22 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



products of incomplete respiration, and their presence operates 

 to prevent the respiration and use of the sugars as a source of 

 energy and as building material. The slow growth at night may 

 therefore be attril)utcd mainly to high acidity impeding respira- 

 tion and the higher rate of water-loss would also tend to lessen 

 the possibilities of increase in length and thickness. 



The disintegrating action of light on these acids would at some 

 time toward midday bring the acidity down to concentration 

 where its impeding effect on respiration would be largely cancelled 

 and growth would then go on at a rapidly accelerating rate until 

 the supply of accumulating sugars was used up. Such exhaustion 

 of the food-material might occur at any time, but in the cacti 

 generally comes at Tucson two or three hours after midday, after 

 which time construction would be largely from sugars diffusing 

 directly from the photosynthetic tracts. The general features 

 of the daily course of growth show some precession with the ad- 

 vance of the seasons and the higher temperature and light maxima 

 at Tucson. The auxographs of Opiintia discata made at Carmel 

 under equable conditions also harmonize with the conclusions 

 given above. 



The above explanation seems to account in an adequate manner 

 for the irreversible changes taking place in the growing plant, 

 whether illustrated by the growing bud or by the large joint in 

 its second year and designated as "mature" though still capable 

 of some growth. It is evident that coincident with growth and 

 extending throughout the active existence of the stem a series of 

 reversible changes in size takes place. This is most noticeably 

 exemplified by the case of Opimtia Blakeana No. 13, which had 

 been subjected to light rays of wave length below .52 mu for four 

 months before growth began. This plant showed extremely rapid 

 accelerations of growth in the forenoon and abrupt slackenings 

 at midday, which soon ran into shrinkages in length as if the 

 hydratation capacity had been altered, following the failure of 

 the supply of building material (fig. 4). 



This superposition of reversible and irreversible changes makes 

 our problem complex, but by no means insoluble. Furthermore, 

 it is reasonably certain that the two types of change ensue in the 

 thin leaf of wheat and in stems as well as in the cacti used in our 

 experiments, this material, however, offering phenomena in which 

 the phases are thrown into high relief. 



