318 OHIO EXPERIMENT STATION; BULLETIN 214 
of parasitic fungi after germinating upon the leaf will likely gain 
entrance into the interior leaf tissues through these openings much 
more readily than by actual boring through the leaf epidermis. 
The illustration, Fig. 4, shows how these openings are distributed 
in the epidermis of a cucumber leaf. These stomates are present in 
_ the leaf covering upon the outside of all green leaves and in the 
epidermis of young growing shoots. In addition to these stomates 
certain classes of plants such as the plants of 
the mustard family (Cruciferae), as cabbage, 
cauliflower, turnip, also the grape, fuchsia, impat- 
iens, etc., are provided with water pores—mar- 
ginal openings through which the excess water 
of the plants isexcreted. ‘These water solutions 
of various materials offer a means of growth for 
organisms, especially of the minuter forms. 
From the culture drops thus formed the para- 
site enters the leaf by the water pores. One of 
the most destructive known diseases of plants is 
the black-rot of cabbage, cauliflower, turnip, 
ruta-baga, etc. This is due to a bacterium which 
gains entrance very largely through the water 
pores just described. So we must bear in mind 
that the very avenues of transpiration or excre- 
tion, so essential toplant growth, are madeameans 
of exposing the plant to the danger of parasitic in- 
vasion. ‘This is analogous to the exposure of 
human subject to diseases of the respiratory 
organs. Atevery turn we find convincing evi- 
Fig. 5. Margin of cab- 
bage leaf showing excreted 
water from water pores 
after cool night. These 
drops contain enough food 
for the growth of the black 
rot bacteria. The motile 
forms may swim through 
the water pores into leaf 
from such drops. Dead 
marginal areas on lower 
fragment show results of 
this bacterial infection. 
(After Smith). 
be maintained, 
dences of the mutual adaptation of parasitic fungi 
to their host plants, in nothing more strongly 
marked than in the limitation of the species of 
plants attacked by a given parasite as discussed 
in the beginning of this paragraph. In view of the 
fact that so long as the leaves of a plant continue 
to function as leaves, these natural openings will 
it will be seen that the risk of exterior infection 
from parasitic fungiis continuous for any given plant; it lasts for its 
whole growing period. 
THE PLANT'S PROTECTION AGAINST PARASITES 
In the case of woody growths we have the development of corky 
epidermis or bark which seems primarily designed to protect the 
interior, living layer from invasions of this sort. 
In a similar 
