nitrogen gas which more particularly serves them for ali- 

 ment. Hence it arises that vegetation is more vigorous 

 when bodies which afford this gas by their decomposition 

 are presented to the plant ; these are, animals, or vegeta- 

 bles, in a state of decomposition, or putrefaction. 



Vital air, or oxygen gas, the other constituent part of 

 atmospheric air, is necessary to the respiration of man, 

 and other animals, and this air is copiously emitted by 

 vegetables : this renovation of the atmosphere being ne- 

 cessary to make up for the great consumption of it by 

 respiration and combustion. 



Hence arises a mutual and essential dependence of the 

 animal and vegetable kingdoms upon each other. Ani- 

 mals, in breathing, consume the oxygen air, but return 

 the nitrogen for the use of the veg. viable ; while the vege- 

 table retains the nitrogen of the air, and the hydrogen of 

 the water, for its own use, and returns the oxygen for 

 ours. How admirable the designs of Providence, who 

 makes every different part of the creation thus contribute 

 to the support and renovation of each other! 



The carbonic air dispersed in the atmosphere, or in 

 water, is also necessary to the vegetation of plants, in 

 order to provide their carbonic principle, which is a con- 

 stituent part of ihejibres, oils, mucilage, and other vege- 

 table principle. It is the basis of all vegetable matter. 

 [See Note Zd, on Carbon.] 



Buds of trees may be truly esteemed individual plants ; 

 and, if one of them be planted in the earth, with a cup in- 

 verted over it, to prevent its exhalation from being at first 

 greater than its power of absorption, it will produce a 

 tree similar to its parent. 



Linnaeus observes, that trees and shrubs are roots above 

 ground ; for, if a tree be inverted, leaves will grow from 

 the root-part, and roots from the trunk part. 



Buds and bulbs are all annual productions, termed, by 

 Linnaeus, the hybernacula, or winter cradles of the plant. 

 And Darwin observes, that the same term might properly 

 be applied to seeds also. 



Each bud has a leaf, which is its lungs, appropriated to 

 it. 



Leaves absorb carbonic acid gas, or fixed air, by their 

 upper surface, and give out oxygen gas, or pure respirable 

 air, by their under surface; as first discovered by Dr. 

 Priestley. 



Dr. Ingenhousz improved upon this discovery, by observ- 



ing light to be necessary to these functions ; remarking 

 that in the dark, leaves give out a bad, or carbonic air, and 

 that fruits and flowers almost invariably give out the 

 last mentioned kind of air, at all times, but especially in 

 the dark. 



It has long been known that light acts beneficially upon 

 the upper surface of leaves, and hurtfully upon their under 

 side ; and, if the latter be repeatedly turned to the light, 

 or forcibly kept in such an unnatural position, the leaves 

 grow sickly, black, or discoloured ; as may be seen in 

 plants trained against a wall. 



Some leaves, if separated from their parent branch, and 

 suspended by a slender thread, will turn their upper sur- 

 face to the light, and vary their position, as the sun pur- 

 sues his course. Sword-shaped leaves are an exception; 

 they have no upper or under surface, but are vertical, and 

 do not alter their position. 



Light is considered as a stimulus or agent which de- 

 composes the various nutritive principles, to be found in 

 the air and water. It seems, in many instances, to be 

 the sole cause of the expansion of flowers and leaves ; for 

 when it is withdrawn, they fold together and droop, as if 

 dying ; this has been elegantly termed by Linnseus, " the 

 sleep of the plants ;" and the Mimosa pudica, or sensitive 

 plant, is a beautiful example of it. This plant, if kept in 

 a dark room, for a considerable lime, will require several 

 minutes' exposure to the solar light before the stimulus of 

 the liyht will dispose it to unfold, or expand its leaves. 



Leaves always turn towards the light ; this is necessary 

 to the formation of their colour, as may be seen by the 

 common practice of blanching celery, endive, &c. by co- 

 vering them from the light ; and by plants raised in dark- 

 ness, which are of a sickly white. 



Vegetables become destitute of smell as well as of co- 

 lour, and lose much of their combustibility by growing in 

 the dark. The celebrated Dr. Robertson, of Edinburgh, 

 gives an account of a plant found in the drain of a coal- 

 work under ground, which was very luxuriant, with large 

 indented foliage, and perfectly white. He had not seen 

 any thing like it, nor could any one inform him what it 

 was. He had the plant with a sod brought into the open 

 air in the light, when in a little time the leaves withered, 

 and soon after new leaves began to spring up, of a green 

 colour, and of a different shape from that of the old ones. 

 On rolling one of the leaves between his fingers, he found 



