42 GEOGRAPHICAL DISTRIBUTION OF PLANTS. 



the distribution of plants, if not so great as that of temperature, is in some 

 cases more striking. In general, plants are as differently constituted in re- 

 spect to water as they are in regard to temperature. The quantity of water 

 absolutely necessary for the nourishment of a plant varies according to its 

 tissue. Plants with large and soft leaves, with little or no pubescence, with 

 many pores or stomata, and with the texture of the entire plant loose and 

 spongy, require most water ; and accordingly this is the description of plants 

 which is found hi marshes, and in lakes or rivers. Plants having their 

 general texture firm and succulent, clothed with pubescence, and having few 

 stomata, grow in dry warm stations. Trees and herbaceous plants, with 

 roots which penetrate into the soil, require least water on the surface, and 

 best resist extreme drought ; and next to these, those that have succulent 

 leaves and few stomata, because they evaporate but little moisture from 

 their surface. Some plants live entirely on water, floating on its surface ; 

 and others immersed in it, and attached to the soil at the bottom of the lake 

 or river : in some, as in river -plants, the water is constantly in a state of 

 motion ; while in lake-plants it is always at rest, except on the surface. In 

 general, all aquatic and marsh plants require the water to be pure ; but in 

 salt marshes, salt steppes, and on the sea-shore, it is strongly impregnated 

 with sea-salt or soda, in which only a small number of vegetables will live. 



145. The influence of soil on the distribution of plants is universally ac- 

 knowledged ; though the difference in the selection of soils by plants depends 

 much more on the condition of that soil with respect to water, than on its 

 chemical properties. By soil, is to be understood that upper coating of the 

 earth's surface, composed of earths or the rust of rocks, and organic matters; 

 and the capacity of this coating for water will depend on the elevation or de- 

 pression of its surface, on its texture, and on the nature and texture of the sub- 

 soil. The relative proportions of the primitive earths do not appear to have 

 much influence on the (distribution of plants; but when a soil has any decided 

 character, such as when it consists almost wholly of sand, of chalk, or of 

 clay, the influence is considerable. In general, the greatest number of species 

 are commonly found on soils having a loose sandy surface ; because their 

 seeds being blown there, or otherwise conveyed, from the plants on ad- 

 joining soils, readily take root ; whereas on chalky and clayey soils, from 

 their greater hardness, and also from their surface being generally more 

 clothed, the seeds which fall on them do not so readily vegetate. Many of 

 the plants which spring up in sandy districts perish for want of moisture, 

 or are blown out by the winds ; but they are nevertheless continually re- 

 newed by the seeds furnished from adjoining surfaces. Those which are 

 indigenous to gravelly soils, much exposed, are chiefly low, compact, or trail- 

 ing plants, which offer but a small surface for the wind to act on, or such 

 as have deeply-penetrating roots. Chalky and clayey soils, on the other hand, 

 from their firm, compact texture, are adapted only to such species as have 

 small fibrous roots, and which do not require any great depth of soil. 



146. A few plants appear to prefer the soils formed by particular rocks, 

 such as limestone, chalk, granite, and slate; yet the same plants which 

 prevail on these rocks are frequently found abounding in districts of a 

 totally different geological character. Thus according to De Candolle, 

 although the box in France is very common on calcareous surfaces, it is 

 found in equal abundance on such as are schistous or granitic. The sweet 

 chestnut grows equally well in limestone soils and clays, in the volcanic 



