Stone : The power of growth of ostrich ferns 223 



exerted on the under surface of the concrete was equal to 699 

 pounds, rupturing occurred in a few hours. In another instance 

 (test no. 2), where a weight of 264 pounds was applied, it required 



ten days to break through, and in still another (test no. 3) it 



required thirteen days for a weight of 1S9 pounds to produce the 



same effect on the concrete as that caused by the ferns. A few 



other tests were tried, but those given are typical of the results 

 obtained. 



In test no. I, where a weight of 699 pounds was used and the 

 concrete was ruptured in a few hours, the pressure applied ex- 

 ceeded that exerted by the ferns, and this is probably also true of 

 test no. 2, in which a weight of 264 pounds was used, although 

 in some instances this might not exceed the work done by the 

 ferns. The force applied in test no. 3 was very similar, as regards 

 length of time required and the manner of breaking through, to 

 that actually accomplished by the ferns. This test, therefore, 

 represents a fairly good duplication of what actually took place, 

 and represents, at least in our estimation, the amount of force re- 

 quired to accomplish the same results shown by the ferns. If we 

 take into consideration the average cross-section area of the frond 

 shoots, which equals in one instance 39.15 square millimeters, 

 we have, since there were six of these, 234 square millimeters as 

 the total cross- section area of the frond shoots. In this test we 

 found that the weight required was 189 pounds, or about 84,672 

 grams. This factor divided by the total area of the frond sec- 

 tions, or 234, equals 361 grams to each square millimeter of cross- 

 section surface, or about 35 atmospheres. From the results of the 

 various tests we are led to believe that this calculation is not an 

 overestimate of the work actually accomplished by these ferns. 

 Employing the same data for test no. 2, we would obtain a pres- 

 sure of 49 atmospheres, which in our estimation, is somewhat in 

 excess of the pressure actually exerted by the ferns. It should 

 be pointed out, however, that tar concrete is much modified by 

 temperature; at the same time the temperature was much higher 

 at the time of the lever experiments than when the rupturing was 

 done by the ferns, which occurred in April and May, while the 

 experiments with the lever were done in May and June. The 

 ferns were observed very closely during the time they were rup- 



