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, c. 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 ooenin?. thev can in tVip 

 vast majority c ' 



because they are mzvyc-n'r. -'■-. -^ « -y;-.^^--^. .;-- . { _ 

 adapted to the 



And even if they nappen to be parasitically in- 

 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 



*FiG. 3.— Young shoots of mulberry inoculated with Bad. 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. 



28 



Fig. 3.^ 



