90 
Effect of Tension on Plants. 
of strain. The observed thickening has been attributed by some inves- 
tigators to this tension, and by others to an observed increase in the 
amount of protoplasm in the cells of the upper side of plants so treated. 
A new series of investigations was begun in order to answer definitely 
the questions involved and if possible to bring the known facts into 
correlation. 
Experiments on many hundreds of plants were made in order to de- 
termine if tension in the vertical direction causes a change in the tensile 
strength of stems or any alteration in their tissues. Young seedlings 
of the sunflower, field bean, castor bean, Vida faba Helleborus differ , 
Mirabilis jalap , and others were used. It is not necessary to enter into 
the minutiae of these experiments. In general, a sling of softest kid 
or chamois skin was devised as a holdfast for the string that supported 
the weights since it was found that the old method of a slip knot in the 
thread very often caused choking of the plant or bruises and abrasions 
of the skin, that would very materially affect the value of the results. 9 
Weights of sizes varying from 2-1000 grams were provided with hooks 
to facilitate attachment to the cord and thus prevent any great shock 
during change. Experiments were made both upon normal and etiolated 
plants. In no case was an increase in the tensile strength of the plant 
thus put under stress other than that which would normally result from 
increase in strength through growth. Seedlings of the bean and the 
leaf stems of Helleborus niger after having been weighted with amounts 
varying from 500 to 3500 grams through periods of four to five months, 
showed no observable abnormal increase in tensile strength nor any 
change in, nor in any case, any appearance of new tissues. 
Thus far the results are purely negative. Not so, however, when the 
plants are in an abnormal position. Proceeding on the hypothesis that 
tension causes the observed thickenings of cell walls when normal 
geotropic curving is prevented, the demonstration of the fact was at- 
tempted. 
A series of experiments were made in which the effects of the two forces 
were studied, when acting alone, bv excluding the one, and also when 
both coincided in their action. 
I. As has already been mentioned, gravitation is known to be the 
cause of hypertrophy in the cell walls of tissues on the upper side of 
stems and limbs of Dicotyledons which stand in an ascending or horizon- 
tal position. In order to eliminate the action of this force, and to ob- 
serve that of stress alone, Elf ving, a Sweedish investigator, placed young 
plants upon the klinostat and allowed them to rotate, after having bent 
9 Townsend, Annals of Botany, 1897, Vol. II, p. 509 ff. shows that small wounds 
act as an irritant, causing more rapid growth in plants. 
