July, 1922] DODGE STUDIES IN THE GENUS GYMNOSPORANGIUM 357 
on stems protected by a layer of cork cells. The mycelium may be confined 
to the leaf two or three years, developing sori each season; the leaf may 
then die off without the fungus having been able to reach the stem tissues 
below. After entering the leaf, the mycelium invades the region between 
the cuticle and the cellulose walls of the epidermis on the proaxial side. 
The effect of the fungus on this part of the cell wall is usually marked 
by considerable swelling, and the disorganized substances take the stains 
very readily (indicated by shaded portions in my figures). The cell walls 
of the hyphae also thicken so that it is often difficult to say where these 
end and the disorganized cuticularized region of the host begins. Haustoria 
are found in epidermal cells (fig. i, C), sometimes even in the guard cells of 
stomata. As noted, the fungus explores and feeds in the cuticularized layer, 
but it may go deeper and invade the palisade mesophyll tissue. None of 
my sections shows hyphae on the abaxial side of the leaf. Figure i, A 
to K, represents about three fifths of that portion of the leaf which shows 
the presence of the fungus. Hyphae are found near the epidermis along 
half the entire length of the leaf. At A, palisade cells have divided and 
the epidermal cells are swollen. At B, only a few hyphae are visible but 
beneath the stoma the mycelium is massed between the palisade cells. The 
cuticle is raised considerably at C, and the cuticularized layer beneath it 
is much disorganized. At D, there is a swollen supporting cell of the 
stoma which is shown at the left below. Exploration laterally in the 
cutinized wall is very slow. 
At E in figure 4 the epidermis is normal. All traces of the fungus 
in this section are indicated in the figure, and it is evident that no hyphae 
approach the conducting system F. The attack on the leaf is certainly 
very limited and superficial, but not more so than it is on the young stems 
or old tree trunks, phases which will now be considered. 
Distribution of the Mycelium in Young Stems 
Vigorously growing terminal regions of young stems are the most sus- 
ceptible to infection, either directly or through the invasion of the fungus 
by way of the leaf axils. At the time of the formation of the first sori, 
which will be either about ten months or a year and ten months after 
infection, the stem will still be green and devoid of cork tissue, the greater 
portion being covered by the decurrent leaf bases. The diagrams A~F in 
figure 2 were made from a series of sections of a small branch of plant 
no. 403 which had been infected in August, 1915,"^ the first sori appearing 
in the spring of 1917. The number of sections J /jl thick between those 
diagramed is given in the figure. Beginning with diagram ^, the mycelium 
represented by shading is shown to occupy the three regions in the cortex 
between the decurrent leaf bases (L), not approaching the cambium; even 
the phloem tissue is some distance beyond. Passing up the stem about 
^ See Bull. Torrey Bot. Club 45 : 287-300. 1918. 
