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THE POPULAR SCIENCE MONTHLY 



•nhen 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 ap- 

 proximately 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 unde- 

 veloped cluster of fern fronds. The fulcrum of the lever was one foot 



jfi 



apg-^ 



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 rup- 

 ture the concrete and to determine the amount of weight necessary to 

 do it. It was not intended to apply force enough to cause an imme- 

 diate 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 neces- 

 sary 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- 



