J94 



MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 



in which the proportion of time during the vegetative season in 

 which exposure to light takes place, would apparently indicate that 

 the action of light actually accelerates growth, at least when under a 

 certain degree of intensity, and that the amount of growth made by 

 a plant would be correspondent to the actual length of time of illu- 

 mination which it undergoes. The increased photosynthetic activity 



Fig. 173. Alarchaniia poh'inorpha. A, transverse section of thallus cultivated 

 in an atmosphere containing carbon dioxide. ^S', section of thallus grown in an at- 

 mosphere lacking carbon dioxide. After Teodoresco. 



of the plant under constant illumination would offer some features 

 different from those of specimens grown in atmospheres enriched in 

 carbon dioxide. In the latter instance a greatly accelerated produc- 

 tion of carbohydrates takes place during the day, and at a rate wholly 

 beyond the capacity of the translocating and assimilatory apparatus, 

 in consequence of which the leaves are not carried to their maximum 

 size on the one hand, and such plants actually become pathological 

 after a time. The increased photosynthetic activity of the plant with 

 a lengthened day is not due to an increased rate, but to a continua- 

 tion of the process at a diminished rate during the entire twenty-four 

 hours — at a rate that allows the entire product to be handled in the 

 usual manner by the assimilating and conducting mechanisms. 



As to the difference in results between continuous and discontinuous 

 illumination, the only solution of the problem that presents itself is 

 the supposition that, the alternating action of light and darkness, 

 rather than continuous light, constitutes the stimulus that sets up the 

 morphogenetic action ascribed to the action of illumination. 



The stature or relative mechanical positions assumed by the 

 various members of the shoot under etiolative conditions presents a 

 phase of the subject not directly tested in many of the experimental 

 observations. It is noteworthy however, that nearly all stems ex- 



