60 CARNEGIE INSTITUTION OF WASHINGTON. 



which can not pass through it, will be adequate for the purpose of 

 the present discussion without reference to its inadequacies in some 

 respects. 



The greater part of the mass of the protoplast is at first a fairly- 

 homogeneous mass mechanically, with the exception of the large 

 nucleus. No distinct "cavities" or large hiatuses in the gelatinous 

 matter are to be seen. The only forces which might cause the cell 

 to enlarge in this condition would be the swelling of imbibition and 

 the osmotic action of the colloids. Definite infoniiation on this 

 latter action in the plant-cell is not available, but it is safe to assume 

 that the molecular aggregates of the colloids would act much in the 

 same manner as single molecules or smaller associations of them. 



The presence of hexoses and various salts soon results in the accumu- 

 lation of water in places in the mass and these irregular vacuoles become 

 the seat of a much greater osmotic pressure than that displayed by the 

 colloids. The protoplast soon attains a stage in which it presents the 

 mechanical features of a sac of denser material with irregular lighter 

 strands and sheets of cytoplasmic gel separating clearer spaces or 

 vacuoles, the nucleus somewhat diminished in size and variously held 

 in the lighter cytoplasm. The cavities or vacuoles undoubtedly con- 

 tain colloidal material which in its high state of dispersion carries its 

 capacity for swelhng with hydration. 



The picture of the cell as thus described includes the principal 

 features by which it and thousands of its fellows expand and thus give 

 rise to the enlargements measurable as growth and to the shrinkages 

 or diminution of volume which have hitherto received but little atten- 

 tion. The state of distension or turgidity of the cell has been chiefly 

 attributed to osmotic pressure and it has been customary to assume 

 that the expansive force of growth was practically'' identical with its 

 action. The use of isotonic solutions in the measurement of turgor is 

 based on this assumption, and the acceptance of the freezing-point 

 of the sap as a measure of the pressure of the cell rests on the same 

 conception. 



It is clear, however, that imbibition or hydration of the colloids in 

 addition to their osmotic pressure is also a force to be reckoned in such 

 detenninations. Interpretations of the action of external agencies, 

 such as temperature on growth, must take into account not only the 

 effect exerted upon respiration, enzymatic action, and other reductions 

 and oxidations, but also the effect of concentration and the state of the 

 membrane in osmosis. In addition, the colloids (including those of 

 the cell-wall, the hning layer, the cytoplasmic sheets, and the nucleus) 

 are undergoing changes in mass as well as in imbibition or hydra- 

 tion capacity. The hydration phenomena are not those of a single 

 gel, nor is the resultant a mechanical one, for the admixture of two 

 or more colloids produces a substance which, as seen in the case of 



