3 o RESEARCHES ON FUNGI 



developed on the side of an older hypha in response to a stimulus 

 sent out by the younger hypha. 



Successive stages in the formation of a hypha-to-peg fusion are 

 shown diagrammatically in Fig. 14. At A, the end of a younger 

 hypha (a) is seen approaching by chance the side of an older hypha 

 (6). At B, the end of the younger hypha has approached within 

 a certain distance of the side of the older hypha. At this stage of 

 development the older hypha, either chemotropically or possibly by 

 the emission of radiations, causes the younger hypha to alter its 

 direction of growth so that the younger hypha grows directly toward 

 the older hypha. An early stage in this tropism is shown at C. In 

 response to a morphogenic stimulus received from the end of the 

 younger hypha, the older hypha now sends out a very short peg-like 

 hypha (c), as shown at D. Next, the younger hypha and the peg 

 grow toward one another until their apices meet, as shown at E. 

 Finally, the end of the younger hypha and the end of the peg flatten 

 out against one another, the cylindrical walls of the hypha and peg 

 become continuous with one another, the two walls appressed 

 together become dissolved, a compound cylindrical tube is thus 

 formed, and the protoplasm of the younger hypha becomes con- 

 tinuous with that of the peg, as shown at F. 



Very often, when the end of a hypha meets the end of a peg, their 

 axes are inclined to one another at an obtuse angle {cf. Fig. 14, E), 

 with the result that, after a hypha and a peg have fused with one 

 another, a more or less S-shaped twist can be seen about the point 

 of their union. This S-shaped twist frequently aids one in recognis- 

 ing as a hypha-to-peg fusion an anastomosis which one has found 

 after it has teen formed. 



Peg-to-Peg Fusions. — A peg-to-peg fusion takes place between 

 the ends of two very short peg-like hyphae produced opposite to 

 one another on the sides of two older hyphae. We may call the 

 compound hypha formed by the union of two pegs a bridging hypha 

 or bridge. 



Successive stages in the formation of a peg-to-peg fusion are 

 shown diagrammatically in Fig. 15. At A are seen two older 

 hyphae running more or less parallel to one another. These hyphae 

 in some way stimulate one another morphogenically so that at 



