THE FORMATION OF HYPHAL FUSIONS 27 



the word hypha is meant an ordinary vegetative hypha of some 

 length which has grown freely in the culture medium before becoming 

 " attracted ' by another hypha and undergoing growth changes 

 leading to a hyphal fusion ; by the word peg is meant a very short 

 special fusion hypha which has never grown freely in the medium by 

 itself but has been stimulated to come into existence by another 

 hypha, another peg, or a hook with which it is destined to fuse ; 

 and by the word hook is meant a very short curved hypha such as is 

 always produced during the formation of a clamp-connexion. A 

 peg may be regarded as a fusion organ. 



Hypha-to-Hypha Fusions.— A hypha-to-hypha fusion takes 

 place between two hyphae of greater or less length which at first 

 grow freely and independently of one another in the culture medium. 

 As growth proceeds, these hyphae, by chance, come within a certain 

 distance of one another (about 7-15 n). Then their ends change 

 their direction of growth so that they grow directly toward one 

 another. Soon, their apices meet and flatten out against one 

 another. Next, the appressed walls at the ends of the two hyphae 

 break down and disappear and, at the same time, the cylindrical 

 side-walls of the hyphae become continuous with one another so as 

 to form a compound tube. Finally, the two masses of protoplasm 

 at the ends of the two hyphae meet and fuse, with the result that 

 protoplasmic continuity from one hypha to the other becomes 

 established. 



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

 are shown diagrammatically in Fig. 13. At A, owing to apical 

 growth, two long independent branch-hyphae happen to be approach- 

 ing one another. At B, they have come near enough to one another 

 for mutual stimulation to take place. At C, as a result of this 

 stimulation, the ends of the two hyphae have altered their direction 

 of growth and are growing toward one another. At D, the two 

 hyphae are rapidly approaching one another. At E, they have met 

 and their ends are touching. The ends of the hyphae now flatten 

 against one another, the cylindrical walls fuse together, and the 

 dividing double wall dissolves and disappears. Finally, as shown 

 at F, with the disappearance of the double wall, a passage-way is 

 formed from one hypha to the other, and the two masses of 



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