40 BULLETIN 879, U. S. DEPAETMEXT OE AGEICTJLTTJEE. 
All the above conclusions are merely tentative, however, particu- 
larly in regard to the r61e of the phloem in the distribution of the 
virus. The vascular elements, however, seem to offer the most 
probable channel for the rapid distribution of infectious material 
through the plant. 
INCUBATION PERIOD. 
The incubation period of cucurbit mosaic varies somewhat with 
the age of the plant inoculated. In young cucumber plants which 
are growing rapidly the first visible signs of the disease may appear 
within 4 to 5 days following inoculation, and the incubation period 
in such plants will rarely exceed 8 or 9 days. In the case of older 
plants which are growing less rapidly, the period of incubation is 
often longer and the first symptoms may not develop for 12 or 14 
days, although these plants generally do not require over 10 days 
for the appearance of the symptoms. 
The incubation period for plants of the same age and vigor of 
growth is quite constant, and where several plants are inoculated at 
the same time those which are infected show the signs of the disease 
within 3 to 4 days after the earliest symptoms appear. Plants 
which do not develop mosaic symptoms within this period usually 
remain healthy. 
The incubation period of the other cucurbits is very nearly the 
same as that of the cucumber and varies within approximately the 
same limits. In the case of large squash and gourd plants the incu- 
bation period is slightly longer, ordinarily, than that of most other 
cucurbits, and usually extends over 12 or 15 days. On the younger 
plants of all species, the first symptoms will normally develop within 
7 to 9 days after inoculation. The general incubation period may be 
said to lie between 4 and 15 days, with the average period varying 
from 7 to 12 days for all species. s 
MOSAIC TRANSMISSION. 
Many mosaic diseases are of such a nature that field operations, 
in which healthy and diseased plants are handled in succession, 
tend to further disseminate the disease. This has been demonstrated 
by Hunger (15) for tobacco mosaic and is particularly true with the 
mosaic on cucumber, the method of growing and harvesting the crop 
being such as to favor the spread of infection throughout the season. 
As infection may occur whenever the juices of a mosaic plant come in 
contact with slight wounds in healthy vines, any thinning, training, 
or other handling of mosaic and healthy plants may produce infec- 
tion. That mere handling of the plants may result in disease trans- 
mission has been proved by bruising the leaf of a mosaic vine between 
the fingers and then handling the leaves of healthy plants in a simi- 
