THE DISEASE. 5 



compacted, aborted, and enlarged into a fleshy edible mass, might well be regarded 

 as a diseased condition, but it is not so regarded for the purposes of this book. 

 On the contrary, a soft rot of the cauliflower head is regarded as a disease. Bacterial 

 diseases of plants usually involve both functional and structural changes. 



Inasmuch as the word " symptoms " has a subjective as well as an objective 

 connotation in medical terminology, the writer has preferred to substitute the word 

 "signs " for those objective characters which serve to distinguish one plant disease 

 from another. 



Fig. 2* 



The student will, naturally, first turn his attention to a careful study of the 

 disease. Under this head should be considered : (i) Previous literature ; (2) 

 Geographical distribution ; (3) Signs of the disease ; (4) Pathological histology ; 

 (5) Direct-infection experiments. 



* FIG. 2. Cross-section of a raw carrot, showing wedging apart of parenchyma cells by Bacillus 

 carotovorus Jones; from paraffin-infiltrated material. The carrot was fixed in strong alcohol 72 

 hours after placing on its cut surface one loop of a fluid culture. The inoculation was made in Che 

 .middle of a cross-section of die whole root, I cm. thick, placed in a sterile Petri dish. The surface 

 of the root was sterilized in mercuric chloride water. This section was made several millimeters 

 below "the inoculated surface. A small portion of it at X is 'shown more highly magnified in fig. 3. 

 This section was stained with carbol-fuchsin and bleached in 50 per cent alcohol. Drawn under Zeiss 

 16 mm. apochromatic objective with No. 4 compensating ocular and the Abbe camera. 



