6 British Uredinece and Ustila^ine<2. 



especially when they occur upon the stems or cauline 

 sheaths of their graminaceous host-plants. 



The presence of the mycelial hyphae 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 i^Aicidiiim grossitlaricE, berberidis, Rastelia 

 cancellatd). 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 Maha 

 sylvestris and Althcea 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- 

 cearuni, 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 hyphae 

 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. S8. 



