266 Sanford. — Anato7ny of the Common 
will be found a thin spot in the wall. Fig. 4 shows a number 
of these cells in longitudinal section ; and Fig. 3, b , shows 
others as they appear in section just beneath the epidermis 
of the knot. 
The epidermis, as before stated, bears little resemblance 
to that of the normal leaf. It consists of a layer of corky 
cells, about four cells in thickness, and covered externally 
by what appears to be the flattened and shrivelled-up re- 
mains of similar cells, forming a thin layer over the surface. 
These cork-cells are shown in cross section in Fig. 3, a. It 
is the epidermal tissue that is raised up and broken through 
by the spore-masses, Fig. 1, c\ and it is this tissue, together 
with the thick masses of mycelium that collect beneath it 
at these points, which form the rings at the bases of the 
spore-masses, Fig. 1, d. 
The mycelium of the fungus penetrates to the interspaces 
between the parenchymatous cells of the leaf, and developing 
there in great abundance, in some way stimulates the tissues 
to this abnormal growth, Fig. 4. The mycelium is rather 
coarse, contains many yellow oil-globules, has very definite 
cell-walls, and branches very frequently. It is a continuous 
tube without septa, save at the places where it collects to 
form the spore-masses, Fig. 2, c. The fungus draws nourish- 
ment from the cells of the knot by means of rather large 
haustoria which penetrate through the walls and develop 
within the cells ; their contents are very granular, and their 
walls are not so firm and definite as are the walls of the 
mycelium, Fig. 5. At the bases of the spore-masses the 
mycelium forms even more of the substance of the knot than 
do the cells of the host, which are here quite small, with 
often very indefinite cell-walls, and themselves appear to be 
completely isolated from one another by the mycelium which 
surrounds them. Fig. 6 is a section through one of these 
places, a the line along which the spore-mass was attached 
(corresponding to c in Fig. 2), b some epidermal cells of the 
knot lying at the edge of the opening made by the protruding 
mass, and c the parenchymatous cells of the host. At these 
