62 
Journal of Agricultural Research 
Vol. XXIV, No. I 
grown at the different temperatures, it is impossible to offer any definite 
explanation for these differences. However, by comparing figure i with 
figures 3 and 4, it will be seen that the higher percentages of yellows 
occurred at and slightly above the temperature at which the drop in dry 
weight of the plant occurred, which was also the temperature at which 
the fungus grew most rapidly in culture. It is probable that both 
reduced vigor of the plant and optimum temperature for growth of the 
fungus played a part in producing a higher percentage of yellows at these 
temperatures. 
There is a marked difference in the external characters of the root 
systems of the plants grown at different temperatures. Roots grown at 
14°, 17°, and 20*^ C. are coarse, light in color, have a thick cortex, and 
spread uniformly throughout the soil mass. Beginning at the roots 
Fig. 4.—Comparison of dry weights of shoots and roots of healthy Commercial Hollander seedlings given in 
Table IV, grown 53 days from seed during November and December. 
are finer, darker in color, and shorter, until at 35° the number of roots 
and the extent of the system are greatly reduced and the color becomes 
dark brown. A comparison of the shoots and roots of plants grown at 
20^^, 26°, and 32° is shown in Plate i. All of these variations suggest that 
fundamental differences occur in the anatomical structures of the tissues 
involved and possibly in their chemical composition, but anal)rtical 
studies on these differences have not as yet been undertaken. It seems 
evident, however, that such variations in external appearance of parts 
of the host at different temperatures may be regarded as indices of the 
more important internal effects of temperature. They are probably 
accompanied by a difference in the rate of manufacture of carbohydrates 
and proteins and in the disposition of these products. In turn, these 
differences in assimilation and physiological balance could affect the 
cell walls and protoplasm in a manner which would make the plant more 
susceptible to the attacks of the fungus. 
