LINNEAX SOCIETY OF LONDON. 53 



Pond No. I began to germiuate ou June 3rd. Growth with 

 these seedlings was, at first, slower than with those in the 

 shallow water ; but at the end of August the seedlings were 

 as strong and healthy as those in Pond II. The little plants 

 in Ponds I and II are now faded and have passed into a state of 

 rest which will endure until next spring. I have removed and 

 show you to-night dried specimens from each of the little ponds 

 and from the bottles. A personal examination will tell you 

 better than any words of mine the featui'es which distinguish 

 the normal seedlings from the " bottle seedlings." 



Intensity and Direction of Light as Factors in Phototropism. 

 By Hahold AVager, D.."Sc., F.1\.«., F.L.S. 



[Read 29tli November, 1917.] 



In this communication an account is given of experiments made 

 to determine the influence of the intensity and the direction of 

 light in effecting phototropic responses in foliage leaves. In 

 accordance with the teachings of physics we may assume that it 

 is only the light absorbed by the photo-sensitive elements of a 

 stimulable plant organ that is capable of setting up a phototropic 

 stimulus. The light rays penetrate the tissues unequally but 

 symmetrically. The extent to which this takes place can be 

 shown by means of a simple piece of apparatus which may be 

 called a phytentoscope. 



Bv keeping the photo-sensitive region of a foliage leaf in a 

 fixed position it can be shown that the orientation to light is 

 determined by the varying extent to which the rays of light 

 penetrate the photo-sensitive elements. The photo-sensitive 

 elements of a foliage leaf probably consist of parenchymatous 

 cells in the apical region of tlie leaf-stalk. Experimental evidence 

 is brought forward to show that it is probably mainly in the 

 cortex that the stimulus is received and transmitted. There is no 

 evidence to show that the epiderinis and pith take any large part 

 in the process. 



It the phototro])ic I'espouse is due to the physico-chemical 

 changes set up by the light absorbed, then it follows that : — 



1. The intensity of the physico-chemical changes depends 



upon the amount of light absorbed. 



2. The extent to which the light is absorbed depends upon 



(a) the degree of penetration of the light, and {!>) upon 

 the selective absorption of the photo-sensitive elements. 



3. The degree of penetration depends upon (a) the intensity 



of the light that impinges upon the stimulated region, 



and (h) upon the angle or angles at which the incident 



rays strike it. 



We see therefore that rhe distribution of the physico-chemical 



activities in the photo-sensitive tissues is dependent upon both 



