4 S etc hell. — An Examination of the Species 
one another only at the places where the pits are situated, 
thus leaving a vast network of small connected intercellular 
spaces throughout this layer, in direct communication with 
the series of spaces in the layers of spongy parenchyma 
below. The mycelium of the fungus is found traversing these 
spaces in all parts of the leaf, but is especially abundant in the 
regions where the spots occur. The mycelium is composed of 
slender hyphae, 2 /x to 3 /x thick, branched at short intervals, 
and septate. The septa are not readily seen until the contents 
are removed by some clearing reagent (Fig. 70). The hyphae 
are full of very small oil-globules. They apply themselves 
closely to the cells along their course, but a careful search 
failed to detect haustoria of any kind. 
The sori are situated in both layers of the leaf, but occur 
most frequently in the palisade-layer. They occupy the large 
dome-shaped cavities in this layer just under the stomata 
(Fig. 68). They are nearly globose as a usual thing, vary 
from 120 ju, to 180 /x in diameter, and are of a light brown 
colour. The spores are rather loosely packed together, glo- 
bose, ellipsoidal, or somewhat polyhedral in shape, and possess 
rather thick, very light-coloured walls. They are from 8 /x to 
]o fi in diameter, and their light, shining, granular contents 
include from one to four or five large drops of oil. The cortical 
layer is readily seen even with a simple lens. It is composed 
of radially elongated cells which measure 1 2 /x to 20 ju by 4 /x 
to 10 /x. They are comparatively uniform in shape and size, 
forming a contrast to the two following species in this respect. 
They are light brown and destitute of solid contents (Fig. 69). 
The development of the sorus, as far as it has been studied, 
seems to agree in its details with that of D. sagittariae as 
described by Fisch (Ber. Deutsch. Bot. Gesell., Bd. II, p. 41 2). 
The hyphae collect in a loose strand in the air-space beneath 
a stoma, and, by means of numerous interlacing short branches, 
are soon formed into an irregular bunch nearly filling the cavity. 
The cells of the hyphae of the interior of the bunch now be- 
come swollen, filling out the spaces between the strands and 
causing the bunch to increase in diameter. A cross-section 
