MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 327 



the maximum rate being attained by i or 2 P.M. and thereafter maintained, 

 with fluctuations, until 6 P.M., when the rates again fall to the night values. 

 The afternoon rates are great enough to more than make up for the negative 

 behavior of the morning, except, as above stated, under unusual conditions. 



That light cannot be held to account for the retardation of growth during 

 the morning hours as above indicated has been shown to be an untenable view, 

 since it was found possible experimentally to alter the rates both positively 

 and negatively quite independently of the constancy, increase or decrease of 

 illumination, even when this has been increased with respect to the growing 

 part by insolation from three directions. There seems indeed to be no maxi- 

 mium insolation normally occurring in the field at this locality which can cause 

 any cessation or inhibition of growth when conditions which insure water 

 supply to the growing part obtain. Thus, when a cessation of growth is 

 apparent, it can be checked, and high rates instituted, by the removal of leaves 

 (which divert the water supply), by increasing the vapor tension in the 

 vicinity of the growing part, or by merely increasing the temperature when 

 the volume of the growing part is small (as when the internode under obser- 

 vation is young). These positive changes may occur coincidentally with in- 

 crease of illumination from the blue or red portions of the spectrum to full 

 insolation. 



A similar action may occur in the inactivity of green opuntias 

 in the open, btit certainly does not apply to the daylight retardation. 

 On the other hand the checking of growth or shrinkage of etiolated 

 members in darkness and of green shoots at high temperatures may 

 well be due to transpiration or modification of imbibition capacity. 



Water-Absorbing Capacity of Plant Tissues. 



Growth is essentially the irreversible enlargement of embryonic 

 cells, by the appropriation of material of which 98 or 99 per cent, is 

 water. The process depends upon the availability of the building 

 material which enters into the structure of the protoplast, its inclu- 

 sions and its envelopes, and upon the continuance of reactions, such 

 as enzymosis and respiration, which maintain an unsatisfied absorp- 

 tive capacity. 



The incorporation of the solutions in the colloids of the proto- 

 plast is essentially a hydration process which is usually designated 

 as imbibition. A stable colloid takes up a fixed solution at a rate 

 expressible by a regular curve. The protoplast is a complex mixture 

 of both emulsoids and suspensoids in which there is almost unceas- 

 ing change. Its structure may be modified by the uneven action of 

 the metabolic plexus which may also result in the accumulation of 



