when the solidity of the concrete and the force needed to rupture it are taken into consideration.
Being interested in this phenomenon, we endeavored to learn approximately the power required by the ferns to rupture the concrete. In the experiment, some of the soil underneath was first excavated and a lever arranged in such a way that force could be applied in practically the same manner as was done by the ferns, i. e., a round piece of wood was placed on the end of the lever of the same dimensions as the undeveloped cluster of fern fronds. The fulcrum of the lever was one foot
Fig. 5. Showing method of demonstrating power of growth. Growing flower stalk of tulip placed in 10 per cent, solution of potassium nitrate, which causes the stalk to shorten. The stalk is then stretched to its original length and the power of growth determined.
from the point of contact with the concrete, and weights were placed on the other end of the lever at different distances, as the case required. Our object in this test was to ascertain how long it would take to rupture the concrete and to determine the amount of weight necessary to do it. It was not intended to apply force enough to cause an immediate rupturing of the concrete, or even in a few hours, but in perhaps ten or fifteen days—the same length of time usually required by the ferns. A number of tests were made, care being taken to have all the conditions as nearly like those under which the concrete was broken by the ferns as possible. A weight of 699 pounds broke the concrete in a few hours. Next a weight of 262 pounds was applied, which required ten days, while in still another test a weight of 189 pounds broke through in thirteen days. Other tests were made, but it is not necessary to give them here. A weight of 189 pounds, therefore, seemed to rupture the concrete in about the same time as was done by the ferns; and in our estimation this test represents a fairly good duplication of the fern phenomenon. If we consider the average cross section area of the six fern fronds and divide this by the total weight lifted, we find that the cells of the young fronds exerted about 35 atmospheres to over-
