42 



HISTORY OF THE VEGETABLE KINGDOM. 



tliouglit, to conclude tliat leaves are equivalent 

 to the lungs of animals. Another argument in 

 support of the doctrine was deduced from Du 

 Hamel's experiment of besmearing the surface 

 of the leaf with oil, in consequence of which 

 treatment it soon died, owing, as it appeared, to 

 the exclusion of air. Modern chemistry has af- 

 forded us many other proofs of the functions of the 

 leaves of plants. The experiments of Priestley, 

 Ellis, Decandolle, and others, have fully estab- 

 lished both the absorption and exhalation of 

 gases through the medium of the leaves. 



From some recent experiments also it appears 

 that vegetables, by decomposition of moisture, 

 can supply an atmosphere to themselves ; and 

 thus, that plants will grow and thrive when in- 

 closed in glass cases, perfectly impervious to ex- 

 ternal changes of atmosphere, simply by decom- 

 posing the water of the moist soil, with which 

 they are famished.* 



The decomposition of the carbonic acid ab- 

 sorbed from the air is effected in the parenchyma 

 of the leaves, as well as in all the other green 

 and herbaceous parts of the vegetable. When 

 vegetables are exposed to the action of the sun, 

 they are decomposed, the air retaining the carbon, 

 and disengaging the oxygen. The reverse takes 

 place when they are withdrawn from the influ- 

 ence of light, in which case they extract from 

 the air a portion of its oxygen, which they re- 

 place by disengaging an equal quantity of car- 

 bonic acid gas. It is well known that vegetables, 

 when removed from the influence of the sun, 

 become blanched ; in other words, lose their green 

 colour, are rendered soft and watery, and contain 

 a larger proportion of saccharine principle. But 

 we shall speak more particularly of the phe- 

 nomena of absorption and transpiration, when 

 we come to treat of nutrition in plants. 



The leaves are susceptible of certain motions 

 evidently depending upon the irritability of 

 which they are possessed. This property in 

 plants is clearly established by numerous and 

 authentic facts. If a branch, stiU attached to 

 its stem, be so placed that the lower surface of 

 the leaves is turned upwards, these will be seen to 

 tui-n gradually round and resume their natural 

 position. This fact may be daily observed in 

 pruning and palisading espaliers; such as the 

 peach, the vine, &c. 



Compound and articulated leaves, or those whose 

 leaflets are attached by a joint to the common 

 petiole, present the most remarkable motions. 

 Thus the leaflets of many legumines, whose 

 leaves are all articulated, have a different posi- 

 tion at night from that which they occupy in 

 the day. Linnaeus called this singular pheno- 

 menon the sleep of plants. The leaflets of the 



* Tlie apparatus fur this purpose shall be described 



aflerwurds. 



acacia, for example, are extended nearly in a 

 horizontal direction at sunrise ; but as the day 

 advances, they gradually rise, and at length be- 

 come almost vertical, falling again as the day 

 declines. Otlier plants present similar pheno- 

 mena, depending upon the influence of light, as 

 may be inferred from the ingenious experiments 

 of 'Decandolle. That excellent botanist having 

 placed some plants with compound leaves in a 

 dark cellar, changed their hours of sleeping and 

 waking, by depriving them of light during the 

 day, and exposing them to a strong light at 

 night. 



The leaves of certain plants also pei-form mo- 

 tions depending upon irritability, and which can- 

 not be attributed to the influence of light alone, 

 as in the sensitive plant. The slightest shock, the 

 least agitation of the air, the shadow of a cloud 

 or of any other body, the action of the electric 

 fluid, heat, cold, irritating vapoura, such as 

 chlorine, or nitrous gas, each of these is sufficient 

 to cause its leaflets to perform the most singular 

 motions. If one of them be touched, it raises 

 itself against the one which is opposite to it, and 

 presently all the other leaflets of the same stalk 

 perform a similar motion, until at length they 

 cover each other like tiles on the roof of a house. 

 The leaf itself soon after bends towards the 

 ground. But, in a' short time, if the exciting 

 cause has ceased to operate, all these parts which 

 seemed withered, resume their natural aspect and 

 position. 



That singular plant the hedt/sarum gyrans, 

 which is a native of Bengal, presents very re- 

 markable motions. Its leaves are single, and 

 have two small lateral stipules. The two stipules 

 perform a twofold motion of bending and twist- 

 ing upon themselves, which in the one appears 

 to be independent of that of the other; for one of 

 them sometimes moves with rapidity, while the 

 other continues at rest. This motion takes place 

 without the intervention of any external stimulus, 

 and is not suspended at night. The motion of 

 the leaflet, on the contrary, appears to depend 

 upon the action of light, and ceases when tlie 

 plant is withdrawn from it. 



The fly-trap (^Dimueci, 

 Muscipula, ) a native of 

 North America, has two 

 lobes connected by ^n inter- 

 mediate hinge, at the extre- 

 mity of its leaves. When 

 an insect or any other body 

 touches and irritates one of 

 the small glandular bodies 

 which are observed on their 

 upper surface, the two lobes 

 quickly rise, approach each 

 other, an'3 seize the insect by which they were 

 irritated. From -this circumstance, the plant 

 has received tlie vulgar name oi fly-trap. But it 



