1917] 
FREIBERG—MOSAIC DISEASES 217 
toxins, or enzymes from one species may not affect closely 
related species. The constitutional or physiological differ- 
ences between two species make these organisms two differ- 
ent species, and the ability of certain plants to resist the in- 
fluence of certain mosaic extracts can be accounted for on 
physiological grounds. It is the physiological difference be- 
tween hosts which makes some of them immune to a disease 
while others are susceptible, and it is likewise a physiological 
difference among parasites which makes ‘‘physiological 
races’’ physiological races. The term is used primarily in 
connection with the parasitism of rusts, mildews, etc., which 
may be detected with the naked eye and are adequately de- 
scribed. The term should not be used in connection with the 
infective principle of mosaic diseases which has never been 
seen, never been described, and the ‘‘properties’’ of which 
are only partially known. If the term is used at all it should 
be used only as a matter of convenience, and this is dis- 
couraged since it tends to lead to confusion. 
From the above it is obvious that when injecting the in- 
fective substance obtained from a diseased plant into a 
healthy plant, we are handling an enzyme and not an organ- 
ism. Although nothing is known as regards the nature of 
this enzyme, it is probably, judging from its reaction with 
formaldehyde, of the nature of an aldehydase. If formalde- 
hyde is one of the first products of photosynthesis, one can 
easily conceive of a physiological origin of mosaic diseases. 
A probable relation with photosynthesis is furthermore 
brought out by the observation on the carbohydrate content 
of the lighter and darker areas of diseased leaves, as was 
pointed out in the microchemical work reported above. The 
problem has, in the light of these facts, assumed a somewhat 
different aspect. Although nothing is known as to the nature 
of the enzyme, the main issue of the problem is this: How 
does this enzyme originate and what are the factors which 
induce its formation? As was pointed out by the writer 
earlier in this paper, and as has also been shown by Allard, 
this infective principle is not found, at least in active form, 
in healthy plants grown under normal conditions. If it is 
