2S6 SCIENCE OF AGRICULTURE. Part II. 



apple and horsechestnut, by means of circular incisions, so as to leave detached rings of 

 bark with insulated leaves remaining on the stem. He then placed them in coloured 

 infusions obtained by macerating the skins of very black grapes in water ; and, on 

 examining the transverse section at the end of the experiment, it was found that the 

 infusion had ascended by the wood beyond his incisions, and also into the insulated 

 leaves, but had not coloured the pith nor bark, nor the sap between the bark and wood. 

 From the above experiment, Knight concludes that the sap ascends through what are 

 called the common tubes of the wood and alburnum, at least till it reaches the leaves. 

 Thus the sap is conveyed to the summit of the alburnum. But Knight's next ob- 

 ject was to trace the vessels by which it is conveyed into the leaf. The apple tree and 

 horsechestnut were still his subjects of experiment. In the former the leaves are 

 attached to the plants by three strong fibres, or rather bundles of tubes, one in the 

 middle of the leaf-stalk, and one on each side. In the latter they are attached by means 

 of several such bundles. Now the coloured fluid was found in each case to have passed 

 through the centre of the several bundles, and through the centre only, tinging the tubes 

 throughout almost the whole length of the leaf-stalk. In tracing their direction from the 

 leaf-stalk upwards, they were found to extend to the extremity of the leaves ; and in 

 tracing their direction from the leaf-stalk downwards, they were found to penetrate 

 the bark and alburnum, the tubes of which they join, descending obliquely till they 

 reach the pith which they surround. From their position Knight calls them central 

 tubes, thus distinguishing them from the common tubes of the wood and alburnum, and 

 from the spiral tubes with which they were every where accompanied as appendages, as 

 well as from a set of other tubes which surrounded them, but were not coloured, and 

 which he designates by the appellation of external tubes. The experiment was now 

 transferred to the flower-stalk, and fruit -stalk, which was done by placing branches 

 of the apple, pear, and vine, furnished with flowers not yet expanded, in a decoction 

 of logwood. The central vessels were rendered apparent as in the leaf-stalk. When 

 the fruit of the two former was fully formed, the experiment was then made upon the 

 fruit-stalk, in which the central vessels were detected as before ; but the colouring matter 

 was found to have penetrated into the fruit also, diverging round the core, approaching 

 again in the eye of the fruit, and terminating at last in the stamens. This was effected by 

 means of a prolongation of the central vessels, which did not however appear to be accom- 

 panied by the spiral tubes beyond the fruit-stalk. Such then are the parts of the plant 

 through which the sap ascends, and the vessels by which it is conveyed. Entering by the 

 pores of the epidermis, it is received into the longitudinal vessels of the root by which it 

 is conducted to the collar. Thence it is conveyed by the longitudinal vessels of the albur- 

 num, to the base of the leaf-stalk, and peduncle ; from which it is further transmitted 

 to the extremity of the leaves, flower, and fruit. There remains a question to be 

 asked intimately connected with the sap's ascent. Do the vessels conducting the sap 

 communicate vsath one another by inosculation or otherwise, so as that a portion of their 

 contents may be conveyed in a lateral direction, and, consequently, to any part of the plant; 

 or do they form distinct channels throughout the whole of their extent, having no sort of 

 communication with any other set of tubes, or with one another ? Each of the two 

 opinions implied in the question has had its advocates and defenders : but Du Hamel and 

 Knight have shown that a branch will still continue to live, though the tubes leading 

 directly to it are cut in the trunk ; from which it follows that the sap, though flowing 

 the most copiously in the direct line of ascent, is at the same time also diffused in a trans- 

 verse direction. 



1545. Causes of the sap's ascent. By what power is the sap propelled ? Grew states 

 two hypotheses : its volatile nature and magnetic tendency, aided by the agency of ferment- 

 ation. Malpighi was of opinion that the sap ascends by means of the contraction and 

 dilatation of the air contained in the air-vessels. M. De la Hire attempted to account for 

 the phenomenon by combining together the theories of Grew and Malpighi ; and Borelli, 

 who endeavoured to render their theory more perfect, by bringing to its aid the influence 

 of the condensation and rarefaction of the air and juices of the plant. 



1546. Agency of heat. Du Hamel directed his efforts to the solution of the difficulty, by endeavouring 

 to account for the phenomenon from the agency of heat, and chiefly on the following grounds : because 

 the sap begins to flow more copiously as the warmth of spring returns ; because the sap is sometimes found 

 to flow on the south side of a tree before it flows on the north side, that is, on the side exposed to the 

 influence of the sun's heat sooner than on the side deprived of it ; because plants may be made to vegetate, 

 even in the winter, by means of forcing them in a hot-house ; and because plants raised in a hot-house 

 produce their fruit earlier than such as vegetate in the open air. There can be no doubt of the great 

 utility of heat in forwarding the progress of vegetation ; but it will not therefore follow that the motion 

 and ascent of the sap are to be attributed to its agency. On the contrary, it is very well known that if 

 the temperature exceeds a certain degree, it becomes then prejudicial both to the ascent of the sap and 

 also to the growth of the plant. Hales found that the sap flows less rapidly at mid-day than in the 

 morning ; and every body knows that vegetation is less luxuriant at midsummer than in the spring. So 

 also, in the case of forcing, it happens but too often that the produce of the hot-house is totally destroyed 

 by the unskilful application of heat If heat is actually the cause of the sap's ascent, how comes it that the 

 degree necessary to produce the effect is so very variable, even in the same climate ? For there are many 

 plants, such as the arbutus, laurustinus, and the mosses, which will continue not only to vegetate. 



