1900} THE HAUSTORIA OF THE ERYSIPHEE 179 
palisade cells, they are still at the very source of that supply of 
food by which the plant maintains its vigor and activity. The pro- 
portion of cases to be found in which the intercellular hyphe do 
develop haustoria in these more favorable regions is larger than 
casual observation would lead one to think. By a study of the 
cells of the host-leaf in sections preceding and following the 
section which contains the haustorium, it may often be deter- 
mined that the fungus actually has placed its absorbing organs 
in one of the favorable cells. It should not be thought, how- 
ever, that this is done invariably. In Xanthoxylum the leaves 
are so constructed that the bundles are near the lower epidermis 
through the stomata of which the fungus finds access. All of 
the mesophyll cells, therefore, have a surplus of food. This fact 
would account for the number of mesophyll cells which contain 
haustoria. 
If it be demonstrated that the mildew on Xanthoxylum does 
not possess typical haustoria under any conditions, it will be 
important to discover whether the apparently stunted haustoria 
can absorb food for the fungus as well as typical organs. If 
they are not effective, how far is the fungus nourished by sapro- 
phytic methods? In the first place, the intercellular spaces of 
the leaves of this particular host are abundantly supplied with 
organic material. On the part of the parasite itself it is notice- 
able that, when under \the stomata, the intercellular hyphe 
encounter parenchyma cells which block the way, the cells of 
the hyphz are short, thick, and vigorous; not only when the 
haustoria are produced near, but also when the hyphe have 
haustoria at some distance from the stomata. But the hyphe 
which do not encounter such cells, but run a comparatively unin- 
terrupted course before constructing haustoria, are more OF less 
attenuated and are less vigorous in appearance. If some cell of 
the hypha in such a case lies in contact with a parenchyma cell 
(fig. 33, penultimate cell), that cell of the hypha is short, thick, 
and vigorous. Biisgen’s results would seem to show that Ery- 
siphe does not succeed as a saprophyte. At least, he found that 
its germinating tubes had a limited growth under the conditions 
