350 MacDOUGAL— HYDRATION AND GROWTH. 



tion of the semi-permeable membrane to provide a mechanism for 

 the translocation of complex material from cell to cell, and the in- 

 corporation of new material in a growing mass has long been recog- 

 nized. That osmotic pressure however may 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. It would be a mistake to conclude that the vacuole is 

 simply a sac charged with electrolytes, as these cavities invariably 

 hold proteins and carbohydrates in a colloidal condition in which the 

 degree of dispersion may vary widely, but still absorb water. A cor- 

 rect estimation of the manner in which osmosis and imbibition inter- 

 lock in growth is one of the tasks demanding the immediate attention 

 of the physiologist. 



The Effect of Organic Acids and their Amino-Compounds on 



Growth. 



The accelerating effects of acids or of the hydrogen ion concen- 

 tration on hydration of proteins is well known, and something of the 

 retarding efifect on the swelling of mucilages or pentosans have been 

 described in recent papers. 



The effect of bases (cations) on these processes has not yet been 

 measvired, nor has the method by which the amino-compounds act 

 in this matter been determined. 



The hydrogen ion concentration of the liquids in the plant cell 

 remains constant within limits and in succulents examined by Jenney 

 Hempel may be expressed by Ph 3-9-7, as determined by electro- 

 meter and titration methods. The presence of the acids of which 

 malic is the more abundant, and its combinations with such bases as 

 potassium, sodium, calcium, magnesium, iron and aluminium makes 

 a " buffer " by which the degree of dissociation is controlled. 



In addition to this comparative stability or narrow range of the 

 concentration of hydrogen ions, amino-compounds are invariably 

 present, and their relative amount probably varies but little. 



A series of tests were planned in which a comparison would be 

 made possible between the action of some of the commoner organic 

 acids and of their amino-compounds. 



