Book I. DISTRIBUTION OF VEGETABLES. 267 



of waters, with their roots always moistened or soaked in it, others again live in soil 

 slightly humid or almost dry. Vegetables which resist extreme drought most easily 

 are, 1 . Trees and herbs with deep roots, because they penetrate to, and derive sufficient 

 moisture from, some distance below the surface ; 2. Plants, which, being furnished with 

 few pores on the epidermis, evaporate but little moisture from their surface, as the suc- 

 culent tribe. 



1703. The qualities of water, or the nature of the substances dissolved in it, must neces- 

 sarily influence powerfully the possibility of certain plants growing in certain places. 

 But the difference in this respect is much less than would be imagined, because the food 

 of one species of plant differs very little from that of another. The most remarkable 

 case is that of salt-marshes, in which a great many vegetables will not live, whilst a 

 number of others thrive there better than any where else. Plants which grow in marine 

 marshes, and those which grow in similar grounds situated in the interior of a country, 

 are the same. Other substances naturally dissolved in water appear to have much less 

 influence on vegetation, though the causes of the habitations of some plants, such as 

 those which grow best on walls, as peltaria, and in lime-rubbish, as thlaspi, and other 

 cruciferaj, may doubtless be traced to some salt (nitrate of lime, &c.) or other substance 

 peculiar to such situations. 



1704. The nature of the earth's surface affects the habitations of vegetables in different 

 points of view : 1. As consisting of primitive earths, or the debris of rocks or mineral 

 bodies ; and, 2. x\s consisting of a mixture of mineral, animal, and vegetable matter. 



] 705. Primitive surfaces affect vegetables mechanically according to their different de- 

 grees of moveability or tenacity. In coarse sandy surfaces plants spring up easily, but 

 many of them, which have large leaves or tall stems, are as easily blown about and 

 destroyed. In fine, dry, sandy surfaces, plants with very delicate roots, as protea and 

 erica, prosper ; a similar earth, but moist in the growing season, is suited to bulbs. On 

 clayey surfaces plants are more difficult to establish, but when established are more per- 

 manent : they are generally coarse, vigorous, and perennial in their duration. 



1706. With respect to the relative proportions of the primitive earths in these surfaces, 

 it does not appear that their influence on the distribution of plants is so great as might 

 at first sight be imagined. Doubtless different earths are endowed with different degrees 

 of absorbing, retaining, and parting with moisture and heat ; and these circumstances 

 have a material effect in a state of culture, where they are comminuted and exposed to the 

 air : but not much in a wild or natural state, where they remain hard, firm, and covered 

 with vegetation. The difference, with a few exceptions, is never so great but that the 

 seeds of a plant which has been found to prosper well in one description of earth, will 

 germinate and thrive as well in another composed of totally different earths, provided 

 they are in a nearly similar state of mechanical division and moisture. Thus De Can- 

 doUe observes, though the box is very common on calcareous surfaces, it is found in as 

 great quantities in such as are schistous or granitic. The chestnut grows equally well 

 in calcareous and clayey earths, in volcanic ashes, and in sand. The plants of Aira, a 

 mountain entirely calcareous, grow equally well on the Vosges or the granitic Alps. 

 But though the kind or mixture of earths seems of no great consequence, yet the presence 

 of metallic oxides and salts, as sulphates of iron or copper, or sulphur alone, or alum, or 

 other similar substances in a state to be soluble in water, are found to be injurious to all 

 vegetation, of which some parts of Derbyshire and the maremmes of Tuscany (Chateau- 

 vieux, let. 8.) are striking proofs. But excepting in these rare cases, plants grow nearly 

 indifferently on all primitive surfaces, in the sense in which we here take these terms ; 

 the result of which is, that earths strictly or chemically so termed, have much less in- 

 fluence on the distribution of plants, than temperature, elevation, and moisture. Another 

 result is, as De Candolle has well remarked, that it is often a very bad method of cul- 

 ture to imitate too exactly the nature of the earth in which a plant grows in its wild 

 state. 



1707. Mixed or secondary soils include not only primitive earths, or the debris of rocks, 

 but vegetable matters not only the medium through which perfect plants obtain their 

 food, but that food itself. In this view of the subject the term soil is used in a very ex- 

 tensive acceptation, as signifying, not only the various sorts of earths which constitute 

 the surface of the globe, but every substance whatever on which plants are found to 

 vegetate, or from which they derive their nourishment. The obvious division of soils in 

 this acceptation of the term is that of aquatic, terrestrial, and vegetable soils ; corres- 

 ponding to the division of aquatic, terrestrial, and parasitical plants. 



1708. Jquatic soils are such as are either wholly or partially inundated with water, 

 and are fitted to produce such plants only as are denominated aquatics. Of aquatics 

 there are several subdivisions according to the particular situations they affect, or the 

 degree of immersion they require. 



1709. One of the principal subdivisions of aquatics is that of marine plants, such as the fuel and many 

 of tb algae, which are very plentiful in the seas that wash the coasts of Great Britain, and are generally 



