose). 

 Of course water is produced by the opposite process (e.g., the polymerization of 

 glucose to form cane sugar), and also by the complete oxidation of carbohy- 

 drates, fats, etc., in respiration. But water apparently disappears in the earlier 

 chemical steps of respiration (see page 228 and compare page 220). 



1 Kraus, Gregor, Ueber die Wasserverteilung in der Pflanze. Halle, 1 879-1 884. [This vol. is reprinted 

 from Naturforscherges Halle; Festschr. (1879). 71 P-; Abhandl. 15: 49-120 (1880); 15: 229-319 (1881); 

 16: 141-205 (1884).] Babcock, S. M., Metabolic water: its production and r61e in vital phenomena. Univ. 

 Wisconsin Agric. Exp. Sta. Research Bull. 22. 1912. (Also Ann., rept. Wisconsin Agric. Exp. Sta. 292: 

 87-181. 1912.) 



2 [The "true acidity" of a solution depends, not upon the total quantity of acid present, but upon the 

 concentration of hydrogen ions; similarly, the "true alkalinity" depends upon the concentration of hydroxyl 

 ions. Which concentration is in excess determines the reaction of the solution. It is necessary to remem- 

 ber that ion concentrations may be different in different parts of the same cell; for example, the protoplasm 

 is generally alkaline while the cell-sap is acid. On the reactions of cell solutions see: Michaelis, L., Die 

 allgemeine Bedeutung der Wasserstofnonenkonzentration fur die Biologie. In Oppenheimer's Handbuch 

 der Biochemie des Menschen und der Tiere. Jena, 1909-11. Erganzungsband, 1913. (See Erganzbd.. 

 p. 10.) Sorensen, S. P. L., Ueber die Messung und Bedeutung der Wasserstoffionenkonzentration bei 

 biologischen Prozessen. Ergeb. Physiol. 12 : 393-532. 1912.] 



v This section appears for the first time in the 7th Russian edition. — Ed. 



