1889.] MICROSCOPICAL JOURNAL. 173 



opposition to this idea, that at the time of the germination of the teleuto- 

 spores, the stomata of the delicate spring growth of these plants present 

 every facility for the entrance of the sporidia which are of much less 

 diameter than those openings. It seems much more probable that the 

 change of hosts is brought about by requirements of the fungus not 

 dependent upon such slight mechanical hinderances, but upon inherent 

 wants of the parasite not to be satisfied by one of its hosts alone. If 

 ease of entrance into the host is all that determines heteroecism, there 

 would be slight cause for the fungi quitting their aecidium hosts to attack 

 the Gramineas, for those plants as a rule are possessed of non-silicious 

 cuticles. That the last stages appear upon the grasses is, we think, 

 not so much a matter of choice as of necessity. 



Mycelium. — This term applies in common to the vegetative portion 

 of all the spore forms. Usually it is essentially a network of anasto- 

 mosing tubular filaments variously septated and branched, having a 

 diameter of from 2 to 6 p.. In the formation of the stroma or bed from 

 which the spores arise, however, the hyphas often become closely united 

 by fusion* so as to form what may fitly be termed a false tissue. The 

 hyphal walls are hyaline and are quite delicate while young. As they 

 grow older the walls thicken and the granular pi-otoplasm with which 

 they are at first filled disappears as the spores are formed. 



As in most uredines, the mycelium is localized, the results of one 

 infection being confined to a quite limited area, as may be determined 

 by single artificial infections on carefully isolated plants. In the case 

 of the aecidium of Puccinia graminis on the leaf of barberry (fig. 6 a) 



Frc 6. — Inferior side of barberry leaf, showing at (a) an 

 aecidium spot arising from single infection. Natural size. 

 Original. 



it will be found that the result of one infection affects but a slight area 

 of the leaf. On the Gramineae, however, the localization is much less 

 marked. In these hosts the leaves, sheaths, and often large portions of 

 the haulm, may be ramified throughout, the fruiting pustules appearing 

 over the whole diseased area. 



When a hypha, say one from a germinating uredospore, penetrates 

 the tissues of the host, branches are given oft' in every direction, which 

 continue to branch in a monopodial manner. The result is a complete 

 ramification of the parenchymatous tissues of the host in the immediate 

 region of infection, the hyphae being at first principally confined to the 

 intercellular spaces. They apply themselves closely to the cell walls 

 and create passage-ways through and around the cells by a process of 

 solution. As the fungus develops the branching becomes more profuse, 

 and, in the vicinity of forming fruit — i. e., pustules or sori — the host 



*See paper on " Sub-epidermal Rusts," Bot. Gaz., 1889, p. 139. 



