X. IMBIBITION AND GROWTH OF OPUNTIA. 



Living cells and colloidal masses are never in a state of perfect 

 equilibrium with the environment, and the possible adjustments in 

 growing cells in which the colloidal substances are increasing, changing 

 in constitution, and varying in condition, may be great both as to 

 velocity and ampUtude. Artificially compounded colloids, such as 

 the agar-protein mixtures which have been so freely used in this work, 

 show a similar delicate series of adjustment and correlations as they 

 pass from the dry state to one of almost completely satisfied hydration; 

 but when they reach this stage they are in the general condition of 

 mature cells or tissues and then show only the minute adjustments 

 which follow the modifications of the environment very closely. 

 Living cell-masses are to be considered as masses of colloid the inti- 

 mate substance of which is being constantly altered by metabolism 

 and by the incorporation of new material. The actual capacity of a 

 gel for water and its consequent state of swelUng may be practically 

 satisfied at any stationary temperature. Such equable temperatures 

 do not occur in natural environments and may be observed only in 

 control chambers. The pen of the instrument employed in methods 

 of accurate measurement of a hydrated mass of colloid indicates con- 

 stant variations in volume, due to the solvation or dispefsion of some 

 of the mass in the water in which it may be immersed. 



The swelling of a cell-mass is to be considered as determined by the 

 hydration capacity of its colloids at the beginning of immersion plus 

 whatever additional capacity or variation may be developed by the 

 rearrangement of its material, reformation of its compounds, and 

 migrations of its molecular aggregates. Of the increased volume char- 

 acterized as growth, 98 per cent results from hydration. The grow- 

 ing cell-mass, however, in addition to the initial hydration capacity 

 of its mass, is continually adding material which has hydration ca- 

 pacity, and metabolic activities result in the accumulation of acids 

 and other substances which affect the coefficient of swelling. The 

 major procedure in growth would theoretically be imitated if minute 

 particles of powdered colloid could be continuously introduced into a 

 swelling mass. 



Measurements of hydration in plants were made with disks about 

 12 mm. across, cut from the flattened joints of an opuntia, which 

 ranged from 5 to 20 mm. in thickness (fig. 23). Such sections 

 consisted of the indurated epidermal layers, between which was a 

 cylindrical mass of parenchymatous cells, the outer ones being chlo- 

 rophyllose. An anastomosed network of thin, fibrovascular strands 

 was included in the parenchymatous mass, and this mechanical tis- 

 sue checked expansion, so that care was necessary not to include the 

 larger, firmer strands in the section. Three of such disks about 12 mm. 



128 



