172 Hydration and Growth. 



protein ratio, the hydrogen-ion concentration, the action of salts, and 

 the effect of the amino compounds. 



The dehcate balance between water-loss and absorption as revealed 

 . by measurements of growing organs of all kinds is very striking. The 

 rate at which water is received is generally so httle in excess of the 

 transpiration that a rise of 10 to 15 degrees centigrade may extinguish 

 the balance. At the same time, such rise in temperature may also 

 result in a lessened hydration capacity, so that by the action of the 

 acids at the higher temperature, water may theoretically be forced out 

 of the colloidal complex. 



It is plainly evident that growth consists of two fimdamental 

 features — ^hydration of the colloidal material of the plasma and the 

 arrangement of additional material in colloidal structures with the 

 entailed additional capacity for adsorbing water. The first may 

 occur without the second, and increase in voliune might occur 

 in a pentosan-protein colloid at any time by the action of its 

 own metabolic products, such as the hydrogen-ion concentration 

 or the proportion of amino-compounds formed. Growth by acces- 

 sion of solid material without a corresponding absorption of water 

 is characteristic of cell organs or walls, and such deposition of 

 material can only result in changes in volume which would not be 

 measurable by auxographic methods. 



Hydration consists, in the first instance, of the union of molecules of 

 water with the molecules of soUd material in the colloidal masses, and 

 it is this action which is entailed in the initial and almost instantaneous 

 enlargement of dried sections when water is poured on them. No serious 

 reason has yet been advanced, however, against the extension of the 

 term to apply to the accompanying and subsequent adsorption of an 

 indefinite number of molecules on the surfaces of the molecular aggre- 

 gates. Cell-masses are already in an advanced stage of hydration, and 

 all of the tests with living material are simply modifications of such a 

 condition. The swelling of dried sections of plant tissue may include 

 some chemical action, or some vuiion of water with the soUd material 

 in definite proportions. 



The manner in which hydration ensues, or rather the character of the 

 process, will natiu-ally depend upon the character of the cell colloids. 

 If these are albxmiinous, swelUng will be largely determined by 

 the hydrogen-ion concentration of the solution. It also follows that 

 any cell-organ or cell-mass which is dominantly proteinaceous will 

 show such increases of hydration capacity with acidity, modified by 

 other facts, including the presence of salts or bases. 



These effects are modified or reversed in colloidal material which 

 consists more largely of carbohydrate material. The pentosans 

 represented by various gums and mucilages are abundant in plant 

 cells, and these present some variety of composition and differences 



