E. B. MAINS 
The pustules make their appearance first on the lower side of the leaf 
and are more abundant and larger there, often becoming confluent. 
The development of the rust within the host can be followed by 
sectioning day by day after inoculation. The method employed, 
which gave very good results, consisted in sectioning the infected leaf 
between pith. Very thin sections can be obtained in this way, if the 
leaf is cut up and a number of thicknesses of the leaf are placed between 
pieces of pith. The sections obtained in this way were mounted in 
Stevens's chloral hydrate and iodine (Stevens, 191 1). The sections 
are cleared by this mixture so that the hyphae of the rust stand out, 
showing clearly the haustoria in the cells of the host. Chlorophyll, 
which in untreated sections interferes with the determination of the 
mycelium, is thus bleached out, and any starch which may be present 
can be recognized. At the end of the third day, mycelium was already 
found to be well developed. At this time, the amount of starch, which 
was normally present in the parenchyma sheaths of the vascular 
bundles, was only slight, or it was entirely absent. By the fourth day, 
the mycelium had formed dense masses in the intercellular spaces of 
the mesophyll of the leaf. None was found sending haustoria into 
the epidermal cells, nor was any mycelium found in the vascular 
bundle and its parenchyma sheath. The cells of the parenchyma 
sheaths in the infected areas did not show at this time so large a quan- 
tity of starch as those in the non-infected areas. By the fifth day, the 
epidermal cells were invaded by haustoria from neighboring hyphae, 
and the amount of starch was noticeably less in the parenchyma sheaths 
of the infected areas. About the sixth day, pustules appeared. These 
were formed from masses of mycelium just below the epidermis. No 
mycelium was found in the vascular bundles or their parenchyma 
sheaths at this time. Some starch was found in the parenchyma 
sheaths of the infected areas, but these did not stain so darkly with 
iodine as those of the non-infected areas. 
The development of the rust progresses by a continued spread of 
its mycelium and the production of more pustules. The infected areas 
are however always limited in extent, varying from a millimeter to 
about a centimeter in diameter. Often, in the case of heavy infec- 
tion, whole leaves may become covered with pustules due to the union 
of many infected areas. Mycelium is found throughout the inter- 
cellular spaces of the infected areas, where it sends its hyphae into the 
neighboring cells, forming the branched, finger-like haustoria, which 
