200 SCIENCE OF GARDENING. PAOT II. 



921. Great anomalies are found in the comparative height in which the same plant will 

 grow in different circumstances. In countries situated under the equator, the two sides of 

 the mountain are of the same temperature, which is solely determined by elevation ; but in 

 countries distant from it, the warmest side is that towards the south, and the zones of 

 plants, instead of forming lines parallel to the horizon, incline towards the north. The 

 reason, in both cases, is sufficiently obvious. In the temperate zone we find the same 

 plants frequently on low and elevated situations, but this is never the case between the 

 tropics. 



922. Altitude influences the habits of aquatics; thus some aquatics float always on the 

 surface of the water, as lemna, while others are either partially or wholly immersed. 

 Such aquatics as grow in the depths of the sea are not influenced by climate ; but such as 

 are near the surface are influenced by climate, and have their habitations affected by it. 



923. The moisture, or mode of watering natural to vegetables, is a circumstance which 

 has a powerful influence on the facility with which plants grow in any given soil. The 

 quantity of water absolutely necessary for the nourishment of plants, varies according to 

 their tissue ; some are immersed, others float on its surface ; some grow on the margin 

 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. 



924. 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 

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

 peculiar to such situations. 



925. 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. 



926. 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. 



92"^. With respect to the relative jrroportions of the jrrimitive 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 nafcwal 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- 

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

 /rreat 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 



