374 Catalogue of Works on Gardening, fyc. 



in which saltpetre exists in the soil in the greatest abundance. Nitrate of 

 soda, also, in this country, has been found wonderfully to promote vegetation 

 in many localities ; and it is a matter of frequent remark, that vegetation 

 seems to be refreshed and invigorated by the fall of a thunder-shower. There 

 is, therefore, no reason to doubt that nitric acid is really beneficial to the 

 general vegetation of the globe. And since vegetation is most luxuriant in 

 those parts of the globe where thunder or lightning is most abundant, 

 it would appear as if the natural production of this compound body in the air, 

 to be afterwards brought to the earth by the rains, were a wise and beneficent 

 contrivance by which the health and vigour of universal vegetation is intended 

 to be promoted. 



" It is from this nitric acid, thus universally produced and existing, that 

 plants appear to derive a large, probably, taking vegetation in general, the 

 largest portion of their nitrogen. In all climates they also derive a portion 

 of this element from ammonia; but less from this source in tropical than in 

 temperate climates. (For fuller information on this point, see the Author's 

 Lectures on Agricultural Chemistry and Geology, Part I.)" 



Plants derive a portion of their nourishment from the atmosphere; which 

 consists of a mixture of oxygen and nitrogen gases, with a minute quantity of 

 carbonic acid, and a variable proportion of watery vapour. The carbonic 

 acid affords an important part of their food to plants ; and the watery vapour 

 aids in keeping their surfaces in a moist and pliant state. The various 

 substances which constitute the food of plants are decomposed in the interior 

 of their vessels, and recompounded so as to form new substances. The 

 leaves of plants are spread out in the air for the same purpose for which the 

 fibres are extended through the soil; and while the roots suck in chiefly 

 liquid food, the leaves inhale almost solely gaseous matters. " In the sun- 

 shine, the leaves are continually absorbing carbonic acid from the air and giving 

 off oxygen gas. When night comes, this process ceases, and they begin to absorb 

 oxygen and to give off carbonic acid. It has been ascertained, that in our 

 climate on an average, not less than from one third to three fourths of the 

 entire quantity of carbon contained in the crops we reap from land of average 

 fertility is really obtained from the air. 



" We see, then, why, in arctic climates, where the sun once risen never sets 

 again during the entire summer, vegetation should almost rush up from the 

 frozen soil, the green leaf is ever gaining from the air and never losing, ever 

 taking in and never giving off", carbonic acid, since no darkness ever interrupts 

 or suspends its labours." 



In the growth of plants from seed, the starch is changed into sugar ; and 

 when the shoot first becomes tipped with green, the starch is again changed 

 into the woody fibre. The seed also contains gluten; and, neither the gluten 

 nor the starch being soluble in water, it is so arranged that when the seed 

 first shoots, there is produced at the base of the germ, from a portion of the 

 gluten, a small quantity of a substance called diastase, which has the power 

 of rendering the starch soluble in the sap. 



" This change of the sugar of the sap into woody fibre is observable more 

 or less in all plants. When they are shooting fastest the sugar is most 

 abundant ; not, however, in those parts which are growing, but in those which 

 convey the sap to the growing parts. Thus, the sugar of the ascending sap of 

 the maple and the alder disappears in the leaf and in the extremities of the 

 twig; thus the sugar-cane sweetens only a certain distance above the ground, 

 up to where the new growth is proceeding; and thus, also, the young beet and 

 turnip abound most in sugar, while in all these plants the sweet principle 

 diminishes as the year's growth draws nearer to a close. 



" In the ripening of the ear also, the sweet taste, at first so perceptible, 

 gradually diminishes and finally disappears; the sugar of the sap is here 

 changed into the starch of the grain, which, as above described, is afterwards 

 destined, when the grain begins to sprout, to be reconverted into sugar for 

 the nourishment of the rising germ. 



