174 BOTANICAL GAZETTE [MARCH 
epidermal cells. igs. 22-26 all represent cells on the under 
side of the leaves. The upper epidermal cells of the willow 
leaves in autumn are so filled with deeply staining products as 
to make the cells of this epidermis unfavorable for study. 
As far as they have been observed the haustoria of Uncinula 
Salicis are not occasionally septate as in Erysiphe communis. In 
the possession of a single nucleus, surrounded by spongy proto- 
plasm, and in its general shape, the haustoria of Uncinula are not 
peculiar. The subepidermal haustoria are also surrounded by 
sheaths such as Erysiphe communis has. But in the epidermal 
cells the sheaths are very frequently absent. The explanation 
of this absence of sheaths lies in the fact that Uncinula is capa- 
ble of greater enzymic activity than Erysiphe, so that the sheaths 
are dissolved away. But by the time the tube has penetrated a 
palisade cell its supply of ferment is either exhausted or weak- 
ened (or both), so that the sheaths remain. The typical collar 
may sometimes be seen around the necks of the haustoria (fig: 
23), but for the subepidermal haustoria it can be demonstrated 
at times, from the size and reaction to stains, that the penetrat- 
ing tube is accompanied entirely across the epidermal cell by the 
cellulose ingrowth from the outer wall (jigs. 22, 24). . 
De Bary does not mention the fact that the lobed appres- 
sorium of Uncinula may give rise to several haustoria (jigs. 22 
24). Other appressoria, as far as they have been observed, pro- 
duce absorbing organs singly, but on the willow leaf two or more 
tubes from the same appressorium pierce the epidermis close 
together. The confusion and crowding of haustoria in the cells 
is due to this fact, rather than to the fact that the extremely 
dense mycelium sends many single organs into the leaf. On the 
other hand, the lobed character of the appressoria seems to be 
produced by the contortion of the filaments necessitated by the 
origin of several haustoria in one place. The significance of the 
term appressoria consists in the fact that the expansions of the 
filaments which rise to haustoria are commonly closely appt essed 
to the surface. But this habit is not uniform in Uncinula Salicts- 
Figs. 23, 25, and 26 show that the appressoria may stand at some 
the A ivi aes 
