macdougal: mechanism and conditions of growth 23 



It will be best to take up this phase of the matter at the time 

 of day when growth has come down to a low rate in the afternoon. 



If it be assumed that a slackening does ensue by reason of the 

 practical exhaustion of the reserve supply of material it would 

 then be seen at once that construction might proceed only at the 

 expense of photosynthetic products diffusing directly from the 

 chlorophyllous layer to the enlarging tracts. The disproportion 

 of the growing mass to the chlorophyll layer in a cactus joint is 

 so great that it might readily be seen that growth may not be 

 supported for any extended period or to any great extent at the 

 expense of coincident photosynthesis. It has long been known that 

 rapidly extending organs such as leaves may not be built up by 

 material derived from their own reducing processes. 



The enlargement of a joint having been carried through the 

 earlier part of the day by actual construction and continued late 

 in the afternoon by increased swelling power of the deacidified 

 tissues, would decrease and come to zero with the falling tempera- 

 ture of evening, and this effect may even be carried so far that 

 the capacity of the colloids might be reduced below the amount 

 acquired at the high afternoon temperatures and some water might 

 be actually extruded. So much for the direct temperature effects. 

 The indirect effects may be quite as large or important. The 

 absence of light and the failure of its disintegrating effects, to- 

 gether w4th the low temperature, would result in an increased 

 accumulation of acids reaching a maximum at daybreak, and this 

 increase would also lessen water-holding capacity and osmotic 

 pressure with the result that all forms of distensive action would 

 be lessened, cancelled or reversed, tending to check elongation 

 or to produce actual shortening. This ready yielding of water 

 resultant from the above action would. It seems, facilitate water- 

 loss to some extent, and that It does so is suggested by the apparent 

 coincidence of low hydratatlon capacity and high transpiration 

 rate. It may be well to emphasize the fact that a joint of a cac- 

 tus, a simple stem, or the still more rudimentary Phycomyces 

 sporangiophores are not homogeneous as to chemical composition 

 or simple as to mechanical qualities. The agents affecting hydra- 

 tatlon may not cause Identical effects in cell-wall, in protoplast, 

 and in accessory slimes or mucilages. When such dissimilarly 

 reacting elements are mechanically bound together as In the stem, 

 the resultant change In volume may not be easily analyzed. 



