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, 
and on old wood, forming elffused 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 Dendryphium, the threads 
and spores are very similar, except 
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- 
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 
Fia. 25.—-Helminthosporium molle. 
* Cooke, ‘‘ On Microscopic Moulds,” in ‘‘ Quekett Journal,” vol. ii. plate 7. 
+ See ‘‘Dendryphium Fumosum,” in ‘‘ Quekett Journal,” vol. ii. plate 8; or, 
‘¢ Corda Prachtflora,” plate 22. 
