92 



THE FACTORS 



[Part 



Saltpetre also induces a xerophilous structure, but only at a higher co 

 centration, and even then less decidedly than common salt. This differen 

 favours the opinion that the decided nature of the xerophilous structure 

 ordinary halophytes, must partly afford protection against poisonous salii 

 action, and therefore appears sooner than in the case of saltpetre, which 



injurious only when more highly concentrate 

 Such a concentration is not usually attained 

 places rich in nitrates, at any rate not in t 

 case of plants, such as many Solanaceae, Cru( 

 ferae, Chenopodiaceae,Fumaria,Sambucus nigi, 

 and others, that have a tendency to store sa 

 petre in their tissues, and usually exhibit a rai 

 growth in such places. The nitrate fields 

 America, however, on account of their extrer 

 dryness possess a decided xerophilous flora. 



5. SERPENTINE. 



F 



Serpentine, a very slightly soluble silicate 

 magnesium, acts in such a way on two Centi 

 European species of fern, Asplcnium viride a 

 A. Adiantum-nigrum, that they are changl 

 into different forms which have for some tit 

 been taken for distinct species. Sadebe 

 succeeded in obtaining a reversion to the 

 ginal form by cultivating them in ordina 

 soil, but not until the sixth generation ; t 

 attempt to induce any corresponding trat- 

 formation in the two Asplenia by cultivati 

 them on serpentine however failed. It is the: 

 fore evidently a case of an extremely si 

 progressive action. The deviations from f 

 normal type are apparently purely morpl 

 logical without any ascertainable use to t 

 plant. 



Asplenium adulterinum, Milde (Fig. 52), the 

 pontine form of A. viride, assumes in many respe 

 an intermediate form between that species and 

 Trichomanes. For instance, the rachis is bro 

 below and green a'bove. Its peculiar habit is 1 



to the extreme convexity of the leaflets and their perpendicularity to the rachis; 



two allied forms have somewhat long plane leaflets, parallel to the rachis. 



according to Liirssen, this characteristic is not constant. Asplenium serpent 



Tausch, differs from the typical A. Adiantum-nigrum by the segments be 



Fig. 52. Asplenium adulteri- 

 num. Natur.il size. After Liirs 

 sen. 



