STEVENS: DISEASES OF GRASSES. I25 



spongy parenchyma, and for their liberation the under layers of dense 

 parenchyma must be broken as well as the cuticle, so that the mere 

 mechanical injury done to the host is not inconsiderable. This is 

 somewhat clearly shown by the lower section of the figure {loc. cit.). 

 The mycelium from which the spores arise is traceable as anastomosing 

 brown filaments penetrating throughout the parenchyma. The chlo- 

 rophyll corpuscles are still present in many of the parasitized cells, 

 but they are not so numerous or so large as in the unaffected portions 

 of the leaf. Some of the cells bordering adjacent fibro-vascular bun- 

 dles are lined with thick brown incrustations. These do not appear 

 in unaffected leaves, and are evidently due to the fungus. The spore 

 clusters on the specimens studied sometimes reached a length of 

 .8 m.m., but the majority were mere points, not exceeding .i m.m. 

 in length. These were so closely grouped that as many as three in 

 the space of a millimeter were not infrequent. It is evident from the 

 way these spore-groups or sori are thickly ranged up and down 

 between two fibro-vascular bundles, while adjacent interspaces are 

 free from them for considerable distances, that many sori are fre- 

 quently produced from one mycelium developed from a single spore, 

 and that the woody fibres of the bundles are, as a rule at least, effect- 

 ual barriers against the lateral extension of the mycelium. Uromyces 

 is a genus closely related to Puccinia. Fig. 2, plate XI, shows U. 

 spartincB parasitic on Spartina stricta. The leaves of this grass are 

 densely ribbed on the upper or inner surface, the ribs extending to a 

 height equal to or exceeding the thickness of the body of the leaf. At 

 the bases of the ribs are fibro-vascular bundles bordered by large and 

 mostly hexagonal cells. Against these, and extending to the cuticle, 

 is a row of delicate palisade parenchyma. The mycelium of the para- 

 site is confined in these parenchyma cells, forming there a dense net- 

 work. In the figure (2, plate XI) the ridges of the leaf are seen to be 

 thrust apart by the development of the spores. The mycelium pene- 

 trating the parenchyma is seen as a shaded narrow strip extending to 

 the under surface of the leaf, where another spore cluster is forming. 

 The sori in this instance are from two to six millimeters in length. In 

 leaves of Panicuni varigatum the parenchyma cells are arranged radi- 

 ally about the bundles. When parasitized by Uromyces graminicola the 

 mycelium extends throughout the parenchyma and surrounds the 

 bundles. The cells, although penetrated and lined on their interior 

 by the mycelial filaments, retain an apparently normal condition. The 

 relation of Puccinia plvagmitis to Spartina cynosuroides is well shown 

 in figure i, plate XII. Here the ridges of the inner surface of the leaf 

 are capped with a thicker layer of sclerenchyma than is the case with 

 S. stricta, and the bundles are larger and project farther into the 



