THE DISEASE. 



( iicoo RAPHICAL DISTRIBUTION. 



Geographical distribution is an exceedingly interesting problem to many 

 naturalists. The writer shares this feeling and has made ever}' effort to determine 

 it, as far as possible, for each disease. There are, however, still many gaps in our 

 knowledge the whole subject is so new, and information from all parts of the 

 world is desired. The inner temperature of plants conforms nearly or quite to that 



of the surrounding medium, and 

 we might therefore expect, in some 

 cases at least, to find a rather more 

 sharply restricted distribution than 

 in diseases of the warm-blooded 

 animals. From theoretical con- 

 siderations we should expect the 

 distribution of plant diseases to be 

 more like that of diseases of fish 

 and other cold-blooded animals. 

 Whenever the bacterium is able to 

 endure as wide a range of temper- 

 ature as the host-plant, we should 

 expect to find it as widely distrib- 

 uted. 



SIGNS OF THE DISEASE. 



Great care should be exercised 

 in the description of the physical 

 signs and of the lesions due to the 

 parasite, so that the disease may 

 be identified from these alone, if 

 necessary. A great many cases 

 should be examined and the signs 

 must be recorded in detail and with 

 great accuracy. It should be 

 remembered that here is a frequent 

 opportunity for error to creep in, 

 lg ' ' since the plant may be affected by 



two distinct diseases which have been confused. Good figures are always desirable, 

 but are not absolutely essential. If possible, however, photographs, pen or pencil 

 drawings, and good water-color sketches should be secured. 



*Fic. 4. Cross-section of a turnip root, showing vessels occupied by Bacterium cainpestre as the 

 result of a pure-culture inoculation hy means of needle-pricks on the leaves. Material fixed in strong 

 alcohol, infiltrated with paraffin, cut on the 'microtome, stained with safranin-picro-nigrosin, and 

 the differential washing stopped at just the right stage. The bacteria are confined to the vessels 

 and their immediate vicinity. They do not occur in the phloem, a small portion of which is shown 

 at the top of the picture. Section made from the same root as fig. 6, but lower, dn the tapering 

 part. Drawn from a photomicrograph. X 85. 



