————— 
=e 
THE DISEASE. 7 
GEOGRAPHICAL DISTRIBUTION. 
Geographical distribution is an exceedingly interesting problem to many 
naturalists. ‘The writer shares this feeling and has made every 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 
Nee atabe tetera ‘2 2 S< SS . ‘ 
oe ISES SZ 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 
4 distribution of plant diseases to be 
e 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 
Bs remembered that here is a frequent 
DEST ANY opportunity for error to creep in, 
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. 
Fig. 4.* 
*Fic. 4—Cross-section of a turnip root, showing vessels occupied by Bacterium campestre as the 
result of a pure-culture inoculation by 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, in the tapering 
part. Drawn from a photomicrograph. X 85. 
