THE FORMATION OF HYPHAL FUSIONS 43 



weaken the supposition that hooks and clamp-connexions are 

 homologous structures. 



According to M. and Mine Moreau, 1 the asci in species of the 

 lichen genera Parmelia, Physcia, and Anaptychia are formed from 

 terminal cells of ascogenous hyphae, and these hyphae resemble the 

 hyphae of the diploid mycelium of the Hymenomycetes in that they 

 consist of a chain of binucleate cells with a clamp-connexion between 

 every two successive cells. M. and Mine Moreau used fixed and 

 stained material and did not watch the " ascogenous clamp- 

 connexions ' actually being developed by the living hyphae. 

 Therefore, there is the possibility that each " clamp-connexion ' 

 arises in the first place by the Pyronema confluens hooking method 

 and that the binucleate penultimate cell of the hook, instead of 

 developing into an ascus, grows forward as a hypha, becomes hooked 

 at its end in its turn, repeats the process of " clamp-connexion '' 

 formation just described, and so on. In this way, by sympodial 

 branching, there might be produced along a multicellular hypha a 

 series of structures resembling in appearance the clamp-connexions 

 of the Hymenomycetes but having a distinctly different mode of 

 origin. If, on the other hand, the " ascogenous clamp-connexions " 

 observed by M. and Mme Moreau are formed not from the curved 

 end of a hypha but from a special branch-hypha which grows back- 

 wards from the middle of a straight terminal cell, there still remains 

 the possibility that the clamp-connexions of the Hymenomycetes 

 and of the Lichen-fungi under discussion may have arisen, not by 

 inheritance from a common ancestor, but by parallel evolution. 



Biological Significance of the Hook of a Clamp-connexion 

 growing Backwards instead of Forwards.— When studying the 

 development of the mycelium of Coprinus stercorarius , Brefeld 2 

 observed that, when a clamp-connexion is being formed, the hook 

 always grows backwards. Subsequent observations have taught us 

 that this rule holds for Hymenomycetes and Gastromycetes in 



1 M. and .Mine F. Moreau : (1) " llecherches sur quelques Lichens des genres 

 Parmelia, Physcia et Anaptychia," Rev. gen. de Bot., T. XXXVII, 1925, pp. 385-417 ; 

 (2) " Le mycelium a boucles chez les Ascomycetes," Compt. rend., 18 avril, 1922 ; 

 and (3) " Crochets et anses ascogenes," Bull. Soc. Myc. France, T. XLI, 192<>, 

 pp. 469-471. 



2 O. Brefeld, Untersuchungen iiber Pilze, Heft III, Leipzig, 1877, pp. 17-18. 



