268 SCIENCE OF AGRICULTURE. Part II. 



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

 culent tribe. 



1739. The qualities of water, or the nature of the substances dissolved in it, must neces- 

 sarily influence pow^erfully 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 Pelt^ia, and in lime-rubbish, as Thlaspi, and other 

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

 peculiar to such situations. 



1740. 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. As consisting of a mixture of mineral, animal, and vegetable matter. 



1741. Primitive surfaces affect vegetables mechanically according to their different 

 degrees of movability or tenacity. On 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. On fine, dry, sandy surfaces, plants with very delicate roots, as Protea and 

 jErica, 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 

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



1742. With respect to the relative proportions of the piimitive 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 e:sposed 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 gre? 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, Decan- 

 dolle 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 Jura, 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 except in these rare cases, plants grow with nearly 

 equal indifference 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 

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

 result is, as DecandoUe has well remarked, that it is often a very bad method of 

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

 state. 



1743. 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 

 extensive 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 ; corre- 

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



1744. Aquatic 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. 



1745. One of the principal subdivisions of aquatics is that of marine plants, such as the Fuci and many 

 of the A'lgx, which are very plentiful in the seas that wash the coasts of Great Britain, and are generally 

 attached to the stones and rocks near the shore. Some of them are always immersed ; and others, which 

 are situated above low-water mark, are immersed and exposed to the action of the atmosphere alternately. 

 But none of them can be made to vegetate except in the waters of the sea. Another subdivision of aqua- 

 tics is that of river plants, such as Chara, Potamogfeton, and A^ympha^a, which occupy the beds of fresh, 

 water rivers, and vegetate in the midst of the running stream ; being for the most part wholly immersed, 

 as well as found only in such situations. 



