THE FORMATION OF HYPHAL FUSIONS 33 



to one another, a single peg is produced by one of them and a pair 

 of pegs facing the single peg by the other (Fig. 27, A, p. 53). As all 

 the three pegs grow in length, both the pegs in the pair bend toward 

 the opposing single peg, but the single peg bends towards one only 

 of the pair of pegs and finally fuses with it alone (Fig. 27, B). As 

 soon as fusion has taken place, the unpaired peg, as a rule, ceases to 

 grow in length. In Pleurage anserina, a few triple-peg fusions have 

 been found in which, apparently, the single peg produced by one 

 older hypha fused with both of two pegs produced by the other 

 older hypha (Figs. 30, D, and 31, B, p. 56). 



When two hyphae in a mycelium run parallel to one another for 

 some distance, they may become connected by several peg-to-peg 

 fusions formed at intervals along their length and thus with their 

 bridges they may come to have a scalariform appearance. Some 

 scalariform hyphae in a mycelium of Pleurage curvicolla are to be 

 seen in the photomicrograph reproduced in Fig. 1 (p. 5). 



The two parallel hyphae about to form a scalariform structure 

 stimulate one another at a, distance from one another and at intervals 

 along their length, and it is in reponse to this telemorphic stimulation 

 that successive pairs of opposing pegs are produced. The two pegs of 

 any pair then stimulate one another tropically, and it is in response 

 to this zygotropic stimulation that they grow toward one another 

 and meet. It used to be supposed that the scalariform structure 

 which is formed during the process of scalariform conjugation in 

 Spirogyra originates like the scalariform hyphae just described ; 

 but the recent observations of Czurda * and Saunders 2 have shown 

 that this supposition is erroneous, for it has been found that con- 

 jugation in Spirogyra begins only after the two filaments concerned 

 have come into contact with one another side by side and have become 

 glued together by mucilage. 



Hook-to-Peg or Clamp-connexion Fusions. — Clamp-connexions 

 are characteristic of the secondary mycelium of the Hymenomycetes, 

 Gastromycetes, Ustilaginaceae, and Tilletiaceae. 3 In certain species 



1 V. Czurda, " Zur Kenntnis der Copulationsvorgange bei Spirogyra," Archiv f. 

 Protistenk. Bd. LI, 1925. Cited from Saunders's paper, p. 233. 



2 Hazel Saunders, " Conjugation in Spirog3 r ra," Annals of Botany, Vol. XLV, 

 1931, pp. 239-242, 255. 



3 Cf. these Researches, Vol. IV, 1931, p. 287. 



VOL. V. L> 



