TROPICAL VEGETATION. 161 



in their natural wild and normal condition, we must procure an 

 artificial atmosphere, and we must add to the soil the ingredients 

 in which it is deficient; 



Very unequal quantities of nourishment must be furnished to 

 different plants in a given time, in order to procure a free and 

 unimpeded growth. On arid sands, simple calcareous soils, or on 

 naked rocks, few plants flourish,- and those that do are generally 

 perennial. These, growing slowly, require only small quanti- 

 ties of mineral ingredients, so that soils sterile for other kinds of 

 plants are still able to furnish to them mineral ingredients in 

 sufficient quantity. The annuals, particularly summer plants, 

 reach complete maturity in comparatively a short time, so that 

 they do not flourish on a soil poor in the mineral substances ne- 

 cessary for their growth. 



The food contained in the atmosphere does not suffice to 

 enable these plants to obtain their maximum of size in the short 

 period of their life. If the object of culture is to be attained, 

 there must be present in the soil itself an artificial atmosphere 

 of carbonic acid and ammonia, and this excess of nourishment, 

 which the leaves cannot get, must be conveyed to corresponding 

 organs existing in the soil. 



But the ammonia with carbonic acid does not suffice to enable 

 itself to become a constituent of a plant fit for the food of animals. 

 Albumen cannot be formed without alkalies, and vegetable fibrin 

 and casein cannot be produced without the aid of phosphoric acid 

 and of earthy salts. We know that phosphoric acid is indis- 

 pensable for the production of the seeds of our cereals and culi- 

 nary vegetables, although the same acid is found in large quan- 

 tity in an excrementitious form in the bark of woody plants. 



How very different, in comparison with summer plants, are 

 the characters of evergreens, of mosses, ferns, and pines ! 

 During every part of the day, both in summer and in winter, 

 they absorb by their leaves carbonic acid, which the sterile soils 

 cannot yield : their fleshy leather-like leaves retain with great 

 tenacity the water absorbed, and lose very little of it by evapora- 

 tion, in comparison with other plants. And yet how very small 

 is the quantity of mineral substances which they abstract from 

 the soil during the whole year of almost perpetual growth, when 



