310 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



which residual acids accumulate at low temperatures and in dark- 

 ness. Acidosis decreases imbibition. Growth beginning with sun- 

 rise shows an acceleration parallel to the disintegration of the clogging 

 acids and the rising capacity for imbibition, till midday only. The 

 retardation after this may not be ascribed to lessened power of imbi- 

 bition or to increased transpiration as water-loss is not greater during 

 this time and the capacity of the plant continues to increase until 

 near the end of the daylight period. The cause of the retardation 

 cannot be identified with the direct action of light, nor does it seem 

 warranted to assume that the " supply of building material " becomes 

 exhausted, as was previously suggested by the senior author. The 

 nature of the stoppage suggests the inhibiting action of respiratory 

 products or the destruction of an enzyme. Respiration in Opiintia 

 is profoundly afifected by light as has been shown by its effect on 

 acid-accumulation and destruction. Yet no immediate effects were 

 secured by exposure of growing members to the action of mercury 

 vapor quartz lamps with an intensity equivalent to normal sunlight 

 .at 2 meters distance, for periods of one to three hours. It is note- 

 worthy that the characteristic retardation or stoppage does not take 

 place in the first few days of the development of the bud, and that 

 the leaves of Mesembryanthemum exhibit a similar behavior. The 

 young shoots of Opiintia in this stage are not more than 8 to 12 mm. 

 in length, i to 2 mm. in thickness and are all but hidden by the 

 slender conical leaves. The joint as well as the leaves are in a state 

 of extreme imbibition. The character of the respiration under such 

 conditions is in all probability such that acids do not accumulate and 

 other by-products are modified with the result that the daily decrease 

 in imbibition capacity is not experienced. A similar behavior attends 

 the development of the flower buds. That retardation and stoppage 

 as observed in hundreds of instances could not be ascribed primarily 

 to temperature seemed to be established by the great variation in the 

 point at which growth might begin or cease. 



Growth began on rising temperatures at 9° C. to 25° C. in the 

 same green plants on diff'erent days at Tucson and was noted at 

 50° C. in flower buds. The continued rise of the temperature 

 resulted in a stoppage of elongation at temperatures between 26° C. 



