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1907] HIBBARD—FORMATION OF MECHANICAI. TISSUE 379 
August 11. The final weights held by the plants under tension varied 
from 1800 to 20008, 
The results of a comparison of two plants of Ricinus communis, 
selected with reference to similarity in all respects except in the 
matter of tension, are added below. The results here obtained are 
in general the same in the other sixty-seven plants used in the 
experiment. 
The main root of the tension plant was straight throughout its 
whole course, that of the control undulate. The main roots had the 
same diameter at the upper ends. At 5™™ below the first lateral 
root the diameter of the tension plant was 4™™, while that of the 
control was 3™™. The control plant had a cluster of 18 lateral roots 
at the upper end with a diameter of 1™™ or Jess; while the tension 
plant had at its upper end a cluster of 23 lateral roots with a diameter 
of 1™™ or more. The lateral roots in the tension plant were much 
longer than those of the control plant. The root system of the tension 
plant was noticeably stronger in general appearance. 
The results of these experiments on seedlings of Ricinus communis 
show how different the root systems of the plants under tension are 
from those growing under normal conditions. In every respect the 
differences are quite marked. Similar experiments conducted in the 
greenhouse with both Helianthus and Ricinus plants show like results. 
From the above data one may conclude that tension causes a 
thickening and strengthening of the roots of Helianthus annuus and 
Ricinus communis grown in the open or in the greenhouse. 
2. INTERNAL ROOT STRUCTURE 
For a study of the internal root structure, freehand sections of 
the main and lateral roots were made. Sections of the main root 
were generally taken in two places, one near the top and the other 
some distance below. Sections of the lateral roots were made 1 to 
2™™ from their insertion on the main root. The lateral roots to be 
sectioned were taken from the upper cluster of rootlets or from a 
position on the main root not lower than 1°™ from the first large 
lateral root. More than one lateral root on each plant was sectioned 
so as to obtain average results. The strongest and the average root- 
lets were always selected for comparison. No rootlet was chosen 
