VOLUME XLIII NUMBER 6 
BOTANICAL GAZETTE 
JUNE 1907 
THE INFLUENCE OF TENSION ON THE FORMATION 
OF MECHANICAL TISSUE IN PLANTS! 
RUFUS PERCIVAL HiIBBARD 
The radical change from a theory of self-regulatory development of 
mechanical tissue in plants subjected to tension by pull, to one imply- 
ing no reaction whatever, necessitates detailed evidence before the 
latter can be accepted. The following investigation was directed to a 
more complete knowledge of the reactions of a plant to tension, to 
determine if possible what influence this exerts on the formation of 
mechanical tissue. Heretofore the root system has been entirely 
ignored, but this together with the stem has been included in the 
work here recorded. 
I. Historical 
For some time previous to 1891, physiologists had generally 
assumed that a plant reacted to a gradually increasing strain by a 
development of its mechanical tissue. HEGLER (PFEFFER ’91) in 
this year subjected several stems and leaves to a longitudinal pull. 
In response to this method of experimentation, the plant, according to 
HEGLER, withstood greater strain than one grown under normal con- 
ditions. For example, a seedling of Helianthus annuus whose original 
breaking strength was 160%" had a breaking strength of 250%" after 
two days under the influence of a pull of 150%. The petioles of 
Helleborus niger, which at first could withstand a weight of only 4008”, 
after five days, during which the weights had been gradually increased, 
held without breaking 3.5*%, while those under normal conditions 
gained but little strength in the same time. Marked differences were 
* Contribution 92 from the Botanical Laboratory of the University of Michigan. 
361 
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