[Vor. 8 
50 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
was infected with the mycelium of P1 obtained from a pure 
culture. All the leaves used in the experiment were previously 
washed in running water for an hour, and finally rinsed with 
sterilized distilled water in a culture room. All the dishes were 
then placed in an incubator. 
After 2 days some discoloration appeared in the leaves in- 
oculated with mycelium, but practically no change was noticed 
in the leaves treated with mycelium extract. Microscopical 
investigation also clearly demonstrated that the tissues of the 
leaves infected with the mycelium was more or less invaded by 
the fungus and disorganized to a certain extent, while in the 
other practically no change had taken place. So far as present 
information goes, this fact may be cited in support of the me- 
chanical theory proposed by Blackman and Brown. 
Fig. 5. P 1a, hypha not entering through stomata; b, swelling of hypha in con- 
tact with epidermal cells of pea stem; c, penetration of cell wall of pea root by 
invading hypha (H), young stage of sclerotia formation (S). (Camera lucida 
ings.) 
If the fungus in question attacks the plant in a purely mechan- 
ical way, as discussed above, it would be logical to believe that 
the infection of the plant by the fungus might take place much 
easier at the roots than anywhere else, since the epidermis of 
the roots, unlike that of leaves and stems, has no cutinized walls, 
especially in the young stage. My attention was, therefore, 
turned to this point. 
For the purpose of this study the following was devised: 
Young pea plants (more than 4 inches high) grown in pure cul- 
tures in test-tubes were pulled out by means of forceps in such a 
