[Vor. 1 
300 ANNALS OF THE MISSOURI BOTANICAL GARDEN 
fungus, but so far as the writer's observation indicates, infection 
in these cases takes place through the twig, or, in some cases, 
through another plum with which it is in contact and which in 
turn is infected through the twig. Nevertheless, field observa- 
tions also verify the laboratory work in that plums (especially 
certain varieties, such as Wood) when approaching maturity 
may be infected in the field without being in contact with other 
fruits and without having any visible punctures or wounds іп 
the skin. All these experiments and observations point to the 
conclusion that penetration of the cuticle is a very important 
factor in the infection of fruits, especially immature fruits; that 
infection of very green fruits without punctures is rare; and, on 
the other hand, that maturing fruits without punetures may be 
readily infected both by spores and by a mycelial felt in the 
field and in the laboratory. 
PENETRATION 
The nature of penetration and the course of the hyphe of 
parasitie fungi in piercing host tissue is an interesting and im- 
portant question in connection with a study of the nature of 
parasitic action. In the case of the brown-rot fungus growing 
on the plum it is of importance to know whether or not the 
hyphe merely follow the middle lamellæ or whether they enter 
the cells wherever they come in contact with them. Previous 
investigators differ very widely in their opinions as to the nature 
and course of the penetration of the fungus in question, a condi- 
tion which is perhaps partly explained by the fact that different 
hosts were employed in the various investigations. Further- 
more, it appears that the methods employed in some of the 
researches were not of such a character as to readily yield com- 
plete information concerning all the facts in the case. 
In my own work a study of the penetration of the host tissue 
by the fungus was made by examining a number of sections of 
infected tissue in which the disease had reached various stages 
of development, and comparing them with sections of healthy 
tissue from the same fruit. For this purpose a special method 
was employed. 
Methods and Results.—Small pieces of fruit composed of 
diseased and sound tissue were cut from plums inoculated with 
