154 RESEARCHES ON FUNGI 



another hypha of considerable length. The branch hypha, as if 

 in response to a chemotropic stimulus, then grows directly toward 

 the other hypha and meets it at a greater or smaller angle (often 

 at right angles). The apex of one of the two hyphae thus comes 

 to be pressed against the lateral wall of the other hypha. The 

 cell-walls separating the two hyphae from one another are then 

 dissolved, presumably by an enzyme, and fusion is complete : the 

 two hyphae are now united by their cell- walls and there is an open 

 passage between them. This type of hyphal fusion is illustrated 

 for the myceUum of Panus stypticus at h and m in Fig. 176, D, in 

 Volume III (p. 414) and again in this chapter for most of the hyphal 

 fusions shown for Coprinus sterquilinus in Figs. 87, 88, and 89, 

 and for C. lagopus in Fig. 96. 



(2) Two hyphae in the course of their development have crossed 

 so that at one place they are very near to one another but not in 

 contact. At the place which marks the shortest distance between 

 the two crossed hyphae, one of the hyphae, on the side nearest 

 to the other hypha, sends out a lateral branch which grows directly 

 toward the other hypha and fuses with it. Such a special branch 

 hypha, since it comes to form a bridge between two other hyphae, 

 may be called a bridging hypha. Bridging hyphae are illustrated 

 for the mycelium of Panus stypticus at c in B and C and at d in 

 D, Fig. 176, Volume III (p. 414) and again in this chapter for 

 Coprinus lagopus at/ and g in Fig. 96 (p. 170). Two bridging 

 hyphae in the mycelium of a Pyrenomycete, Pleurage anserina, are 

 well shown in Fig. 102, B (p. 178). 



It seems possible that the growth of one hypha toward another, 

 which it meets unerringly and with which it subsequently fuses, 

 is due to a chemotropic stimulus provided by some substance 

 excreted by the non-growing hypha ; but this has not been proved, 

 and it may be that the phenomenon has an entirely different explana- 

 tion. In this connexion an experimental enquiry is desirable. 



In the life-history of the Hymenomycetes in general there are 

 two kinds of mycelia : the haploid and the diploid. A haploid 

 mycelium comes into existence when a spore germinates. It is 

 characterised by having isolated nuclei — often only one in each 

 cell — and simple cross- walls. A haploid mycelium is transformed 



