CHAP. VII. BOTANICAL GEOGRAPHY. 299 



provided with long pubescence, but have few stomata. 

 An excess of water is apt to corrupt and dissolve the 

 outer texture, and hence we find many aquatics, as the 

 pondweeds (Potamogeton), protected by a superficial 

 varnish. Many Monocotyledons are coated with a 

 siliceous pellicle, and afford useful materials for thatch- 

 ing, as the common reed. 



(306.) Influence of Soils. Most soils are a very 

 heterogeneous mixture of different earths and other mat- 

 ters ; and hence it is not likely that any very decided 

 feature will be often impressed upon the flora of a given 

 district, by any peculiarity in the purely chemical 

 qualities which soils possess. That some chemical 

 action takes place in certain soils cannot be positively 

 denied, but has probably been greatly exaggerated. 

 For though certain plants seem to prefer particular 

 geological districts marked by the prevalence of pecu- 

 liar rocks, some especially abounding on limestone 

 and chalk, others on slate-rock ; yet it not unfre- 

 quently happens that many of these plants also occur 

 in equal abundance in some other localities where 

 the rocks possess a totally different mineralogical cha- 

 racter. It seems, therefore, more likely that such effects 

 may be attributed to mechanical rather than to chemical 

 causes; especially to the mode in which different rocks dis- 

 integrate, and are rendered capable of retaining a greater 

 or less abundant supply of moisture. It may indeed be 

 said, that these mechanical properties are generally the 

 direct result of the peculiar chemical qualities which the 

 rocks possess, though in some cases rocks of very different 

 mineralogical character certainly disintegrate in much the 

 same manner. Hence we find the same lichens and 

 some other plants growing on schistose rocks, whether 

 they happen to be argillaceous or cretaceous in their 

 composition. Various soils may be stated as generally 

 retaining moisture in proportion to the quantity of alu- 

 mina which they contain, and parting with it more rea- 

 dily in proportion as they abound in silica. Siliceous 

 tracts require most rain, and clay soils least, to become 



