1900 | THE HAUSTORIA OF THE ERYSIPHEA 173 
some of the powdery-mildews, and Palla (24, p. 68) has recently 
shown that Phyllactinia has intercellular hyphe. Uncinula Salicts 
also offers a striking contradiction to that old conception, but 
the method by which this fungus reaches the interior tissues of 
the leaves is different from the one Phyllactinia employs. 
Uncinula is amphigenous on the leaves of the willow. Its 
appressoria are lobed, as De Bary affirmed. The mycelium is 
entirely external. On the upper surface of the leaves the lobed 
appressoria give rise to penetrating tubes which enter the epi- 
dermal cells. All of these tubes do not develop in the epi- 
dermal cells into haustoria. An examination of a cross-section 
of the willow-leaf shows that the epidermal cells are very abun- 
dantly infested with haustoria, but in addition, numerous slender 
bars can be observed reaching from the outer walls across the 
epidermal cells to their inner walls. The bars give the cells the 
appearance of possessing trabecule. The haustoria in the cells 
frequently hide the outer end of these structures, but in follow- 
ing their course to the inner wall, haustoria may sometimes be 
observed in the palisade cells of the leaf. The bars are the 
penetrating tubes, or the necks, of these subepidermal haustoria. 
When the penetrating tubes reach the palisade cells they enlarge 
into haustoria not unlike the ones described for EZ. communis, 
possessing the sheaths and nuclei. Several tubes may pierce the 
outer epidermal wall close together and take different directions 
across the cells. Of the tubes which penetrate the outer wall of 
the epidermal cell close together, some may enlarge immediately 
into haustoria, while others may pass to the palisade cells. This 
crowding and confusion makes it difficult to discern the true 
Structure. At first glance it looks at times as if the short necks 
of the epidermal haustoria give rise to one or more branches 
which develop into haustoria. The deception arises from the 
crowded position of the organs. 
On the under side of the leaf the epidermal cells are like- 
wise penetrated by the slender tubes, some of which immedi- 
ately enlarge into haustoria, and some of them (alittle less than 
half), penetrate into the mesophyll cells immediately under the 
