- 5 6- 



But silica in plant economy is not considered a nutriment. 

 Sachs •« regards it as certain that this substance is superfluous for 

 the chemical processes of nutrition as well as for the molecular 

 movements connected with growth."* Its office is better de- 

 scribed as mechanical, a strengthening material which gives hard- 

 ness and stiffness when deposited in the cells of their tissues, or 

 still better a hindrance to excessive transpiration when left in the 

 cortical cells or epidermis. This check on evaporation is a great 

 benefit to plants growing on rocks like many of the ferns, where 

 the supply of water is scanty. 



If we take account of the mineral ingredients which are di- 

 rectly nutritious of plants, such as lime, potash, soda, magnesia, 

 iron, the rocks mentioned, either in their pure or impure condi- 

 tion, contain most of these in some measure. They enter into the 

 silicates which are variously combined and make up the mass of 

 granite, granitoid, syenitic and volcanic rocks, from whose decom- 

 position come the common mineral foods the soil supplies to 

 plants. 



That ferns as well as other plants like a particular mineral 

 food is well known. Sadebeck showed this experimentally. He 

 took specimens of Asplenium Serftenti?ii and A. adult erinum % 

 which have a preference for serpentine or silicate of magnesia, and 

 cultivated them in soil that did not contain a trace of serpentine. 

 By the sixth generation they had lost their identity and were 

 changed to two other spleen worts, A. adiantum-nigrum and A. 

 viride. Warming, from whose Manual of Ecological Plant-Geog- 

 raphy these facts are taken, uses them to show that soils may be 

 decisive factors in originating species. In this case the characters 

 of the two ferns had not become permanently fixed. They were 

 evidently derived from the two into which they were changed, 

 since time enough had not elapsed for the serpentine rocks to gain 

 such a mastery over their denizens that they would not return 

 when placed under less favorable conditions to maintain their 

 peculiarities. 



Chicago, 111. 



* Physiology of Plants, p. 2887. 



— According to a writer in the May number of Meehari s 

 Monthly the Indians of Nevada use the outer layers of the Brack- 

 en's root-stock in basket making, its dark color serving well for 

 making various figures in the pattern. 



