326 MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 



have the effect of a flattening of the curve that is of slowing down 

 growth. The action becomes more pronounced until a stage is 

 reached when more and more of the elongation of the forenoon is 

 retracted in the afternoon (see Fig. 3). 



Such negative action might 'be due to the reduction of an enzyrne 

 concerned in the renewal of the constructive material below the 

 eft'ective amount, or to the clogging action of accumulated products, 

 or as has been previously suggested, to transpiration counterbalanc- 

 ing imbibition and accretion of suspended material. Cessation of 

 growth at 26° C. to 30° C. would be difficult to reconcile with the 

 assumption that it might be due to a destruction of an enzyme, since 

 all known bodies of this kind do not begin to show a rapid rate of 

 disintegration until a much higher temperature is reached. An 

 accumulation of the products in some part of the chain of reactions 

 might well take place, however. Similar retardations in photo- 

 synthesis are known to occur when translocation of the carbohy- 

 drates is prevented. 



As to the third suggestion it is to be said that the stoppage or 

 slackened growth of green plants in the open in the hours imme- 

 diately preceding daylight coincides with a condition of lessened 

 im'bibition capacity due to high acidity and accompanied by the most 

 rapid transpiration displayed by the plant. The low temperatures 

 at this time might also cause a decreased absorption. The rate of 

 absorption of green plants would be greatest in the afternoon, and 

 as water-loss at this time has been found to be actually less than in 

 early morning, it is to be seen that the decreased growth character- 

 istic of this- part of the day may not be attributed to excessive trans- 

 piration. Acidity is near the minimum at this time and the imbibi- 

 tion capacity of the growing joint is greatest. That transpiration 

 may actually check or neutralize growth has been demonstrated in 

 Eriogoniim by Lloyd :^ 



The daily march of growth is as follows: During the early daylight hours 

 until about 8, there is usually a slight rise in growth rate. After that hour 

 the rate falls to a low Value, or, much more frequently there ensues an actual 

 shrinkage. This is the period during which the loss of water by transpiration 

 is rapidly increasing, reaching its maximum at about noon. Coincidentally 

 with the checking of transpiration, the growth rates rapidly increase in value. 



3 Report Dept. Bof. Research, Carnegie Inst, of Washington for 1916. 



