292 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



With increase in size vacuoles begin to appear. The active cell 

 is usually conceived as a sac with irregular strands of cytoplasm 

 extending from the peripheral layers of protoplasm, the nucleus 

 being variously placed in this irregular mass. The vacuoles or 

 spaces appearing colorless in living cells and clear in preparations 

 are taken to be sacs containing electrolytes or other dissolved mate- 

 rial. The capacity of these dissolved substances to absorb water 

 osmotically tends to increase their volume and cause distension 

 resulting in turgidity or swelling of the cell and in rigidity of the 

 organ when whole tracts or layers act in this manner. Turgidity has 

 hitherto been held to account for the entire expansion of growth as 

 noted above. It is now apparent, however, that we are in a posi- 

 tion to draw a slightly different picture of the mechanical features 

 of the cell in what may be termed the second stage. In addition to 

 the denser colloids of the wall, the lining layer of protoplasm, and 

 the nuclear structures, it is known that even in the clear regions of 

 the cell there are emulsions and that the entire cell is a mass of gels 

 of different composition and varying degrees of dispersion. The 

 cell may take water into the vacuoles by the osmotic action of the 

 -electrolytes, but the entire mass tends to swell as would a mixture 

 'of protein, cellulose, agar, gum arable, starch and other substances, 

 and such masses may be modified by transpiration or direct loss 

 of water. 



The first recognition of the dift'erential action of acidity and 

 alkalinity appears to have been expressed by Spoehr and Estill 

 who say :- 



It has become evident tliat the total swelling of plants like Opuntia 

 blakeana and O. discata in dilute solutions of acids, alkalis, and salts repre- 

 sents the summation of independent reactions of various material to these 

 reagents. Thus, solutions of acids, alkalies and salts influence the swelling 

 and growth of these plants by affecting: (i) the hj^dratation of the proto- 

 plasts ; (2) the material that goes to make up the cell-wall and fibro-vascular 

 system; (3) the permeability and osmotic properties of the plasma-membrane. 

 It has been found that these three factors can act independently and even in 

 opposite directions. Great differences were found in these respects in dif- 

 ferent portions of the same cactus joint and between young and mature ones; 

 the colloidal material of the former showed much greater swelling than the 

 latter in all solutions, and the excess of swelling in acid media above that in 



2 Report Dept. Bot. Res. Carnegie Inst, of Wash, for 1915, p. 66: 



