No. 11. Circulation of matter in the Animal S^^ Vegetable Kingdoms. 845 



know that other elements must be supplied 

 by the earth, if plants are to thrive and mul- 

 tiply. 



The amount of materials contained in the 

 atmosphere, suited to the nourishment of 

 plants, is limited; but it must be abundantly 

 sufficient to cover the vvliole surface of the 

 earth with a rich vegetation. Under the 

 tropics, and in those parts of our globe 

 where the most genial conditions of fertility 

 exist — a suitable soil, a moist atmosphere, and 

 a high temperature, — vegetation is scarcely 

 limited by space; and where the soil is want- 

 ing, it is gradually supplied by the decaying 

 leaves, bark, and branches of plants. It is 

 obvious there is no deficiency of atmospheric 

 nourishment for plants in those regions, nor 

 are these wanting in our own cultivated 

 fields: all which plants require for their de- 

 velopment is conveyed to them by the inces- 

 sant motions of the atmosphere. The air be- 

 tween the tropics contains no more than that 

 of the arctic zones; and yet how diderent is 

 the amount of produce of an equal surface 

 of land in the two situations ! 



This is easily explicable. All the plants 

 of tropical climates, the oil and wax palms, 

 the sugar-cane, &c., contain only a small 

 quantity of the elements of the blood neces- 

 sary to the nutrition of animals, as compared 

 with our cultivated plants. The tubers of 

 the potatoe in Chili, its native country, 

 where the plant resembles a shrub, if col- 

 lected from an acre of land, would scarcely 

 suffice to maintain an Irish family for a sin- 

 gle day. The result of cultivation in those 

 plants which serve as food, is to produce in 

 them those constituents of the blood. In 

 the absence of the elements essential to 

 these in the soil, starch, sugar, and woody 

 fibre are perhaps formed ; but no vegetable 

 fibrine, albumen, or caseine. If we intend 

 to produce on a given surface of soil, more 

 of these latter matters than the plants can 

 obtain from the atmosphere, or receive from 

 the soil of the same surface in its unculti- 

 vated and normal state, we must create an 

 artificial atmosphere, and add the needed 

 elements to the soil. 



The nourishment which must be supplied 

 in a given time to different plants, in order 

 to. admit a free and unimpeded growth, is 

 very unequal. 



On pure sand, on calcareous soil, on naked 

 rocks, only a few genera of plants prosper, 

 and these are, for the most part, perennial 

 plants. They require, for their slow growth, 

 only such minute quantities of mineral sub- 

 stances as the soil can furnish, Vv'hich may 

 be totally barren for other species. Annual, 

 and especially summer plants, grow and at- 

 tain their perfection in a comparatively short 



time; they therefore do not prosper on a soil 

 which is poor in those mineral substances 

 necessary to their development. To attain 

 a maximum in height in the short period of 

 their existence, the nourishment contained 

 in the atmosphere is not sufficient. If the 

 end of cultivation is to be obtained, we must 

 create in the soil an artificial atmosphere of 

 carbonic acid and ammonia; and this surplus 

 of nourishment, which the leaves cannot ap- 

 propriate from the air, must be taken up by 

 the corresponding organs, that is, the roots, 

 from the soil. But the ammonia, together 

 with the carbonic acid, are alone insufficient 

 to become part of a plant destined to the 

 nourishment of animals. In the absence of 

 the alkalies, the phosphates and other earthy 

 salts, no vegetable fibrine, no vegetable case- 

 ine, can be formed. The phosphoric acid of 

 the phosphate of lime, indispensable to the 

 cerealia and other vegetables in the form- 

 ation of their seeds, is separated as an ex- 

 crement, in great quantities, by the rind and 

 barks of ligneous plants. 



How different are the evergreen plants, 

 the oleaginous plants, the mosses, the ferns, 

 and the pines, from our annual grasses, the 

 cerealia and leguminous vegetables! The 

 former, at every time of the day during win- 

 ter and summer, obtain carbon through their 

 leaves by absorbing carbonic acid, which is 

 not furnished by the barren soil on which 

 they grow ; water is also absorbed and re- 

 tained by their coriaceous or fleshy leaves 

 with great force. They lose very little by 

 evaporation, compared with other plants. 

 On the other hand, how very small is the 

 quantity of mineral substances which they 

 withdraw from the soil during their almost 

 constant growth in one year, in comparison 

 with the quantity which one crop of wheat 

 of an equal weight receives in three months ! 



It is by means of moisture that plants re- 

 ceive the necessary alkalies and salts from 

 the soil. In dry summers a phenomenon is 

 observed, which, when the importance of 

 mineral elements to the life of a plant was 

 unknown, could not be explained. The 

 leaves of plants first developed and perfect- 

 ed, and therefore nearer the surface of the 

 soil, shrivel up and become yellow, lose 

 their vitality, and fall off while the plant is 

 in active state of growth, without any visi- 

 ble cause. This phenomenon is not seen in 

 moist years, nor in evergreen plants, and 

 but rarely in plants which have long and 

 deep roots, nor is it seen in perennials in 

 autumn and winter. 



The cause of this premature decay is now 

 obvious. The perfectly developed leaves 

 absorb continually carbonic acid and ammo- 

 nia from the atmosphere, which are converted 



