

70 



BOT ANY. 



Sotfrci. 



Vegetable The simple fact of the increase of vegetables, is a 

 Physiology. Su f5 c j en t proof that some food is received into its sys- 

 ' J tern from the earliest period of its existence. What- 

 ever this food may !><, it seems, in the first instance, 

 to be derived from the cotyledons of the seed, and af- 

 terwards from the surrounding soil. It is also proba- 

 ble, that caCbon is the chief material ; though the man- 

 ner in which it is prepared and transmitted is not quite 

 so obvious. Water, it 13 well-known, is necessary for 

 vegetation ; and it has therefore been considered by 

 Nature.! some as the sole food of plants. Some experiments of 

 Van Helmont seemed to support this opinion ; but 

 more recent observations have shewn their insufficien- 

 cy. Other experiments by Duhamel and Tillet are 

 equally inconclusive. Braconnot too, has lately en- 

 deavoured to establish the old opinion ; but the sources 

 of fallacy are so numerous, that little dependence is 

 to be placed in his results. Plants have been suppo- 

 sed to derive nourishment from the atmosphere. This 

 we deem highly improbable, unless water be consi- 

 dered as their sole food. Various facts tend to dis- 

 prove this hypothesis. 



The soil in which plants vegetate, is the source 

 from which they have been supposed to derive food ; 

 and it has been found, that the proportion of earthy 

 matter varies with the soil. It is also highly 

 probable, that Schrccder's opinion, founded on nu- 

 merous experiments, is correct that plants pos- 

 sess the power of generating particular solid matters, 

 such as the earths and carbon, within themselves. 

 The matters contained in manures affect the plant 

 very considerably ; and it is very singular, that the 

 saline matters most injurious to vegetation are taken 

 up most abundantly, although it has been tolerably 

 well ascertained, that the root does not absorb saline 

 matters indiscriminately. 



Tin; vegetable black mould is particularly fertile, 

 and when treated with an alkaline ley, exhibits phe- 

 nomena exactly resembling those of pit coal. Ani- 

 mal manure probably furnishes the same materials as 

 vegetable manure. In what manner does these sub- 

 stances afford food to the plant ? This is a question 

 of much difficulty, and cannot be answered in the 

 present state of our chemical knowledge. It has, 

 however, been rendered probable by Mr Ellis, that 

 carbon is afforded by " spontaneous changes in the 

 seed" to the young plant, and that the agents which 

 produce these spontaneous changes are heat and mois- 

 ture. This material is, in all likelihood, afforded by 

 soils and manures, and even by substances that do not 

 contain it in any sensible quantity. We have already 

 endeavoured to shew, from Mr Ellis, that carbon is 

 not derived from the atmosphere. From the facts 

 which we possess, we feel disposed to suspect, that 

 carbon and water constitute the food of plants : 

 these, however, are elaborated in a manner much too 

 nice for our senses. We do not pretend to state that 

 the fact is proved. Sucli is the uncertainty of all 

 chemical speculations. 



Whatever may be the food of plants, and in what- 

 ever manner it may be digested, we are assured, that 

 at certain periods of the year a fluid exists abundant- 

 ly in the vessels, already described, and is evidently 

 destined to perform offices of the utmost importance 

 to the life of the plant. This fluid is named the -sap, 



and is observed, in temperate climates, to abound in Vogr.-aM 

 the spring and at the beginning of autumn. In warm Hiysioloj 

 countries it flows from palm trets at all times. The >~' 



sap is a transparent, colourless, and inodorous fluid, 

 almost devoid of taste. It exists in different states, at 

 different seasons of the year, and different parts of 

 the plant. Chemical analysis shews, that it is very 

 various, according to the plant in which it is found. 

 It has also been shewn, by the same means, that the 

 age and exposure of the tree affect the composition 

 of the sap. It is highly probable, from the single 

 fact that a grafted branch bears its own peculiar 

 fruit, that although the food is received by the root 

 it undergoes the great and essential change in some 

 parts of the plant. The circulation of this sap, or 

 its ascent to the leaves, and consequent descent to- 

 wards the roots, have been satisfactorily proved by 

 the experiments of Hilcs and Duhamel. Many hy- Circuit 

 potheses may be framed as to the mode in which this tiou> 

 circulation is effected, but we shall forbear to enter 

 into them, as tlte most perfect detail will add nothing 

 to our actual knowledge. Mr Knight seems to have 

 shewn pretty distinctly, that the s.ap ascends by 

 means of the central and alburnous vessels, in the 

 latter of which it is combined with a quantity of 

 matter deposited in the alburnum for that purpose iu 

 the course of the preceding autumn. After reaching 

 the leaf, where it undergoes some important changes, 

 as is shewn by the phenomena which there take 

 place, it is returned downwards by the cortical ves- 

 sels. In the course of this ascent and descent, after 

 forming all the new parts, the plant is employed in 

 forming new matter, which Mr Knight has shewn ig 

 deposited in the alburnum, where it remains until the 

 following spring, when it affords nourishment to the 

 buds and young leaves. From this fluid (the sap) 

 the peculiar juices of plants are also formed ; proba- 

 bly by some living action in the vegetable, similar to 

 those that produce the various secretions of the living 

 animal. See CHEMISTRY. 



Thus we find, that the perfect plant is not only Recapi 

 composed of solid parts, containing vessels which lation. 

 convey certain fluids, but that each, of these parts 

 perform determinate functions, which contribute to 

 the well-being of the whole. We have seen, that the 

 root absorbs nourishment ; that the leaves, under cer- 

 tain circumstances, absorb moisture ; that the fluid 

 nourishment taken in by the roots, circulates through 

 the stem, the leaves, and the bark ; that the plant 

 perspires an aqueous fluid ; that it forms peculiar de- 

 positions; that carbonic acid is formed by the union 

 of carbon evolved in a fluid with the oxygen of 

 the atmosphere ; that carbonic acid, taken into the 

 system with the food, and by other means, is decom- 

 posed in the parenchyma of the leaf ; that heat is 

 disengaged by the formation ; that the colouration 

 is effected by the decomposition of the carbonic 

 acid ; and that the processes of assimilation are going 

 on, as is evident to our senses, by the effects. The 

 sum of all our real knowledge in this department of 

 physics is now shewn to be extremely limited : Some 

 of these causes of this imperfection have been already 

 glanced at ; and aa they may be obviated, they fur- 

 nish many reasons that should add fresh vigour to 

 the efforts of those who have it in their power t- 



