ivith Geographic Botutiy. — Soil. 525 



miaeral substances are a necessary part of plants, but it seems 

 more probable that they are foreign matters entering the tis- 

 sues with the nutrient thiids, and of no use in the oeconomy 

 of" vegetation. In mentioning a few instances, we shall pass 

 over altogether the results which have been elicited by culti- 

 vation, as in this state it is frequently the object to obtain an 

 engagement of, or a determination of nutrient matter to, a 

 particular organ or set of organs, very different to what is ob- 

 served in natural healthy vegetation. 



The vegetation of chalky soils can nowhere be better studied 

 than in our own country ; chalk being unknown in some of 

 the large continents, though limestone is one of the most 

 abundant rocks. The natural families of Labiates, Orchidece, 

 and some members of Le(juminos(B, display the greatest par- 

 tiality, whilst many other families have species which show a 

 decided preference. DeCandolle gives the following as chalk- 

 loving plants : — Buxus sempervirens, Poteutilla rupestris, P. 

 caulescens, Polypodium calcareum, Gentiana cruciata, Ascle- 

 pias vincetoxicum, Cyclamen eiiropceum, Trifolium montanum, 

 Adonis vernalis, Avith several species of Oxalis, Bupleurum, Se- 

 dum, Lichen, &c. 



As partial to a siliceous soil the same talented botanist men- 

 tions Castanea vesca. Digitalis purpmrea, Sediun villosum, Pte- 

 ris crispa, Polystichum oreopteris, Saxijraga stellaris, Achil- 

 lea moschata, Carex pyrenaica. 



Soils im})regnated strongly with saline matter are frequently 

 spread o\'er extensive districts ; a large tract in Mesopotamia 

 is covered with a species of Artemisia ; several genera of Fi- 

 coidece and Chenopodice will grow nowhere else ; and Umbel- 

 liferce, Composit<£, Plantaginece, Polygonea;, Plumbaginece, Nyc- 

 taginea (Abronia, several species,) have all members with a 

 similar bias. The cocoa-nut palm will not thrive out of the 

 influence of the salt air of the ocean, and it is in its happiest 

 vigour among the low coral islands of the Pacific, when its 

 roots are almost bathed by the waves. The sugar-cane also 

 j)refers a saline soil, and some of the plantations of the West 

 Indies occupy land originally overflowed by the sea. The ve- 

 getation of the Natron lakes of Central Africa does not seem 

 very abundant, but on the margins of some date trees thrive, 

 and their marshy borders are invested by grasses and a species 

 of Juncus. A distinction may be made according to the saline 

 constituent; in some cases this is nitrate of potash, as in the 

 maritime parts of Chili and Peru ; natron or carbonate of 

 soda prevails in Egypt; in others it is the chloride of sodium, 

 and this is more frcqiient in saline marshes near the sea. 



