graves: chemotropic reactions in rhizopus 329 



The combinations in A and B correspond to those in table i, but 

 D and E are new. 



E represents the control, for here conditions were made as 

 much ahke as possible in both upper and lower layers. The turnip 

 juice being everywhere practically the same in concentration, 

 no positive chemotropic force is acting. If the number and 

 length of the hyphae were also equal in both films, the turning 

 on both sides should be no more pronounced in one direction than 

 another. But it is practically impossible to prepare films of 

 exactly the same spore number per unit of volume, and it is 

 reasonable to assume that the more mycelium in a given prepara- 

 tion, the more staling substance will be given off by the hyphae. 

 This is doubtless one of the factors which will influence the final 

 percentages in this case. Another factor is the personal error in 

 estimation. In spite of this, however, the method is accurate to 

 within 10 per cent, as shown by tests to be published later. 



The results in A and B correspond with the same combinations 

 in the older .preparations described above, and the same observa- 

 tions apply here. In A, as would be expected in the young stage 

 where the hyphae are very short — averaging 40 /x here in 6^ 

 hours — the effect of the negative chemotropic stimulus is slight. 

 Later, in 73^ and 8 hours, when the hyphae are longer and more 

 staling substance has been given off, the percentage of turning 

 toward the spore-free layer is gradually increased; and as we 

 have seen in the older preparations described above, may become 

 ultimately about 100 per cent. In B, we have of course the 

 positive and negative chemotropic forces working together, which 

 accounts for the pronounced reaction. 



A consideration of D now bears out our interpretation of A 

 and B. For since both films here contain spores, the amount of 

 staling substance is more or less equalized throughout the prepara- 

 tion, and the force of the negative chemotropic stimulus is there- 

 fore practically eliminated. Any reaction which occurs should 

 be due to a positive chemotropism, and as the tables show, there 

 is considerable turning from the plain agar to the turnip juice 

 agar. This last experiment is the one which gives the final 

 clinching evidence for positive chemotropism. 



On the basis of these results we can get some approximate idea of 

 the relative intensity of the positive and negative chemotropic 



