442 
AMERICAN JOURNAL OF BOTANY 
[Vol. 9, 
not retard the growth of roots directly, but that, on the other hand, the 
root increases in growth in the presence of nitrates if carbohydrates are 
also present. This fact shows that carbohydrates may be a limiting factor. 
Further evidence along this line is shown in some unpublished data 
of Mr. W. C. Muenscher of the department of botany of Cornell 
University. Using averages of large numbers of water cultures (12 in one 
case, 25 in another), he found that the ratio of dry weight of tops to roots 
(barley) was always greater for plants grown in the shade than for those in 
the unobstructed sunlight. A brief summary of his data (in dry weights) 
is as follows: 
Sunshine 
Shade 
Time of Year 
(I) 
Ratio, 
Tops 
5.10 
7.84 
Jan. 19 to Feb. 24 
Roots 
(2) 
Ratio, 
Tops 
4.26 
5-96 
Aug. 4 to Sept. 8. 
Roots 
This table shows also that the ratio of tops to roots was higher for plants 
grown in winter months than for those grown in the summer-fall season. 
Working as he was with a full nutrient solution, the production of carbo- 
hydrates seems to be the limiting factor, which accounts for the difference 
under the two conditions. Since the amount of carbohydrates produced 
in the shade or during the season of relatively less sunshine would be rela- 
tively less than that produced in greater light, the tops in the shade or in 
the reduced light of winter would use relatively more of that amount, 
thus leaving less for the roots. 
Results from the pruning experiments of Chandler (6) may also have 
bearing on this point. He has shown that, while pruning is a dwarfing 
process, it checks root growth more than it does top growth. It is possible 
that, in removing a large leaf surface which is not compensated for by new 
growth, the carbohydrate production is cut down, and, as the increased 
vigor of the remaining shoots following pruning tends to use the carbo- 
hydrates manufactured, there is a smaller amount to be carried to the roots. 
In the case of nitrates, it is likelv that, when the concentration of 
nitrates is increased, relatively more of the manufactured carbohydrates 
tend to be consumed in the growth processes of the tops. Little is left for 
distribution to the roots. The roots, therefore, will not make the same 
relative growth as tops, and their growth may be severely checked in this 
way. This will explain also the sparse development of aerial roots in the 
high-nitrate cultures. 
Plants like flax which show no increase in ratio of tops to roots as the 
nitrate content in the solution is increased may not be able to absorb 
nitrates beyond a certain amount or may not be able to utilize them beyond 
