238 SCIENCE OF AGRICULTURE. Part IJ. 



inches in length and one in diameter, for the purpose of introducing a supply of water, but kept alwavs 

 shut except at the time of watering. The holes at the bottom of the pot were also shut, and the pot arid 

 plant weighed for fifteen successive days in the months of July and August : hence he ascertained not only 

 the fact of transpiration by the leaves, from a comparison of the supply and waste ; but also the quantity of 

 moisture transpired in a given time, by subtracting from the total waste the amount of evaporation from 

 the pot. The final result proved that the absorbing power of the root is greater than the transpiring power 

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

 cabbage, whose mean transpiration was found to be 1 lb. 3 oz. per day ; and on some species of evergreens, 

 which were found, however, to transpire less than other plants. The same is the case also with succulent 

 plants, which transpire but little in proportion to their mass, and which as they become more firm tran- 

 spire less. It is known, however, that they absorb a great deal of moisture, though they give it out thus 

 sparingly ; which seems intended by nature for the purpose of resisting the great droughts to which they 

 are generally exposed, inhabiting, as they do for the most part, the sandy desert or the sunny rock. Along 

 with his own experiments Hales relates also some others that were made by Miller of Chelsea ; the result 

 of whiQh was that, other circumstances being the same, transpiration is in proportion to the transpiring 

 surface, and is afi'ected by the temperature of tlie air ; sunshine or drought promoting it, and cold and 

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

 and is least during the night. But when transpiration becomes too abundant, owing to excess of heat or 

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

 day, though they are again revived during the night. For the same or for a similar reason, transpiration 

 has been found also to increase as the heat of summer advances ; being more abundant in July than in June, 

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

 decrease. 



1554. A Jiuid little different 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 are 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. 



1555. Ferceptible 2)erspiratio7i, 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 covers. 



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

 of plants ; so that it is not always merely an exudation of sap, but 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 Mtisrt paradisiaca, or plantain tree ; and such as are sometimes to be seen in hot and calm weather 

 exuding from the leaves of the poplar or willow, and trickling 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 Cistus 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 Linna;us, 

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

 also to be the fact with 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 off a portion at the lea 



1557. The sap is further affected by means of the gases entering into the root along \\'ith 

 the moisture of tlie 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 of this gas effected? Is it assimilated to the vegetable substance immediately 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 when 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, Senebier 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 of 

 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 essential character of the parts decomposing carbonic 

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

 to red, do still exhale oxygen gas. 



1559. Elaboration of 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 



