380 BOTANICAL GAZETTE [JUNE 
unless it looked healthy, reached a fair length, and had a number of 
branches. Often the largest lateral root of the normal plant was not 
so large as the largest in the tension plant. This was sometimes 
taken as evidence of greater growth due to tension, but the results 
were not based on this fact alone. From the tension plant was 
selected a rootlet which was as large and vigorous as the largest 
rootlet in the normal plant. A comparison could then be made and 
the results, whatever they were, could be accepted without further 
allowance or additional estimation. 
Helianthus annuus 
Sections were taken of the main root at various points below the 
first lateral root. These were compared with sections taken at 
corresponding places in the control plant. The diameters of the 
sections of the normal root averaged 1™™ greater, and for this reason 
a better mechanical growth would be expected in the control. As the 
result of careful observations the xylem was found to be greater in 
amount and the cells were thicker-walled in the tension plant than 
in the normal. In cases where the diameter of the sections of the 
normal and tension roots were the same, the tension root always 
showed the greater amount of xylem and thicker-walled cells. 
From a comparison of the rootlets at the crown of the main roots 
of the tension plants with those from the corresponding place in the 
control plants, it was seen that the former had a larger size, and micro- 
scopical study emphasized this fact and showed that the xylem 
cylinder was larger and the walls of the cells thicker in the tension 
: plants. Rootlets of average size taken from tension plants were 
compared with others of the same size, vigor, and growth taken from 
the normal plants. The comparison showed that the tension plants 
had a larger amount of xylem and cells with thicker walls. 
To recapitulate, we may say that, out of five Helianthus plants 
under tension, the main roots of all show a better development of 
mechanical tissue than the main roots of five control plants of similar 
development. With reference to the lateral roots, the majority in the 
experimental plants showed a better mechanical development than 
similar rootlets in the control plants. 
