Book I. PROCESS OF VEGETABLE NUTRITION. 237 



tained fluid into the cylinder immediately above it. But the cylinder immediately above it, when acted on 

 in the same manner, is affected in the same manner ; and thus the fluid is propelled from cylinder to cylin- 

 der till it reaches the summit of the plant. So also when the first cylinder has discharged its contents into 

 the second, and is no longer acted upon by the stimulus of the sap, it begins again to be dilated to its ori- 

 ginal capacity, and prepared for the intro-susception of a new portion of fluid. Thus a supply is constantly 

 kept up, and the sap continues to flow. The above is by far the simplest as well as most satisfactory of all 

 theories accounting for the ascent of the sap. 



1522. Contraction and dilatation. Knight has presented us with a theory which, whatever may be its 

 real value, merits at least our particular notice, as coming from an author who stands deservedly high in 

 the list of phytological writers. This theory rests upon the principle of the contraction and dilatation, not 

 of the sap-vessels themselves, as in the theory of Saussure, but of what Knight denominates the silver 

 grain, assisted perhaps by heat and humidity expanding or condensing thie fluids. (Phil. Trans. 1801.) 

 Keith considers this theory of Knight as beset with many diflSculties, and the agency of the alleged cause 

 as totally inadequate to the production of the effect to be accomplished. 



1523. Elaboration of the sap. The moisture of the soil is no sooner absorbed into the 

 plant than it begins to undergo a change. This is proved by the experiment of making 

 a bore or incision in the trunk of a tree during the season of bleeding ; the sap that issues 

 from the wound possesses properties very different from the mere moisture of the soil, 

 as is indicated by means of chemical analysis, and sometimes also by means of a peculiar 

 taste or flavor, as in the case of the birch tree. Hence the sap has already undergone a 

 certain degree of elaboration ; either in passing through the glands of the cellular tissue, 

 which it reaches through the medium of a lateral communication, or in mingling with 

 the juices contained in the cells, and thus carrying off a portion of them ; in the same 

 manner, we may suppose, that water by filtering through a mineral vein becomes im- 

 pregnated with the mineral through which it passes. But this primary and incipient 

 stage of the process of elaboration must always of necessity remain a mystery to the 

 phytologist, as being wholly effected in the interior of the plant, and consequently beyond 

 the reach of observation. All he can do, therefore, is to trace out its future progress, 

 and to watch its succeeding changes, in which the rationale of the process of elaboration 

 may be more evident. 



1524. The jrrocess of elaboration is chiefly operated in the leaf: for the sap no sooner 

 reaches the leaf, than part of it is immediately carried off by means of perspiration, 

 perceptible or imperceptible ; effecting a change in the proportion of its component parts, 

 and by consequence a change in its properties. 



1525. Hales 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 diameter, 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 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. 3oz. 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 which was that, other circumstances being the same, 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 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. 



1526. A fluid little different from common water is eor/iafcrf according to the experi- 

 ments of Hales and Guettard ; in some cases it had the odor 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. 



1527. 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, wliich it sometimes wholly covers. 



1528. 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 musar arbor, or plantain-tree ; and such as are sometimes to be seen in hot and calm weather ex- 

 uding 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 



