330 MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



Stimuli. Our knowledge of the relation between individual varia- 

 tion in sensitixeness of the germ tul)es and the intensity of the 

 acting stimulus is of course very vague. However, it seems safe to 

 assume that in general a larger percentage of turning means a 

 stronger stimulus. 



If, then, we denote by n the strength of the stimulus causing 

 negative chemotropism and by p that causing positive chemo- 

 tropism, we have in A of the above table, using the per cent of 

 reaction in the oldest preparations, w — ^ = 43; whereas in B we 

 have n -\- p = 94, taking the oldest preparations here also. 

 Since the hyphae in these two corresponding cases were approxi- 

 mately equal in length, and the number of spores in the films 

 was about the same, a comparison is legitimate. By eliminating 

 the />'s we have then, 2n = 137, or n = 68.5: p will then be 25.5. 

 In other words, under the special conditions of this experiment, 

 the positive chemotropic stimulus exerted by the turnip juice is a 

 little more than one third as strong as the negative chemotropic 

 stimulus exerted by the staling substances of the hyphae them- 

 selves. 



Similar experiments were carried on with cane sugar and glucose. 

 Briefly, the results of these showed a slight positive reaction to 

 the sugar or glucose, but so small as to be hardly above the per- 

 centage of error. The invariable uniformity with which it ap- 

 peared, however, is good evidence for the validity of the result. 

 A full account of these experiments, as well as other points, will be 

 left for later i)ul)li('ation. 



Summary 



1. Conclusive evidence has lieen obtained to substantiate the 

 hypothesis put forward b\' I'\ihon and by Clark that many fungi 

 exhibit a negative chemotropism toward their own metabolic 

 products (staling substances). 



2. Positive chemotropism towards such substances as turnip 

 juice, cane sugar and glucose, also exists, but under ordinary 

 conditions of growth, the effect is very much itss than that of the 

 negative chemotropism mentioned above. 



3. The substances present in turnip juice exert a much stronger 

 j:)ositi\'e chemotroi)ic effect tiian, e. g., 10 per cent cane sugar, 

 which suggests that plant juices in general may evoke a fairly 

 high positive chemotropic response. 



