92 



THE FACTORS 



[Part I 



Saltpetre also induces a xerophilous structure, but only at a higher con- 

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

 favours the opinion that the decided nature of the xerophilous structure in 

 ordinary halophytes, must partly afford protection against poisonous saline 

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



injurious only when more highly concentrated. 

 Such a concentration is not usually attained in 

 places rich in nitrates, at any rate not in the 

 case of plants, such as many Solanaceae, Cruci- 

 ferae, Chenopodiaceae,Fumaria,Sambucus nigra, 

 and others, that have a tendency to store salt- 

 petre in their tissues, and usually exhibit a rank 

 growth in such places. The nitrate fields of 

 America, however, on account of their extreme 

 dryness possess a decided xerophilous flora. 



5. SERPENTINE. 



Serpentine, a very slightly soluble silicate of 

 magnesium, acts in such a way on two Central 

 European species of fern, Asplenium viride and 

 A. Adiantum-nigrum, that they are changed 

 into different forms which have for some time 

 been taken for distinct species. Sadebeck 

 succeeded in obtaining a reversion to the ori- 

 ginal form by cultivating them in ordinary 

 soil, but not until the sixth generation ; the 

 attempt to induce any corresponding trans- 

 formation in the two Asplenia by cultivating 

 them on serpentine however failed. It is there- 

 fore evidently a case of an extremely slow 

 progressive action. The deviations from the 

 normal type are apparently purely morpho- 

 logical without any ascertainable use to the 

 plant. 



Asplenium adulterinum, Milde (Fig. 52), the ser- 

 pentine form of A. viride, assumes in many respects 

 an intermediate form between that species and A. 

 Trichomanes. For instance, the rachis is brown 

 below and green above. Its peculiar habit is due 

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

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

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

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



Fig. 52. Asplenium adulteri 

 num. Natural size. After Liirs 

 sen. 



