Book I. PROCESS OF VEGETABLE NUTRITION. 



169 



ascent is to be found in a peculiar species of irritability inherent in the sap-vessels themselves and de 

 pendent upon vegetable life ; in consequence of which they are rendered capable of a certain degree of 

 contraction, according as the internal surface is affected by the application of stimuli, as well as of subse- 

 quent dilatation according as the action of the stimulus subsides ; thus admitting and propelling the kid 

 by alternate dilatation and contraction. In order to give elucidation to the subject, let the tube be sup- 

 posed to consist of an indefinite number of hollow cylinders united one to another, and let the sap be 

 supposed to enter the first cylinder by suction, or by capillary attraction, or by any other adequate 

 means; then the first cylinder being excited by the stimulus of the sap, begins gradually to contract, and 

 to propel the contained 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 pro- 

 pelled from cylinder to cylinder 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 original 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. 



7j0. Contraction and dilatation. Knight has presented us with a theory which, whatever maybe 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 the fluids. (Phil. Trans. 1801.) 

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

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



751. 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. 



752. The process 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. 



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 ex- 

 cept 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 cab- 

 bage, 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 transpire 

 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 List period it begins 

 again to decrease. 



753. A Huid little different from common wa'er is exhaled according to the experi- 

 ments of Hales and Guetard ; 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. 



754 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 



