64 M. C. COOKE ON MICROSCOPIC MOULDS. 



manent in themselves to warrant the prior recognition of Tubercu- 

 laria as a good species until its autonomy was placed beyond a 

 doubt. It would have been a very different thing indeed if Tuber- 

 cularia vulgaris could have been shown to have passed into Tuber- 

 cidaria granulata, or any other species of the same genus. Such 

 an event would have affected the soundness of specific distinctions, 

 which dimorphism does not. 



Passing from Helminthosporium, we may sometimes meet with 

 effused velvety black patches, on decorticated oak, which, although 

 similar to the naked eye ; are very different in character when 

 seen under the microscope. This is Triposporium elegans (Plate 

 viii.). The flocci are also dark coloured, erect, and branched, but 

 the fruit is very different ; it consists of tri-radiate articu- 

 lated spores. Recently, Mr. Broome has found these spores 

 with swollen and rounded tips ; a circumstance from which it 

 would at present be premature to draw conclusions. Noth- 

 ing of the kind is known in Helminthosporium. The Tripos- 

 porium is, then, something like a Helminthosporium with compound 

 spores. Other species are found in other parts of the world, and 

 this seems to strengthen the genus as perfectly distinct from Helmin- 

 thosporium. It will be time enough to doubt its generic value 

 when some one has demonstrated the tendency to a similar form of 

 fruit in any good species of Helminthosporium. 



Amongst the -numerous and interesting fungi which are found 

 growing on dead stems of nettles, there is one which may be al- 

 luded to as exhibiting a variation in the structure of the stem, 

 which is found also in some other genera of the Black Moulds 

 (Dematiei). I allude to Arthrobotryum atrum. The stem is in 

 this instance a complex one, composed of numerous threads, or 

 flocci, which are collected together and combined into a common 

 stem, which seem at first to support a globose head of spores, 

 but upon closer examination each of these spores will be seen to 

 have its own proper stem or thread, and by their agglomeration 

 the spores assume the form of a more or less globose head. In 

 the present instance the threads are swollen above, just at the 

 point of junction with the spores, and the spores themselves are 

 nearly elliptic, divided by transverse septa, with the central por- 

 tion brown, and the terminal joints colourless. A similar species 

 with smaller spores is found on willows, and what I think is an un- 

 de scribed species, on decaying grass and straw. Here, again, in 



