308 



PHY 



THE UNIVERSAL HERBAL; 



PHY 



the atmosphere. Shrubs, trees, and succulent plants in par- 

 ticular, receive most of their sustenance from the air : dew, 

 mist, and rain, are also rapidly absorbed by them. The se- 

 condary vessels properly perform this function ; so do all fine 

 capillary elongations. Thus the green stalk, and the under 

 surface of the leaves, chiefly absorb these moist vapours. 

 Bonnet demonstrated this last fact by a beautiful experiment. 

 He placed a leaf of the White Mulberry tree, with its upper 

 surface upon water, and it continued six days fresh and green. 

 A leaf of the same tree, which was laid with its under surface 

 upon water, remained for six months fresh, and in good 

 condition. Plants imbibe gases also, otherwise it would be 

 impossible to explain whence they derive the great quantity 

 of carbon, of which they principally consist. 



" The function of transpiration is performed by plants 

 through apertures which are surrounded by the lymphatic 

 vessels. Bonnet anointed leaves with oil, by which means 

 the process of transpiration was completely suppressed. 

 They assumed a black colour, and decayed. I observed the 

 same thing in a hot-house plant, the leaves of which being 

 oiled in order to destroy the aphides, all fell off. Plants 

 which have been exposed to the dust, by the continuance 

 of drought, lose the leaves, merely because their pores are 

 obstructed. The number of the pores which are found on 

 the whole surface of a plant, is by no means insignificant. 

 Hedwig enumerated five hundred and seventy-seven in one 

 single quadrate line, on the surface of a leaf of the Lilium 

 bulbiferum. Thus, according to this computation, a square 

 foot would have nine hundred ninety-eight thousand seven 

 hundred and forty-five. How many square feet of surface 

 must a plant present to the air, and how great must be their 

 number in a full-grown oak! The transpiration of plants is 

 twofold, aqueous and gaseous. The aqueous is considerable ; 

 Hales made many experiments, which clearly prove this 

 assertion. A plant three feet high, lost in one hour, 1 Ib. and 

 6 oz. during tUp night: if no dew fell, it sustained a loss of 

 six ounces ; but if dew fell, the leaves had imbibed 4 or 6 

 ounces of moisture ; whereas in the day-time the transpira- 

 tion was always very considerable. Watson exposed a glass 

 of 20 square inches within, to very warm sunshine, in a place 

 where it had not rained for several months, and turning it 

 round upon a plot of mowed grass, he found it full of drops 

 of water, which ran copiously down ; he collected them by 

 an exactly weighed piece of muslin, and repeated this expe- 

 riment for several days, between 12 and 3 o'clock; hence he 

 was enabled to calculate that an acre of ground transpires 

 in 24 hours, 6400 quarts of water. Brugmanns observed a 

 particular kind of aqueous transpiration in the roots of some 

 luxuriant plants; he had put some plants of this kind into a 

 glass filled with earth, and observed at night a drop of fluid 

 in the top of the radicles ; he remarked as soon as such a 

 drop touched the roots of other plants, they dried immedi- 

 ately. If this happened frequently, the plant decayed. 

 Thus Oats, (Avena sativa,) were destroyed in this manner 

 by Serratula arvensis ; Flax, (Linum usitatissimum,') by the 

 Scabiosa arvensis and Euphorbia peplus; Wheat, (Triticum 

 asstivum,) by Erigeron acre; Buck-Wheat, (Polygonum/af/o- 

 pyrum,) by Spcrgula arvensis; Carrots, (Daucus carota,) 

 by the Inula hclenium. Hence he concludes, that weeds 

 with the fluid dropping from their radicles, suppress the 

 growth of the contiguous plants. But might not the weed 

 destroy the cultivated plant, owing to its absorbing the ali- 

 mentary matter with greater rapidity, and expanding sooner, 

 and thus prevent the further growth of the adjacent plant? 

 Drops are also frequently observed on the leaves of quick- 

 growing plants, particularly on the top. The gaseous trans- 



piration of plants was first discovered by Bonnet in 1754, 

 after him by Priestley in 1773, and next by Ingenhous in 

 1779, afterwards by other celebrated philosophers, Senne- 

 bier, Scheele, Achard, Scherer, Succou, &c. &c. No branch 

 of the physiology of plants has produced more numerous 

 experiments. The following are the results of all these labo- 

 rious investigations: Plants in sun-shine emit a great quan- 

 tity of oxygen gas, but at night exhale a kind of air which 

 is unfavourable to animal respiration. The quantity of this, 

 however, is much less than that of the oxygen lost in the 

 day-time. Thus a constant circulation takes place in the 

 atmosphere, the plants improving the air which has been 

 spoiled by the breathing of animals. The surface of leaves, 

 all green stalks, and in general the green part of vegetables, 

 exhale oxygen gas in sun-shine, but particularly green water- 

 plants, Pine-trees, Gramina, and many succulent plants. 

 The leaves of trees emit less of it than herbs. No oxygen 

 gas whatever, even when exposed to the sun, is emitted from 

 Ilex aquifolium, Primus laurocerasus, Mimosa sensitiva, Acer 

 foliis variegatis, the petala, ripe fruits, the bark of trees, 

 the pedicels, and the ribs of leaves. The gas which is emit- 

 ted during night is by far less in quantity, either pure car- 

 bonic acid gas, or, as in most cases, often mixed with hydro- 

 gen, sometimes also with azote. 



" Water is the chief nourishment of plants. They absorb 

 it out of the earth by their roots, and above the earth they 

 imbibe all the moisture which exists in the form of vapour. 

 The light by its stimulus resolves water into its constituents, 

 hydrogen and oxygen. The oxygen combines with the 

 caloric, becomes gaseous, and conducted by the air-vessels 

 runs out from the pores of the leaves. The hydrogen com- 

 bines with carbon, which plants likewise absorb, and with 

 several elements which the vegetable body receives in various 

 proportions, according to its organization, and forms the 

 juices and other substances peculiar to vegetables. At night, 

 when the light cannot effect the decomposition of water, 

 combinations and separations of another kind take place, and 

 for this reason plants then discharge carbonic acid and azotic 

 gases. The little oxygen which remains cannot stimulate 

 the fibre so powerfully, consequently the quantity of trans- 

 pired matter is much less. The stimulus which the oxygen, 

 separated by the light, has exerted upon the fibre, occasions 

 a relaxation, by which the sleep of plants, or folding of the 

 leaves, is produced. Light is absolutely necessary to plants, 

 as it nourishes them by means of its influence. If we except 

 subterraneous plants, and some species of Boletus, the vege- 

 tation of which is regulated by other principles hitherto not 

 investigated, vegetables cannot exist without the influence 

 of light. The direction and proper situation of the parts in 

 every species depend entirely upon it. Plants also, which 

 affect the shade, require light, but that only in a moderate 

 quantity; the rays of the sun would stimulate them too vio- 

 lently. Young plants, as well as most of the Cryptogamous 

 class, require-defence against too powerful light, but cannot 

 live without its influence. Trees, and the most of the Gra- 

 mina, need a great deal of light, and hence all trees have a 

 greater tendency towards the south than towards the north. 

 It is by the decomposition of water that the temperature 

 peculiar to plants is produced. Philosophers, however, are 

 not entirely agreed in their explanations of this phenomenon. 

 Sennebier and Hassenfratz assert that the oxygen, being set 

 free by the decomposition of water, unites with the caloric 

 ofthe vegetable fibre, and flows in a gaseous form from the 

 pores of the vessels. Von Humboldt, again, supposes that 

 plants absorb caloric from the atmosphere, and combine in 

 the air with the oxygen, which is separated by the influence 



