174 PHYSIOLOGY OF NUTRITION 



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 coUoids 

 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 direc- 

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

 and hydroxyl ion (anion) concentrations important in this way.^ 



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 carbohyd- 

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

 chemical steps of respiration (see page 205 and compare page 197). 



§13. The Germination of Seeds.' — In the above discussion, some of the ques- 

 tions concerning various material changes and other physiological processes 

 have been considered with reference to changes that occur in germinating seeds. 

 The important factors in seed germination will now be considered more in detail. 

 Considerable amounts of organic reserve food materials are stored in all seeds, 

 either in the cotyledons or in the endosperm. Consequently, the first phases 

 of germination can occur without light or mineral substances. During 



' [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 m 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 alkalme while the cell-sap is acid. On the reactions of cell solutions, see: MichaeUs, L., Die 

 allgememe Bedeutung der Wasserstoffionenkonzentration fur die Biologic. In Oppenheimer's Handbuch 

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

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

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



• This section is numbered §11 in the German edition. It appears unnumbered, at the 

 end of the chapter, in the 7th Russian edAion.— £d. 



