2S8 8CIENCE OF AGRICULTURE. Part II. 



inches in length .in ! one in duui Introducing a suppl) of water, but kept always 



abut except at the time of watering, Tin- hole» al the bottom of the pot were also shut, and the pot and 

 plant weighed lot fifteen successive days In the xnonths of Jul] and August : hence he ascertained not only 



the fact of trantplral 1>\ the leaves, from a comparison of the supply and waste ; but also the quantity of 



moisture transpired In ■ given time, by subtracting from the total waste the amount of evaporation from 

 the pot The Dnal result proved that the absorbing power of the root is greater than the tram>piring power 

 of the leaves, in the proportion ol five to two Similar experiments were also made upon some species o( 

 cabbage, whose mean transpiration was found to be 1 lb 3 ox. pet <i.iy ; and on some species of e\ ergreena, 



e found, however, to transpire less than other plants Tin- same is the case also with succti 1 

 pl int-. which transpire but little in proportion to their mass, ana which as they become more linn tran- 

 spire less It i- known, how eve r, thai the) absorb a great deal oj moisture, though they give it out thus 

 sparingly ; winch seems intended bj nature tor the purpose of resisting the great droughts to winch they 

 generally exposed, inhabiting, as they do lor the mosl part, the sandy d< serl r the sunnj rock. Along 

 with his own experiments Hales relates ah hers that were made by Miller of Chelsea ; the result 



of whuh was that, other circumstances being the tame, transpiration is in proportion to the transpiring 

 surface, and is affected by the temperature of the air; sunshine or drought promoting it, and cold and 

 wet diminishing or suppressing it entirely. It is also greatest from us o'clock in the morning tit! noon, 

 ami is least during the I 1 hi But when transpiration 1 ecomi ■ too abundant, owing ol heat or 



drought, the plant immediately suffers and begins to languish] and hence the leaves droop during the 



. though they .ire eg on re\ ived during the night. For the same or lor a similar reason, transpiration 

 has been found also! IS the heat of summer advances ; being more abundant in July than in June, 



and still more in August than in eitiier of the preceding months, from which last period it begins again to 

 decrease. 



1554. A fluid little (liferent from common water is exhaled, according to the experi- 

 ments of Hales and Guettard ; in some cases it had the odour of" the plant; but Du 

 Hamel found that it became sooner putrid than water. Such then arc the facts that have 

 been ascertained with regard to the imperceptible perspiration of plants, from which it 

 unavoidably follows that the sap undergoes a very considerable modification in its passage 

 through the leaf. 



1.55.5. Perceptible perspiration, which is an exudation of sap too gross or too abundant 

 to be dissipated immediately, and which hence accumulates on the surface of the leaf, is 

 the cause of its further modification. It is very generally to be met with, in the course of 

 the summer, on the leaves of the maple, poplar, and lime tree ; but particularly on the 

 surface exposed to the sun, which it sometimes wholly cove;-s. 



1556. The physical as veil as chemical qualities of perspired matter are very different in different species 

 of plants ; so that it is not always merely an exudation ol sap, hut of sap in a high state of elaboration, or 

 mingled with the peculiar juices or secretions of the plant. Sometimes it is a clear and watery fluid con- 

 glomerating into large drops, such as are said to have been observed by Miller, exuding from the leaves 

 of the Musa paradislaca, or plantain tree ; and such as are sometimes to be sem in hot and calm weather 

 exuding from the leaves of the poplar or willow, and tinkling down in such abundance as to resemble a 

 slight shower. This phenomenon was observed by Sir J. E. Smith, under a grove of willows in Italy, and 

 is said to have occurred even in England. Sometimes it is glutinous, as on the leaf of the lime tree ; 

 sometimes it waxy, as on the leaves of rosemary ; sometimes it is saccharine, as on the orange leaf; or 

 resinous, as on the leaves of the t'istus creticus. The cause of this excess of perspiration has not yet been 

 altogether satisfactorily ascertained ; though it seems to be merely an effort and institution of nature to 

 throw off all such redundant juices as may have been absorbed, or secretions as may have been formed, 

 beyond what are necessary to the due nourishment or composition of the plant, or beyond what the plant 

 is capable of assimilating at the time. Hence the watery exudation is perhaps nothing more than a re- 

 dundancy of the fluid thrown off by imperceptible perspiration, and the waxy and resinous exudations 

 nothing more than a redundancy of secreted juices; all which may be still perfectly consistent with a 

 healthy state of the plant. But there are cases in which the exudation is to be regarded as an indication 

 of disease, particularly in that of the exudation known by the name of honey-dew, a sweet and viscid 

 substance covering the leaves like a varnish, and sometimes occasioning their decay. Such at least seems 

 to be the fact with regard to the honey-dew of the hop, which, according to the observations of Linnaeus, 

 is the consequence of the attacks of the caterpillar of the ghost-moth injuring the root; and such seems 

 also to be the fact m itli regard to the honey-dew of the beech tree, and perhaps also the honey-dew of the 

 oak. The sap then, in the progress of its ascent from the extremity of the root to the extremity of the 

 leaf, undergoes a considerable change, first in its mixing with the juices already contained in the plant, 

 and then in its throwing ofi'a portion at the leaf. 



I. '>M. The sap is further affected by means of the gases entering into the root along with 

 the moisture of the soil, but certainly, by means of the gases inhaled into the leaf; the 

 action and elaboration of which shall now be elucidated. 



1558 Elaboration of carbonic acid. The utility of carbonic acid gas, as a vegetable food, has been al- 

 ready shown ; plants being found not only to absorb it by the root along with the moisture of the soil, but 

 also to inhale it by the leaves, at least when vegetating in the sun or during the day. But how is the ela- 

 boration ofthi- gas 1 fleeted? Is it assimilated to the vegetable substance imme d iately upon entering the 

 plant, or is its assimilation effected by means of intermediate steps ? The gas thus inhaled or absorbed is 

 not assimilated immediately, or at least not wholly : for it is known that plants do also evolve carbonic 

 acid gas w hen vegetating in the shade, or during the night. Priestley ascertained that plants vegetating in 

 confined atmospheres evolve carbonic acid gas in the shade, or during the night, and that the vitiated 

 state of their atmospheres after experiment is owing to that evolution ; and Saussure that the elaboration 

 of carbonic acid gas is essential to vegetation in the sun ; and, finally, Si nebier and Saussure proved that 

 the carbonic acid gas contained in water is abstracted and inhaled by the leaf, and immediately decom- 

 posed ; the carbon being assimilated to the substance of the plant, and the oxygen in part evolved and 

 in part also assimilated. The decomposition of carbonic acid gas takes place only during the light of day, 

 though Saussure has made it also probable that plants decompose a part of the carbonic acid gas, which 

 they form with the surrounding oxygen, even in the dark. But the effect is operated chiefly by means ol 

 the leaves and other green parts of vegetables, that is, chiefly by the parenchyma ; the wood, roots, petals, 

 and leaves that have lost their green colour, not being found to exhale oxygen gas. It may be observed, 

 however, that the green colour is not an absolutely essentia] character of the parts decomposing carbonic 

 acid ; because the leaves of a peculiar variety of the .1 'triplex horU'nsis, in which all the green parts change 

 to red, do still exhale oxygen gas. 



15 '& Flaboration iff oxygen. It has been already shown that the leaves of plants abstract oxygen from 

 confined atmospheres, at least when placed in the shade, though they do not inhale all the oxygen that 

 disappears , and it has been further proved, from experiment, that the leaves of plants do also evolve a 

 gas in the sun. From a great variety of experiments relative to the action and influence of oxygen on the 



