M. C. COOKE ON SOME REMARKABLE MOULDS. 141 



POLYACTIS DEPR.EDANS, Cooke, MSS* 



Some six or seven years ago I noticed, Allien in Norfolk, that 

 several young trees of Acer jjseado-platanus, growing in a damp 

 plantation, presented an unusual appearance, from the flabbiness 

 and decoloration of the leaves, which induced me to collect some 

 for examination, the results of which I will now endeavour to de- 

 scribe. The green leaves had become flaccid and rotten whilst still 

 attached to the tree ; the whole surface blotched with greyish spots, 

 which were in many cases confluent over a great portion of the 

 leaf. The under surface, under a pocket lens, was spotted with 

 minute white points, like the head of a small pin. These points 

 were most numerous on, and almost confined to, the veins of the 

 leaf. Under the microscope, these minute points were found to be 

 the globose capituli, or heads, of a small parasitic mould, scattered 

 over the under-surface of the leaf, with its delicate mycelium pene- 

 trating into the substance. The heads were loosely scattered, and 

 not collected in tufts, almost wholly confined to the venation. The 

 hyphae, or threads, short, slender, flexuous, and septate, swollen at 

 the apex, where one, or three to four larger cells formed the basis, 

 of the globular head; around these large cells were clustered a 

 number of smaller, elliptical cells, which again were surmounted 

 by somewhat triangular, obtuse-cornered cells, and these divided at 

 the apex in a furcate manner, each fork divided off as a globose 

 hyaline spore. Each capitulum was, in its entirety, about one- 

 tenth of a millemetre in diameter, and the spores 12 micromille- 

 inetres. 



Some of these leaves were placed under glass and kept moist for 

 weeks, when a very peculiar phenomenon was presented, the for- 

 mation of small black round sclerotia on the spots occupied pre- 

 viously by the mould. This took place several times, and was re- 

 ported as a curious circumstance to my friend the Rev. M. J. 

 Berkeley. It would not be surprising for a Polyactis to be de- 

 veloped from a Sclerotium, because this already was known to have 

 taken place, but for a Sclerotium to be developed from a Polyactis 

 seemed to be a reversal of the order of nature. Pressure of other 



* Maculis griseis, determinatis vel confluent ibns. Hyphis assurgentibus 

 septatis, flexuosis, simplicibus, ad apicem cellnlis ellipticis, basidiiforruibus 

 coronatis, capitnlo globoso sub-compacto, conidiis globosis hyalinis, '012 

 mm. Toto albo. 



On under surface of leaves of Acer pseudo-x>latanus, which it destroys* 



Journ. Q. M. C, Series II., No. 12. m 



