Growth in Eriogonum nudum 241 



concentrations as high as those which prevail in the tissues of cacti and 

 of Eriogonum, it is evident that, under such conditions of tonus, even 

 the changes in acid concentration indicated by the figures given just 

 above may have some sHght significance, at least in altering the perme- 

 ability of the protoplasmic membranes, or the physical properties of the 

 cell wall. 



The absence of this sort of evidence makes the question as to the 

 net growth accretion of the early afternoon in Eriogonum difficult of 

 elucidation. The loss of water and the concomitant shrinkages, and 

 the recovery until the original early morning volume is attained, seem 

 to point to the relatively simple relation of income and outgo of water. 

 Beyond this point, however, it seems necessary to assume that other 

 sources of energy are being drawn upon. Increasing soil temperatures 

 in the afternoon is probably a contributory factor in water absorption. 

 We may suppose that the products of photosynthesis accumulated during 

 the morning become available for maintaining the turgidity of the ex- 

 panding cells, and for the complex operations which include enzymatic 

 and other activities conditioning growth in the complete sense. These 

 factors obviously include the role of substances affecting the dispersion 

 of cell-wall and protoplasmic colloids, affecting the extensibility of the 

 former and the permeability of the latter. 



Summary. 



1. The march of growth in Eriogonum, in the particular situation in 

 which the foregoing study was carried on, is the following: The night 

 rates are low and nearly constant, the low night temperatures being 

 the limiting factor. Humidity at night is always high. There is gener- 

 ally a slight acceleration of growth after dawn till 7 hour, though the 

 rate may and frequently does remain constant. From 7 till 10 hour 

 growth slackens, and frequently shrinkage occurs. This is to be referred 

 to excessive transpirational activity, which is checked on wilting by 

 stomatal closure. Recovery of the original volume then sets in, and 

 by noon is usually complete. The earlier portion of the afternoon is a 

 period of net increase in volume, when a net advance in growth is made. 

 The maximum rate may be reached by the 15 hour or as late as the 

 18 hour, more frequently at 16 or 17 hour. Afterwards the rate of growth 

 falls during two or three hours till the night rate is assumed. 



2. It was evident that the most rapid growth rates might be attained 

 during hours of maximum illumination; and experimentally it has been 

 shown that even abnormally high rates might obtain under the most 

 severe conditions of illumination possible by the use of full insolation 



