THE FERX BULLETIN 



5 



avoiding lime, respectively; silicolous, such as live on 

 a silicious substratum, or in a soil abounding in sand, 

 mere dwellers as it were with a somewhat neutral 

 character, since silex as nutrition for plants is on the 

 whole a neutral substance. Some regard a calcipho- 

 bous plant as in a sense the equivalent of one that is 

 silicolous. This is explained by saying that plants do 

 not avoid a calcareous and select a silicious soil, but 

 are driven by competition with other plants away 

 from a soil well supplied with calcium and usually 

 richer, into one with much less of this element, such 

 as a very sandy soil. Here may be found enough of 

 calcium for actual needs, since they can do with less, 

 and in addition are freed from competitors that are not 

 so easily satisfied. This, on the whole, seems a more 

 satisfactory explanation of the relations. As far as 

 experiments upon the relations of plants to their habi- 

 tats have been made, they appear to be determined 

 more by the physical than by the chemical constitution 

 of the soil, provided that it contains the ingredients a 

 plant requires, and does not have in excess those that 

 are injurious. 



In accordance with this explanation, the two ferns, 

 commonly calcicolous but sometimes silicolous, may be 

 more closely related to this underlying rocks physically 

 than chemically. If this need of calcium is greater 

 than that of plants in general, it would have a bearing 

 on their habitats. But P. atropurpurca, at least, since 

 it lives in exposed places, must have a better chance to 

 resist physically adverse conditions affecting its sta- 

 bility on a firm limestone basis than on the more 

 readily disintegrating sandstone. In this way its in- 

 frequency on sandstone rocks may in a measure by ex- 

 plained. 



Chicago, III. 



