MEMOIRS OF THE NEW YORK BOTANICAL GARDEN. 203 



when the reproductive organs are held up in sunlight. Here the 

 phototropic response is made to a stimulus ordinarily associated with 

 a series of complex conditions embracing currents of air, activities of 

 animals useful in dissemination, water, etc., which are actually 

 necessary for the profitable and successful dispersal of the propa- 

 gating bodies. Even the germination of a large number of seeds 

 and spores in light only may be regarded as a similar association of 

 a stimulus with other vegetative conditions. It is true of course that 

 spores of certain pteridophytes must have light-exposure to enable 

 the chlorophyl-apparatus to construct building material for germina- 

 tion and growth, but in the larger number of instances illumination 

 acts simply as a signal indicative of the presence of other necessary 

 factors. 



The intensity of light necessary to constitute a phototropic stimulus 

 varies enormously with different species, and with the developmental 

 state of the individual. Using a normal candle burning 7.78 grams 

 of paraffine per hour at a distance of one meter as a standard it has 

 been found that an illumination of .00033 to .06 meter candle con- 

 stitutes the minimum in seedlings of the most delicately organized 

 species examined. The optimum effect in curvature is obtained in 

 the same plants with an intensity of .11 to .44 meter candle, and these 

 intensities must be increased a hundred to a thousand times to reach 

 the maximum. Increase of the intensity beyond the maximum may 

 xesult in changing the character of the response in such manner that 

 the organism will curve or move away from the source of the rays. 

 The more refrangible rays are chiefly active in such effects, and the 

 amount of increase in the intensity necessary to constitute a stimulus 

 is not more than 18 per cent, in some instances. 



In addition to the reactions described above the plant shows other 

 forms of response to intensities of illumination by photeolic and pho- 

 tolytic movements which bring the cell-constituents into adaptive rela- 

 tions with radiant energy by which injurious activity of the trans- 

 piratory and other functions are avoided. All efforts to establish a 

 connection between the action of light on the enzymes or other cell 

 contents as a primal and direct cause of the reactions in question 

 have failed. 



Light has undoubtedly exerted a predominating influence in the 

 development of the prevailing types of vegetation, the form and 

 structure of the body and its members being largely determined by 



