222 Stone : The power of growth of ostrich ferns 



rapidly. In one case, however, it required two years for a certain 

 group of fronds to break through the concrete. 



The ease with which this is accompHshed and the relative free- 

 dom of the fronds from scars and injuries is remarkable when one 

 takes into consideration the solidity of the concrete and the 

 enormous power necessary for rupturing it. 



Being interested in this phenomenon, we endeavored to ascer- 

 tain approximately the power required to accomplish this ruptur- 

 ing ; therefore we excavated a portion of the soil under the 

 concrete and arranged a lever in such a manner that force could 

 be applied in practically the same way as was done by the ferns ; 

 that is, 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 i foot 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 



r 



this test was to ascertain how long it would take to rupture the 



Figure i. The rupturing of concrete by the young fronds of ostrich ferns 



{^Onoclea Struthiopteris Hoffm. ), 



concrete and to determine the amount of weight necessary to do 

 this. It was not intended to apply force enough to cause a 

 rupturing of the concrete immediately, or even in a few hours, but 

 to determine how much weight it would require to do this in ten to 

 fifteen days ; in other words, the same length of time as was usually 

 required by the ferns to accomplish this. In this way we could 

 obtain some idea of the force exerted by the fern fronds in break- 

 ing through the concrete. 



•In one test, which we will designate no. i, where the pressure 



