GEOGRAPHICAL DISTRIBUTION OP PLANTS. 45 



Some stations, on the other hand, are absolute ; such as maritime, marine, 

 aquatic, marsh, subterranean, and parasitic, and cannot be dispensed with in 

 our attempts at cultivation. 



150. " The habitations of plants" is an expression used to denote the 

 range of country throughout which any particular species is found distri- 

 buted ; the stations being those soils or situations in that country in which 

 alone, or chiefly, the plant is found. (134). For example, a plant may 

 be an inhabitant of mountains, and its station on these mountains may be 

 a peat -bog. The habitations of plants are much less certain than their 

 stations ; for the limits in latitude and longitude within which plants occur, 

 have little relation to those in which, judging from the stations and climate 

 in which they are found, they might extend themselves. Thus we have 

 certain species growing in a particular station and temperature in the 

 northern hemisphere, which are not to be found in stations and temperatures 

 of exactly the same kind in the southern hemispheres. On the other hand, 

 there are some species, such as certain grasses, which are found extensively 

 distributed on both hemispheres ; while some few plants, such as the Stre- 

 litzia, have their habitations so limited as to be found only in one or two 

 stations of very confined extent. Plants of this kind are called solitary, while 

 those which grow in immense masses are said to be social. Those which 

 have been long in cultivation are said to be domesticated ; but this term is 

 not applied to such plants as have been introduced into gardens without 

 undergoing any change in their habits there. 



CHAPTER II. 



SOILS CONSIDERED WITH REFERENCE TO HORTICULTURE. 



151. IN the last section of the preceding chapter we have seen, that 

 though plants are less absolute in the choice of soils than of climates, yet 

 that in the cultivation of plants, soils are much more under our influence 

 than any other element of culture. The term soil is applied to that thin 

 stratum on the surface of the ground which is occupied by the roots of the 

 smaller herbaceous vegetables ; on uncultivated surfaces it varies in depth 

 with the nature of the soil and the plants growing on it ; but on lands in 

 cultivation, the soil extends to the depth usually penetrated by the imple- 

 ments of culture. The principal materials of which soils are composed, are 

 earths formed of the debris of different kinds of rocks, combined with organic 

 matter derived from decomposed vegetables or animals. Earths without 

 organic matter will only support plants of the lowest grade, such as lichens 

 and mosses; and where soils are found supporting the higher classes of 

 plants, endogens and exogens, their vigour will generally be found to be 

 greater or less according to the proportion of organic matter which the soil 

 contains. This organic matter, when supplied by art, is called manure, and 

 constitutes the food of plants ; while the soil may be compared to a stomach, 

 in which that food is digested. The subject of manures will be most conve- 

 niently treated in our next chapter. Here we shall confine ourselves to the 

 consideration of soils, and treat, first, of their origin and kinds, and secondly, 

 of their improvement. 



