52 



HISTORY OP THE VEGETABLE KINGDOM. 



Tlie leaves and greeu parts are the seat of vege- 

 table transpiration, expiration, and excretion. 

 The sap is deprived, in them, of the atmospheric 

 air which it still contains, of its superabundant 

 quantity of aqueous principles, and of the sub- 

 stances which have become foreign or useless to 

 its nutrition. But while it thus loses part of 

 the principles of which it was previously coiisti- 

 tuted, it undergoes a particular elaboration, ac- 

 quires new qualities, and, following a course the 

 reverse of that which it has already perfonned, 

 descends from the leaves towards the roots, 

 through the liber or vegetating part of tlie corti- 

 cal layers. 



The transpiration or aqueous emanation of 

 vegetables, is that function by which the sap, 

 on arriving in the leaves, loses and gives out the 

 superabundant quantity of water whicli it con- 

 tained. 



It is generally in the form of vapour tliat this 

 water is exhaled into the atmosphere. When 

 the transpiration is not great, the vapour is ab- 

 sorbed by the air as it forms ; ])ut if the quantity 

 increases, and the temperature of the atmosphere 

 is low, the liquid is seen transpiring in the form 

 of extremely small drops, which often unite to- 

 gether, and then acquire a considerable size. 

 Thus at sunrise, limpid drops are often observed 

 hanging at the point of the leaves of many 

 grasses. Cabbage-leaves also present them of 

 large size. It was long thought that tliey were 

 produced by dew; but Muschenbroek first 

 proved, by conclusive experiments, that they 

 result from vegetable transpiration, condensed by 

 tlie coldness of the night. He intercepted all 

 communication between a poppy and the ambient 

 air, by covering it witli a bell, and between it 

 and the earth, by covering the vessel in which 

 it grew with a leaden plate. Next morning the 

 drops appeared upon it as before. 



Hales, in like manner, made experiments to 

 determine the proportion existing between the 

 quantity of fluids absorbed by the roots, and 

 that exhaled by the leaves. He reared a sun 

 flower in a pot of earth, till it grew to the height 

 of three feet and a half; he then covered the 

 mouth of the pot with a plate of lead, which he 

 cemented so as to prevent all evaporation from 

 the earth contained in it. In this plate he fixed 

 two tubes, the one nine inches in length, and of 

 but small diameter; left open to serve as a 

 medium of communication with the external air, 

 the other two inches in length and one in diame- 

 ter, for the purpose of introducing a supply of 

 water, but kept always shut, except at the time 

 of watering. The holes of the bottom of the 

 pot were also shut, and the pot and plant were 

 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 wa.ste, but 



also the quantity of moisture transpired iu a 

 given time, by subtracting from the total waste 

 the amount of evaporation from the pot. In a 

 dry and hot day, it transpired the most, and in a 

 damp and wet day, it transpired the least. The 

 mean rate of transpiration being one pound four 

 ounces, that is, seventeen times more in propro- 

 tion than that from the human body. In a hot 

 and dry night without dew, it transpired tlu'ee 

 ounces ; in a dewy night it did not transpire at 

 all ; and in a rainy night or night of much dew, 

 its weight was increased by three ounces. Hales 

 suspected that the quantity transpired was in 

 proportion to the extent of the surface of the 

 leaves, which he regarded as the principal organs 

 of transpiration, and ascertained also the relative 

 proportion of the capacity of the leaves for tran- 

 spiration as compared to the capacity of the root 

 for absorption. The surface of the leaves and 

 the stem of the plant which was the subject of 

 experiment was found to be equal to about S61G 

 square inches, and the surface of the root of 

 the same plant, or rather, of a plant of nearly 

 the same size, was found to be about 228(i 

 square inches ; the latter being to the fonner in 

 the proportion of two to five ; from which it fol- 

 lows that the absorbing power of the I'oot is 

 greater than the transpiring power of the leaves, 

 in the proportion of five to two. Similai' ex- 

 periments were also made upon some species of 

 cabbage, whose mean transpiration was found 

 to be one pound three ounces a 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 tran- 

 spire but little in proportion to their mass, and 

 which, as they become more firm, transpire less. 

 It is known, however, that they absorb a great 

 deal of moisture, though they give it out thus 

 sparingly, which we cannot but regard as a wise 

 institution in nature, f^the purpose of resisting 

 the great droughts to which they are generally ex- 

 posed, inhabiting as they do for the most part the 

 sandydesertorthe sunny rock. Hales also relates 

 corroborative experiments to his own, made by Mr 

 Miller of Chelsea. The result of this was, that, 

 other circumstances being the same, transpiration 

 is in proportion to the extent of transpiring sur- 

 face, and is affected by the temperature of the 

 air, sunshine, state of moisture, and dryness. It 

 is also greatest from six in the morning till noon, 

 and is least during the night; but when tran- 

 spiration becomes too abundant, owing to excess 

 of heat or drought, the plant immediately suft'ei-a 

 and begins to languish ; hence the leaves drop 

 during the day, though they are again revived 

 during the night. For similar reasons, transpira- 

 tion increases as the 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 



