THE DISEASE. 



8 . >f ij^i-f^ 



(rl'XJCiRAPHICAL DlSTRIJiUTION. 



( Tcographical distrilnitioii is an exceedingly interesting problem to many 

 naturalists. The writer shares tliis feeling and has made ever}- effort to determine 

 it, as far as possible, for each disease. There arc, ho\ve\'er, still many gaps in onr 

 knowledge — the whole snlyect is so new, and information from all parts of tlie 

 world is desired. The inner temperature of plants confirms uearh' or quite to that 



of the snrr(>unding niedunn, and 

 \\c might tlierefore expect, in S(.)nie 

 cases at least, t(j find a ratlier more 

 slrirpU' restricted distribution than 

 in diseases of tlie warm-blooded 

 animals. From theoretical con- 

 siderations we should expect the 

 distril)ntion of plant diseases to be 

 more like tliat of diseases of fish 

 and otlier cold-blooded animals. 

 Whenever the bacterium is able to 

 endure as wide a range of temjier- 

 ature as the host-plant, we shoidd 

 ex])ect to find it as wideh- distri!)- 

 uted. 



vSlGNS OK THE DISEASE. 



(xreat care should be exercised 

 in the description of the physical 

 signs and of the lesions due to the 

 parasite, so that the disease ma^- 

 be identified from these alone, if 

 necessar}-. A great mau^' cases 

 should \)(t examined and the signs 

 uuist be recorded in detail and with 

 great accuracy. It should be 

 remembered that here is a frequent 

 opportimit)' for error to creep in, 

 since the plant may be affected by 

 Good figures are alwa)"s desiralile, 





^ 



-^ 



F.g 4' 



two distinct diseases wliicli have been confused. 



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



drawino-s, and good water-color sketches should be secured. 



*pjf; I Cross-section of " turnip root, showing vessels occupied by Bacterium cainpcslrc as the 



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

 alcohol, infiltrated with paraffin, ctvt 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, in the tapering 

 part. Drawn from a photomicrograph. X 8$. 



