1907] HIBBARD—FORMATION OF MECHANICAL TISSUE 373 
tension plant when compared with the xylem ring in the control plant 
showed no appreciable difference. The phloem in each had devel- 
oped to the same degree. Under high power the walls of the cells 
in the tension plant were seen to be no thicker than the walls of the 
corresponding cells of the normal plant. 
A further study, such as has been made with other stems by deter- 
mining differences with the aid of area or weight methods, seemed 
unnecessary. It must be concluded then, as a result of experiments 
on five plants of Brassica oleracea, that tension does not call — 
a self-regulatory development of mechanical tissue. 
Phaseolus multiflorus (greenhouse cultures) 
Four plants of Phaseolus multiflorus were subjected to tension for 
two weeks. These plants were cultivated in the greenhouse and when 
a few centimeters high were put under a strain in the same manneras 
explained for Helianthus. Small weights were first attached, and to 
these were added others until the final weight amounted to 17508”. 
Freehand sections were made at heights on the stem 5°™ above the 
ground. Sections at corresponding places in the control plants were 
chosen for comparison. Control plants, as in all experiments, were 
selected with reference to similarity in growth, size, and vigor. 
In cross-section the stem of Phaseolus shows a very well-differen- 
tiated xylem ring, and it is very easy to note differences if they occur, 
when tension and control plants are compared. In the four plants 
under experimentation, a careful inspection with the microscope 
failed to show any appreciable differences in the size of the xylem 
ting, bundles of phloem, or thickness of walls. For this reason it 
seemed unnecessary to proceed with the various methods as used in 
previous experiments. My conclusion, therefore, with reference to 
the influence of tension on the stem of Phaseolus multiflorus can be 
nothing else than that tension does not induce a self-regulatory devel- 
opment of mechanical tissue. 
: Conclusion 
From the above seven experiments we conclude that tension has 
No influence in the formation of tissue in stems of Helianthus annuus, 
Ricinus communis, Brassica oleracea, and Phaseolus multiflorus, but 
that in Vinca it seems to call forth a regulatory development of sup- 
