1917] 
FREIBERG—MOSAIC DISEASES : 207 
seed was planted directly in Ше boxes. No consistent results 
warranting a definite decision with respect to the transmis- 
sion of the disease through the seed was obtained. Neither 
does it seem possible by the method employed that injury 
can be a very great factor, and it should therefore be re- 
garded as incidental or disregarded entirely. 
It has been the general experience of all workers that the 
seeds of diseased plants usually give rise to healthy plants, 
and this in itself is evidence strongly in favor of the non- 
transmissibility of the disease through seed. If we consider 
this question from the standpoint of an organism or parasite, 
one can only arrive at a satisfactory explanation by assuming 
that the virus present in the placenta (Allard, 715) cannot 
penetrate the integuments of the ovule and is thus filtered 
out. A much better explanation is afforded by the physio- 
logical aspect of the problem. In this case we should not ex- 
pect to find the infective principle present in the seed. Mosaic 
is most pronounced in young shoots where growth, photosyn- 
thetic and metabolic activity is at its height. It is in these 
tissues that plant products are first formed. In the seed the 
physiological functions are entirely different. The relatively 
simple compounds elaborated in the green portion of the 
plant are polymerized there into storage products and no 
initial synthesis whatever occurs. The infective principle, if 
elaborated in young active shoots, may be transmitted to all 
parts of the plant through the general food stream and might 
therefore be present in the placenta. It might even enter the 
ovule, but on account of the specific functions of the ovule, 
embryo, and endosperm, the infective principle or correspond- 
ing enzyme might be altered and its continued formation ob- 
viated. We must bear in mind that in a problem of this kind 
we are dealing, from the standpoint of the host, with an ex- 
tremely complex organism and must not confuse an intricate, 
nevertheless complete, chemical reaction or function exhibited 
by it with such a phenomenon as, for example, multiplication 
of bacteria or ultramicroscopic parasites. This will be dis- 
cussed again later on. 
