1900] THE HAUSTORIA OF THE ERYSIPHEA 169 
dissolution of the epidermal wall of the host about the point of 
penetration has been mentioned. The penetrating tube makes 
its way through a long ingrowth, of cellulose before it expands 
into a mature haustorium. The mature organ shows no unmodi- 
fied host-cellulose surrounding it except the collar around the 
neck of the haustorium. The distal portion of the ingrowth 
partially disappears in the development of the penetrating tube. 
During the disappearance, it undergoes changes which materially 
alter its microscopic appearance as concerns structure and its 
capacity of reacting to stains. It becomes decidedly granular 
and takes an orange stain, whereas, in the beginning, the entire 
ingrowth was of the same consistency as the collar, and took 
the safranin stain slightly. By the time the penetrating tubes 
begin to enlarge, these changes are conspicuous. The tube may 
finally break through the cellulose and develop without any 
sheath. But usually some of the cellulose remains as the granu- 
lar masses of the sheath. It will be shown subsequently that, 
under certain conditions, the haustoria of Uncinula Salicis have 
no sheaths, and the sheaths of Erysiphe graminis on Poa do not 
show any contents in the older stages nor are the sheaths always 
present. 
The extremely minute size of the penetrating tube has been 
mentioned. The amount of fermentative action, of which the 
young haustorium is capable at first, is, therefore, only sufficient 
to provide for the onward growth of the tube. The cell suc- 
ceeds in laying down cellulose ahead of it for a time. The 
circle of the fermentative effect has not at first a long radius. 
The distal end of the ingrowth is the first to show signs of dis- 
solution. It becomes granular there. As the absorbing organ 
attains larger growth the digestive powers of the fungus become 
More effective, and that part of the collar coming within the 
Sphere of influence of the ferment is gradually attacked and 
partly dissolved. 
It is interesting to find that the membrane by which the 
Sheath is bounded in a majority of cases is contributed from the 
plasmic membrane of the host-cell. The plasmic membrane is 
