Growth and Colloidal Reactions. 9 



them, in water or in some solutions may increase from temperatures 

 near the freezing-point to 39° to 46° C. and then fall off above this 

 region, or in acid solutions of a concentration normal to the plant both 

 biocolloids and sections of hving and dried plants may show decreased 

 hydration as the temperature rises above 17° or 18° C.^ 



Nowhere is metabolism more active than in the embryonic growing 

 cell. The dissociations which are usually included in the conception 

 of respiration may be taken to concern molecules of material already 

 present in the more liquid phase of the colloid or newly introduced. 

 The splitting of the sugars results in the formation of acids as one stage 

 of the process, and if the succeeding stages are impeded such material 

 accumulates, acidosis results, with new temperature relations which 

 may affect imbibition and the enlargement constituting growth in a 

 profound manner. Other actions will depend upon the composition 

 of the cell with respect to its carbohydrate and proteinaceous constitu- 

 ents. If it is high in albuminous material, its capacity for absorbing 

 and swelling may be greatly increased, w^hile on the other hand this 

 effect will be greatly lessened if a large proportion of pentosans are 

 present, especially in the presence of salts. No further recapitulation 

 of detail is necessary to emphasize the fact that the products of the 

 reactions within the cell may be responsible for many of its most marked 

 changes in behavior. Such changes do not in any manner give support 

 or approval to vitalistic theories as to the constitution or activities of 

 living matter. 



The other soluble carbohydrates, including the hexoses — sucrose, 

 dextrose — do not occur in the cell in such concentration as to affect the 

 enlargement of the protoplasmic mass directly, but in the vacuoles they 

 may exert an osmotic effect additive to that of the amino-acids which 

 may accumulate in these cavities. It is to the osmotic activity of 

 these substances in the vacuoles that turgidity is due, and a by no 

 means unimportant part in the maintenance of the rigidity of organs 

 and other features is to be ascribed to these turgor stresses and ten- 

 sions. That osmotic pressure may also play an important part in the 

 enlargement of the plant cell may well be concluded from the fact that 

 in the stage following the initial swelling of the embryonic cell a large 

 share of the increase in volume is due to the increase of the vacuoles. 

 The inadequacy of osmotic phenomena and of the conception of the 

 semipermeable membrane to provide a mechanism for the trans- 

 location of complex material from cell to cell, and the incorporation of 

 new material in a growing mass has long been recognized. It would be 

 a mistake to conclude that the vacuole is simply a sac charged with 

 electrolytes, as these cavities invariably hold proteins and carbohy- 

 drates in a colloidal condition in which the degree of dispersion may 



1 MacDougal, D. T. The relation of growth and swelling of plants and of biocolloids to temper- 

 ature. Proc. Soc. for Exper. Biol, and Med., 15, No. 3, p. 48. 1917. 



