6 British Uredinee and Ustilaginee. 
especially when they occur upon the stems or cauline 
sheaths of their graminaceous host-plants. 
The presence of the mycelial hyphe generally acts as 
a local stimulant to the tissues in which they are present, 
as is evinced by the increased thickness which is often 
associated with a concave arching or vaulting of the 
affected places (Zcidium grossularia, berberidis, Restelia 
cancellata). When the stems are affected, this is usually 
shown very markedly by the development of swellings and 
distortions; as a rule these are more or less fusiform, and 
often induce considerable bending of the attacked stem 
(P. difformis, Kze.; Ur. trifolit), Even upon these cauline 
tumefactions may often be traced the concentric arrange- 
-ment of the sori. The stimulation of the affected part 
may be carried to such an extent as to kill the invaded 
tissues; thus we often find the older leaves of Malva 
sylvestris and Althea rosea with numerous circular holes, 
punched as it were out of them. Each of these has been 
the seat of the very localized mycelium of Puccinia malva- 
cearum, which has by its presence killed a circumscribed 
portion of the entire thickness of the leaf tissue, so that, as 
the leaf itself expands, the dead area above described 
becomes separated round its circumference and falls out, 
leaving a circular hole. The above-described perforated 
foliage is most observable after a period of drought; in 
rainy weather the reproduction of the parasite is so rapid 
that the entire leaf tissue is quickly invaded by the fungus 
and totally destroyed. The same dropping out of mycelial 
areas occurs upon the stem; here, however, the hyphz 
only penetrate the external parts, so that when the affected 
spots drop out an elongated or fusiform wound is left, at 
the bottom of which the central woody part of the stem 
is exposed. Schroter* has pointed out that a somewhat 
* Schroter, ‘‘Cohn’s Beitrage,” vol. ii. p. 88, 
