DISEASES OF CULTIVATED PLANTS 319 
manner the external layer or bark of all growing plants, inciading 
herbs, is provided with a protective covering or epidermis. The 
skin of the apple or of the grapeand the covering of the potato stem 
are allfamiliar and serve this function of protection to the inner 
tissues. In young plants there is retained the power of protective 
growth in response or resistance to parasitic attack; thus it happens 
that the potato scab organism induces the growth of cork cells on 
the outside of the potato and makes a roughness. The roughness is 
is not the scab fungus but the corky growth of the tubers in 
response tothe scab attack. Inasimilar manner the attack of the 
scab fungus upon the apple results in the roughening of the apple 
skin through the development of more protective or wound cork. 
The most remarkable example of this multiplication of protecting or 
outer cells in response to the attacks of parasitic fungi is found in 
“eaf-curl” of the peach and in the pockets or “bladders” of the plum, 
where we have such a rapid multiplication of cells in response to the 
stimulus of the fungus as to bring about an entire transformation 
in the form and structure of the parts. While we may think of this 
abnormal development as the result ‘of fungous growth, it is only 
indirectly so. It isin fact a response of the host to the stimulus of 
the invading fungus. The nature of the stimulus or excitation 
exerted by particular parasitic fungi isa highly interesting subject 
for investigation. 
DISEASE RESISTANCE IN PLANTS 
Disease resistance and disease susceptibility are as yet imper- 
fectly understood. ‘The cause of the inherent differences in the 
tendency of this or that variety to suffer, as with the leaf-curl 
in the Elberta variety of peach, the apple scab predisposition 
in White Pippin, Winesap and others, may become in 
practice, varietal weaknesses. Yetsuch is the commercial super- 
iority of some such varieties that they increase in public favor 
despite these weaknesses. The great differences among varieties of 
fruit in susceptibility tothe diseases which prevail under certain 
conditions, is a matter of observation and experience, From the 
difficulties involved in breeding a less susceptible or more resistant 
type of tree fruit belonging to any commercial variety, increased 
resistance is not yet within reach. This applies to established varie- 
ties and yet leaves the field open for new sports to be discovered or 
for its occupation by less desirable sorts which do not suffer so 
severely from disease. This actually happens in the growing of pears 
outside of certain favored districts; owing to the ravages of fire 
blight, a bacterial disease, the ordinary grower selects less popular 
but more resistant varieties for culture. 
