STRUCTURE. 



43 



shall obtain some notion of the prevalent structure. In the 

 former the threads are more or less carbonized, in the latter 

 nearly colourless. One of the largest genera in Dematiei is 

 Helminthosporium. It appears on decaying herbaceous plants, 

 arid on old wood, forming effused black velvety patches. The 

 mycelium, of coloured jointed threads, overlays and penetrates the 

 matrix ; from this arise erect, rigid, and usually jointed threads, 

 of a dark brown, nearly black colour 

 at the base, but paler towards the 

 apex. In most cases these threads 

 have an externally cortical layer, 

 which imparts rigidity ; usually from 

 the apex, but sometimes laterally, the 

 spores are produced. Although some- 

 times colourless, these are most com- 

 monly of some shade of brown, more 

 or less elongated, and divided trans- 

 versely by few or many septa. In 

 Helminthosporium Smithii; the spores 

 much exceed the dimensions of the 

 threads ; * in other species they are 

 smaller. In Dendrypliium, the threads 

 and spores are very similar, except j(y 

 that the threads are branched at their 

 apex, and the spores are often pro- 

 duced one at the end of another in a 

 short chain. t In Septosporium again, 

 the threads and spores are similar, but 

 the spores are pedicellate, and at- ,-, 



1 FJG. 25. Helmmthosporium molle. 



tached at or near the base ; whilst in 



Acrothecium, with similar threads and spores, the latter are 

 clustered together at the apex of the threads. In Triposporium, 

 the threads are similar, but the spores are tri-radiate ; and in 

 Helicoma, the spores are twisted spirally. Thus, we might pass 



* Cooke, " On Microscopic Moulds," in " Quekett Journal," vol. ii. plate 7. 

 f See "Dendryphium Fumosum," in " Quekett Journal," vol. ii. plate 8; or, 

 " Corda Prachtflora," plate 22. 



