354 



THE FARMERS' REGISTER. 



40,000, and in that of Mesembrvanlhenum at 

 70,000, and of an Aloe at 45,000"'; the first in- 

 habiting llie damp ditches ol' India, the last two 

 natives of the dry rocks of the Cape of Good 

 Hope : but the islomates of C.-inuni annabile are 

 among the largest that are ivnown, and liiose ol 

 Mesembryanihe.num and Aloe are among ilie 

 smallest; so that the 70,000 of the lormer are 

 not equal to 10.000 of the Crinum. Again, the 

 Yucca aluifolia has four limes as many siomaies 

 as a species of Cotyledon in my collection, but 

 those of the latter are about the , it; o' a" '"^h 

 in their longer diameter, large and active, while 

 the siomates of the Yucca are not more than 

 ^3*jjf of an inch long in the aperture, and corp- 

 paratively inert. The Yucca, therefore, with its 

 numerous siomates, has weaker powers of per- 

 epiraiion and respiraiion than the Cotyledon. 



A leaf, then, is an appendage of the stem of a 

 plant, consisting of an expansion of the cellular 

 rind, into wiiich^eins are introduced, and en- 

 closed in a skin mrougii which respiration and 

 perspiration take place. It is in reality a natural 

 contrivance lor exposing a large surface to the 

 influence of external agents, by whose assistance 

 the crude sap contained in the stem is altered 

 and rendered suitable ,to the particular wants o! 

 the species, and for returning into the general 

 circulation the fluids in their matured condition. 

 In a word, the leaf of a plant is its lungs and 

 stomach, traversed by a system of veins. 



As the leaf is an extension of the rind of a 

 stem, its epidermis is also an extension of the 

 slcin of the same part; and hence it is that in 

 plants which produce no true leaves, such as 

 the Stapelia, the office of the leaf is performed by 

 the riad and epidermis of the bark. 



The functions of respiration, perspiraiion, and 

 digestion, which are the particular oflices of leaves, 

 are essential to ihe health of a plant ; its healihi- 

 ness being in proportion to the degree in which 

 these functions are duly performed. Consequent- 

 ly, whatever tends to impede the free action of 

 leaves, tends also to diminish the healthiness of 

 a plant. 



These functions are performed by means of 

 the vital forces of vegetation, which we can- 

 not estimate or comprehend, assisted by the in- 

 fluence ol' an external agent, the nature of whose 

 action may be understood li-om its ellects. That 

 agent is solar light. 



It is the property of solar light, when striking 

 upon the leaf ol a plant, to cause: 1. A decom""- 

 position of carbonic acid ; 2. An extiication ol 

 nitroaen; and, 3. Insensible perspiration. By 

 ttyeir vital Ibrces planis appear to decompose 

 water, independentlyof the action of light. 



Carbonic acid is originally introduced into the 

 interior of a plant, eiiher dissolved in the wafer 

 it imbibes by iis roots, or by attraction from the 

 atmosphere, or by the combination of the oxygen 

 obtained by a decomposition of water or oiher- 

 wise, with the carbon in its interior. When a 

 leaf is exposed to the direct influence of the sun, 

 it gives otfoxygen, by decomposing the carbonic 

 acid; whereupon the carbon remains behind in 

 the interior of the leaf in a solid state.* Al- 



* [It remains behind, not in the state oi uncombined 

 iolid. carbon, or eharcoal, as the reader unlearned in 

 lehemistry mio;ht 9ti^)po8e from the statement in the 



though the nature of the air thus extricated cao 

 only be determined by a chemist, yet the extri- 

 cation itself can be easily seen by any one who 

 will plunge a leaf in water and expose it to the 

 sun ; Ibr bubbles of oxygen vtWi be seen to form 

 themselves upon 'he surface of the leal. But, 

 if the same leaf be observed in the lo'al absence 

 of solar light, there will be liitle or no extrication 

 of air, and what Utile is given off will be found 

 to be carbonic acid, which plants exhale at all 

 times in small (juantities ; oxygen, however, 

 which was before expelled, is inhaled. Hence 

 plants decompose carbonic acid during the day, 

 and ibrm it again during the night, the oxygen 

 they inhale at that time entering again into com- 

 bination with their carbon ; and, during the heal- 

 thy state of a plant, the decomposition by day, 

 and recomposition by night, of this gaseous mat- 

 ter, is perpetually going on.* The quantity of 



text, but as one of the chemical components of starch, 

 su^ar, gum, tartaric or malic acid, organic tissue, or 

 other product of the plant, whether solid or liquid. 

 A. G.] 



* [This absorption of oxygen and recomposition of 

 carbonic acid during ttie lught, might perhaps be lelt 

 out of the account m a general view of the subject, 

 except as an explanation ot the manner in which 

 planis are injured or destroyed by the protracted ab- 

 sence of liaht. According to the celebrated chemist 

 from whom the following remarks are cited, this pro- 

 cess is not at all connected with the life or growth 

 of vegetables, but is entirely chemical. "It is true 

 that the decomposition of caibomc acid is arrested by 

 the absence ot light. But then, namely, at night, a 

 true chemical jirocess commences, in consequence 

 of the action ot the oxygen in the air upon the or- 

 ganic substances composing the leaves, biosscms, and 

 Iruit. . . The substances composing the leaves of dif- 

 ferent plants being known, it is a matter of the great- 

 est ease and certainty to calculate which of them, during 

 life, should absorb most oxygen by chemical action, 

 when the influence of light is withdrawn. . . . Whilst 

 the tasteless leaves of Jigave Americana absorb only 

 0.3 of their volume of oxygen, in the dark, during 24 

 hours, the leaves of the Pinus abies, which contain 

 volatile and resinous oils, absorb 10 times ; those of 

 Quercus robur containing tannic acid 14 times; and 

 the balmy leaves of the Popalus alba 21 times that 

 quantity. This chemical action is shown very plainly, 

 also, in the leaves of the Cotyledon calycinum, the 

 Cacalia Jicoides, and others ; for they are sour, like 

 sorrel, in the morning, tasteless at noon, and bitter in 

 the evening. The formation of acids is eliected du- 

 ring the night by a true process of oxidation : these 

 are deprived ot their acid properties during the day 

 and evening, and are changed, by the separation of a • 

 part of tlieir oxygen, into compounds containing 

 oxygen and hjdrogen either in the same proportions 

 as in water, or even with an excess of hydrogen, 

 which is the composition of all tasteless and bitter 

 substances. . . . Most vegetable physiologists have 

 connected the emission of carbonic acid during the 

 night with the absorption of oxygen from the atmo- 

 sphere ; and have considered these actions as a true 

 process of respiration in plants, similar to that of ani- 

 mals, and, like it, having for its result the separation 

 of carbon from some of their constituents. This opi- 

 nion has a very weak and unstable foundation. The 

 carbonic acid, which has been absorbed by the leaves 

 and by the roots, together with water, ceases to be 

 decomposed on the departure of daylight. It is dis- 

 solved in the juices which pervade all parts of the 

 plant, and escapes every moment through the leaves, 

 in quantity corresponding to the water which evapo- 

 rates. . . . Plants during their life constantly possess 



