1917] 
FREIBERG—MOSAIC DISEASES 21% 
now being used іп connection with physiological experiments, 
and want of material has precluded carrying on inoculation 
experiments, though this will be done as soon as possible. 
Plate 17 shows the healthy and diseased avocado plants. 
Discussion oF Recent INVESTIGATIONS 
Whether ог not mosaic diseases are initially ‘‘physio- 
logical’? or are caused by an organism, it is nevertheless 
apparent from the preliminary results on physiological re- 
lations reported above that the physiological side of the prob- 
lem is an extremely important one. After the physiological 
relations are more clearly understood, we will undoubtedly 
be able to account more fully for some of the inconsistencies 
of results, such as failure to get infection after inoculation 
or the spontaneous occurrence of diseased plants among 
checks. This is true regardless of the origin of the disease. 
Since so little is known concerning ‘‘physiological’’ diseases, 
it is impossible to cite conclusive data as regards environ- 
mental influences exerted on their causes. 
In the case of diseases which have been proven to be due 
to well-described organisms, we are able to correlate the effects 
of environmental factors with something tangible and the 
results are always obvious. In any event, such problems must 
be considered from two different aspects, i. e., from the stand- 
point of the host and from the standpoint of the parasite. 
Work on temperature and host relations, covering a period 
of several years, has recently been reported by Gilman (716). 
Our present knowledge on the subject has been admirably 
summarized by him. The importance of physiological path- 
ology has also been brought out by the work of Selby (’99), 
Orton (213), Halsted (298), Balls (708), Reed (710), Earle 
(702), and others. If mosaic diseases are caused by an 
organism, the effect of those climatic conditions favorable for 
their manifestation may be accounted for, first, by the favor- 
able effect exerted upon the organism, and second, by the 
favorable effect on the host which, however, makes it better 
prey for the parasite. If, on the other hand, mosaic diseases 
