310 
Journal of Agricultural Research voi. xxvm, No. 4 
curly-top was not produced in a given period. The periods represented between 
any two of the horizontal lines show the number of insects used on any one day or 
period. Where the columns are very short the fact is to be inferred that the 
insect concerned soon escaped or died. A blank in a column indicates that no 
note was obtained for that period, owing to the fact that the tag label could not 
be found. 
RELATION OF LIGHT TO INFECTION 
A limited amount of experimental work has been done in the attempt to de¬ 
termine whether or not light is necessary for infection to occur. Young beet 
plants were kept in darkness for a period and then inoculated in comparison 
with a series of plants exposed to daylight. In each experiment some of the 
plants were inoculated on the cotyledons and others on the true leaves. The 
details of the procedure and the results are listed in Table VIII. 
Table VIII .—Relation of light to infection of beet plants tested in 1920 
Experiment and 
date 
No. 1, Mar. 24 
No. 2, Apr. 1__ 
No. 3, Apr. 24- 
No. 4, Nov. 25 
No. 5, Dec. 4. 
Length of time 
Point of inoculation 
Number 
of plants 
inoc¬ 
ulated 
Diseased 
In the 
dark be¬ 
fore inoc¬ 
ulation 
Insects 
on 
plants 
In the 
dark 
Hours 
Hours 
Hours 
! 
I 48 
72 
120 
Cotyledons... 
5 
a 2 
1 48 
72 
120 
First true leaves. 
b 1 
/ 26 
22 
48 
Cotyledons... 
3 
3 
\ 26 
22 
48 
First true leaves.. 
3 
1 
f 34 
21 
55 
Cotyledons... 
8 
3 
1 34 
21 
55 
First true leaves... 
7 
1 
1 ( c ) 
21 
Cotyledons... 
8 
2 
l ( c ) 
21 
First true leaves_ 
7 
4 
f 19 
26 
65 
Cotyledons... 
6 
6 
1 19 
26 
65 
First true leaves.... 
6 
3 
1 ( c ) 
20 
Cotyledons___ 
4 
4 
l ( c ) 
20 
First true leaves.. 
4 
4 
f 22 
20 
42 
Cotyledons... 
4 
2 
1 22 
20 
42 
First true leaves... 
4 
0 
1 ( c ) 
20 
Cotyledons .. 
4 
1 
l ( c ) 
20 
First true leaves... 
4 
2 
o Only four lived. 
& Only two lived. c Not placed in the dark. 
In each test plants of the same age were used throughout, and they were 
grown under conditions as nearly similar as possible except in regard to light. 
If any significance may be attached to the results of these few tests, they indicate 
that, with young plants which have been kept in darkness for about 24 hours 
prior to inoculation, infection can be more readily effected on the cotyledons 
than on the leaves. Plants exposed to usual daylight, on the other hand, seem 
about as easily infected on the true leaves as on the cotyledons. If these facts 
be true, it would seem logical to presume that the presence of food substance as 
stored products in the cotyledons or from the process of photosynthesis in all 
green parts facilitates infection. It seems worthy of note that in test No. 1, 
where the plants were kept in darkness for five days, only two of the five leaf- 
hoppers caged on the true leaves survived, while all five of those on the cotyledons 
lived. Probably those on the true leaves were starved. 
