On the Food of Plants. 
503 
Tlie rimount of humus present in different lands varies very 
jireatly ; from the loose blown sand of the "dunes" by the sea- 
side, to peat-earth, of wliich it forms the principal constituent. 
The fertility of a soil is dependent on other thing:s besides its 
chemical composition; we must consider that the office it performs 
is twofold, namely, to retain the plant firmly in the position most 
favourable to its growth, and to supply a certain amount of food : 
hence mechanical texture becomes a matter of great importance ; 
it must be firm enough to afford the proper degree of support, 
and at the same time loose enough to allow the delicate fibres of 
the rootlets to extend themselves, and also access of air to take 
place, without which the plant cannot live ; it must be of such a 
texture as to retain for a considerable period the water which 
falls on it, and at the same time porous enough to suffer the 
excess to drain away, otherwise the roots of the plant will rot; 
it is for these reasons that the nature of the solid substratum at 
some depth beneath the soil must be borne in mind : these, and 
many other things, such for example as the condition of the sur- 
face with respect to its absorbent power for heat, all tend gi'eatly 
to complicate the subject, and render decisions concerning the 
comparative value of different lands founded on merely chemical 
evidence exceedingly prone to error. 
Tlie Composition of Plants. 
When speaking of the composition of any substance, chemists 
are in the habit of drawing a distinction between what are termed 
its "proximate" and its "ultimate" constituents. For example, 
the salt called acetate of morphia is considered to be composed of 
two different bodies of opposite chemical energies, namely, acetic 
acid and the organic base morphia ; but both these are themselves 
compound ; the first contains carbon, hydrogen, and oxygen, and 
the second, these elements with the addition of a fourth, nitrosren : 
Ave call therefore the acetic acid and the morphia the proximate, 
and the carbon, hydrogen, oxygen, and nitrogen, the ultimate, 
elements of the salt in question. Just in the same manner the 
juice of a plant may contain a number of proximate principles, 
gum, sugar, albumen, &c., and its solid tissue may be made up 
of others, such as lignin, starch, &c., all being themselves com- 
pounds of a very few simple ultimate elements. 
Now, these ultimate elements are not numerous; the most 
important are the four mentioned above, and of these all the 
strictly organic portions of the plant are composed, however much 
these latter may differ in properties. Other substances are also 
present in smaller proportions, such as ammoniacal salts, silica, 
sulphuric and phosphoric acids in combination with lime, mag- 
nesia, traces of oxides of iron and manganese, &c. Some of 
