DISEASES OF CULTIVATED PLANTS 317 
grape downy mildew and the common white molds of the mustard 
family. ‘The powdery mildews by reason of the appearance upon 
the surface of parts attacked, are descriptively named ‘‘mildews.” 
A definite system has been followed in most cases of naming plant 
diseases and I trust the results will not be altogether disappointing. 
The differences between the species of parasitic or other fungi 
are as strongly marked as those of higher plants, even though 
microscopic examination is necessary to distinguish these char- 
acters; it shows, therefore, that a discriminative system of naming 
diseases has a secure foundation. 
THE PLANT OR HOST IN RELATION TO DISEASE 
As stated elsewhere only closely related plants are usually 
subject to attack by a para- 
sitic organism, thus it hap- 
pens that the tomato as 
well as the potato plants 
are attacked by the downy 
mildew or late blight fungus 
of the potato. In general 
the true parasites among 
our fungi are limited toa 
rather narrow range of host 
plants; thus we may expect 
the potato Phytophthora to 
attack several plants of the 
potato family (Sodanaceae). 
The writer proved thissame 
was true of the attacks of 
downy mildew (Plasmopara) 
upon a number of species 
belonging to the cucumber 
family (Curcurbitaceae). 
Since our cereal grains 
belong to the same great Fig. 4. Avportion of the epidermis from the upper 
1 he grasses (Gram- surface of a cucumber leaf, showing the breathing pores 
family ast g ( “i (stomates) surrounded by guard cells containing chloro- 
-gneae), we expect, and find phyll grains, much magnified. These guard cells, which 
that there is a development control the opening and closing of the stomates, are the 
Z only epidermal cells that contain this green substance, 
of the same diseaSeS Upon _ the others being colorless. 
many of them and upon the ; 
grasses growing nearby. In this connection it must be remembered 
that clover and alfalfa are not grasses, but legumes. 
‘The leaves of the host plant provided as they are with stomates 
or breathing pores, minute openings through the epidermal cover- 
ing of the leaf, will be attacked through these openings. The spores 
