THE FORMATION OF HYPHAL FUSIONS 69 



Imperfecti, the vegetative fusions which are formed in such large 

 numbers are in no way connected with sex ; for they are formed 

 between the hyphae of one and the same haploid mycelium and 

 between the hyphae of any two haploid mycelia of the same sexual 

 kind ; and, in the Basidiomycetes, they are also formed between the 

 hyphae of one and the same diploid mycelium and between the 

 hyphae of any two diploid mycelia. 



While the telemorphic and zygotropic phenomena observed in 

 vegetative hyphal fusions may be initiated by the excretion of 

 chemical substances, it is not easy to formulate a precise theory, 

 based on the assumption of chemical stimuli, which will account for 

 all the observed reactions. 



If on a substratum of agar or gelatine a pollen-grain is set a little 

 distance away from a stigma, when it germinates its pollen-tube 

 grows directly toward the stigma, i.e. it grows from a region where 

 the chemical excretion of the stigma is in lesser concentration to a 

 region where it is in greater concentration. 



If in a vegetative hypha-to-hypha fusion of a fungus the tips of 

 both the hyphae were to excrete one and the same chemical sub- 

 stance, then if we may judge by what has been observed in the 

 chemotropism of pollen-grains the two hyphae could not approach 

 one another, for there would be no gradient of weaker to stronger 

 concentration of the substance extending from each hyphal tip to 

 the other hyphal tip. It therefore appears that the excretion of one 

 and the same chemical substance by two hyphae bending toward 

 one another zygotropically is inadequate to account for the growth- 

 movements observed. 



If one substance is not enough for our chemical theory, let us 

 try two. Let us suppose that, in a hypha-to-hypha fusion, the tip 

 of one of the hyphae excretes one chemotropic substance and the 

 tip of the other another chemotropic substance and that each hypha, 

 while unable to react chemotropically to the substance which it 

 itself excretes, is yet chemotropically sensitive to the substance 

 which the other hypha excretes. Under the conditions here assumed 

 it is theoretically possible for the two hyphae in a hypha-to-hypha 

 fusion to grow toward one another until they meet. 



However, the assumption that in a hypha-to-hypha fusion two 



