INFLUENCE OF SOIL. 127 



should be inversely proportionate to the distance of the countries they 

 naturally inhabit from the equator; 3d, plants with annual stems re- 

 quire more moisture than those with ligneous stems ; and 4th,the amount 

 of atmospheric moisture required by plants at rest is inversely propor- 

 tionate to the water they contain at the time. Thus succulent plants 

 when at rest require a dry air. Much attention has been paid to 

 ventilation and other particulars connected with the growth of plants, 

 but these do not fall within the scope of the present chapter. 



The influence of soil on plants, in a general point of view, is well 

 known. Its consistence, as a predominant quality, varies in different 

 parts of the earth. The light and drifting soil of deserts can, at best, 

 support only humble shrubs, while that of a compact nature, which 

 distinguishes large tracts of country, sustains plants of a large and 

 lofty growth. Extremes of loose and compact soils are therefore 

 equally devoid of vegetation. Water or wind may equally affect the 

 one while the other is impermeable to the roots of plants. The chem- 

 ical nature of soils, or the rocks which compose them, is the general 

 cause of the local distribution of plants. These are composed of 

 the constituents of neighboring mountains. Hence, as a general rule, 

 the character of the soil may be known by that of rocks composing 

 the mountains which would most naturally, from their situation, con- 

 duct water upon the soil. The materials for this will be such as con- 

 stitute the elements of plants flourishing upon it. Other soils are de- 

 posites from water which have formed at distant periods various kinds 

 of earths from distant situations, either as a running stream or as a 

 portion of the sea. Vast tracts of country are deposites from the sea, 

 thus forming immense deltas, islands, etc. The peculiarities of soil 

 are consequently as various as were the causes which produced them ; 

 and the combination of materials which those causes must have effected 

 in the progress of many ages, was therefore greatly varied and singular. 



How diversified soever the agents may be which contribute to the 

 growth of plants, it is certain that they will not grow in any climate, 

 if the soil be devoid of the elements essential to their composition. 

 Soil must be so constituted as to absorb water more or less readily, retain 

 it with more or less force or part with it with greater or less facility. 

 In a dry climate it should have the property of absorbing moisture 

 most readily, and in a humid atmosphere of resisting it to a great ex- 

 tent ; yet this is to be considered in reference to the nature of the 

 plants grown on the soil. Every kind of soil has these properties in 

 a greater or less degree. Some are more or less readily pulverized 

 and others, from their nature and color, are more or less readily heated ; 

 or they retain their heat longer or shorter. 



The influence of the atmosphere both upon the growth and distribu- 

 tion of plants is of the greatest importance, for they can no more exist 

 without it than without a soil. It is from the atmosphere that they 



