BACTERIA ON THE SURFACE OF PLANTS. 
OBSTACLES TO THEIR ENTRANCE INTO PLANTS—OBSTACLES TO THEIR MULTIPLICATION IN PLANTS. 
The young vegetative parts of plants are covered by the epidermis, a skin of close-set 
cells interrupted here and there by stomata, but not easily permeable to water. This 
epidermis when unbroken offers great resistance to the entrance of harmful micro-organisms. 
Its surface is often reinforced by cutin a still more 
resistant layer which is sometimes developed to 
a very marked degree. Some plants also turn 
aside water and whatever that may contain, by 
a waxy bloom, e. g., the cabbage. A dense layer 
of soft hairs may have the same function, as on 
the surface of a peach fruit or the stem of a com- 
posite. These devices render it difficult to wet 
the actual epidermis lying under the cutin, wax, 
or lanugo. 
In older parts the epidermis is displaced by 
cork a many-layered, close-celled, very imper- 
vious, very indestructible covering which keeps 
out fluids and also keeps them in so perfectly 
that the special kind found on the cork-oak is 
used by civilized man everywhere for this very 
purpose. Its use to the plant is obvious. Only 
through wounds or through certain natural open- 
ings, known as lenticels, can bacteria pass this 
very perfect barrier (see fig. 3). 
The plant then is naturally very well pro- 
tected against bacteria, except as I will point out 
in a following chapter. 
The surface of plants, as we shall see a little 
later, is often covered by a variety of bacteria 
and some of these are likely to find their way 
into the tissues whenever they are wounded, but 
if they do gain an entrance either through wounds 
or through some natural opening, they can in the 
vast majority of cases take no advantage of it 
because they are saprophytes, 7. e., they are not 
adapted to the conditions present in the plant. 
And even if they happen to be parasitically in- 
Fie.3* clined they are often debarred from further pro- 
gress by the fact that the wounded plant does 
not contain enough water for their needs. In such cases they make either no growth or 
such a very slow growth that the plant has time to erect a physical barrier to further 
progress in the form of a cork-layer cutting out the affected tissues from the body of the 
plant. This happens very frequently in potato-tubers attacked by various soft-rots. It 
*Fic. 3.—Young shoots of mulberry inoculated with Bact. mori, showing cirri of bacterial slime oozing to surface 
through lenticels. Inoculated by needle-pricks Jan. 4, 1909. Photographed (enlarged) Jan. 9. The dark stripe is a 
sunken diseased area. 
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