342 MICROSCOPIC STBUCTURE OF HIGHER CRYPTOGAMIA. 



constitute a group of special interest to the Microscopist ; of 

 which a few of the chief examples may here be noticed. The 

 mildew, which is often found attacking the straw of Wheat, shows 

 itself externally in the form of circular clusters of pear-shaped 

 spore-cases (Fig. 125), each containing two compartments filled 

 with sporules ; these (constituting the 

 Fio- 125. Puccinia graminis) arise from a filamen- 



tous tissue constituting the mycelium, the 

 threads of which interweave themselves 

 with the tissue of the straw; and they 

 generally make their way to the surface 

 through the "stomata" or breathing- 

 pores of its epidermis. The rust, which 

 makes its appearance on the leaves and 

 chaff-scales of Wheat, has a fructification 

 that seems essentially distinct from that 

 just described, consisting of oval spore- 

 cases, that grow without any regularity 

 of arrangement from the threads of the 

 mycelium ; and hence it has been con- 

 sidered to iDelong to a different genus and 

 _ . . species, Uredo ruhiqo. But from the ob- 



servations ot Jrrot. Henslow, it seems cer- 

 tain that the "rust" is only an earlier form of the " mildew ;" 

 the one form being capable of development into the other, and 

 the fructiffcation characteristic of the two supposed genera having 

 been evolved on one and the same individual. Another reputed 

 species of Uredo (the U. segetum) it is, which, when it attacks the 

 flower of the wheat, reducing the ears to black masses of sooty 

 powder, is known as " smut" or " dust-brand." The corn-grains 

 are entirely replaced by aggregations of spores ; and these being 

 of extreme minuteness, they are very easily and very extensively 

 diifused. The "bunt" or " stinking rust" is another species of 

 Uredo (the U.fcetida), which is chiefly distinguished by its dis- 

 gusting odor. The prevalence of these "blights" to any con- 

 siderable extent, seems generally traceable to some seasonal in- 

 fluences unfavorable to the healthy development of the wheat 

 plant ; but they often make their appearance in particular locali- 

 ties, through careless cultivation, or want of due precaution in 

 the selection of seed. It may be considered as certain that an 

 admixture of the spores of any of these fungi with the grains, 

 will endanger the plants raised from them ; but it is equally cer- 

 tain that the fungi have little tendency to develope themselves in 

 plants that are vegetating with perfect healthfulness. The wide 

 prevalence of such blights in bad seasons is not difiicult to ac- 

 count for, if it be true (as the observations of Mr. John Marshall, 

 a few years since, rendered probable) that there are really very 

 few wheat-grains, near the points of which one or two sporules 

 of Fungi may not be found, entangled among their minute hairs; 



