23a SCIENCE OF AGRICULTURE. Part II. 



readi 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 colored, 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 coloring 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. It was 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 with 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 difiused in a trans- 

 verse direction. 



1519. Causes of the sap^s ascent. By what power is the sap propelled ? Grew states 

 two hypothesis : 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 rarification of the air and juices of the plant. 



1520. Agenci/ of heat. Du Hamel directed his efforts to the solution of the difficulty, by endeavoring 

 to account for the phenomena 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 ; and 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, that will continue not only to vege- 

 tate, but to protrude their blossoms and mature their fruit, even in the midst of winter, when the temper- 

 ature is at the lowest. And in the case of submarine plants the temperature can never be very high ; so 

 that although heat does no doubt facilitate the ascent of the sap by its tendency to make the vessels 

 expand, yet it cannot be regarded as the efficient cause, since the sap is proved to be in motion even 

 throughout the whole of the winter. Du Hamel endeavours, however, to strengthen the operation of 

 heat by means of the influence of humidity, as being also powerful in promoting the ascent of the sap, 

 whether as relative to the season of the year or time of the day. The influence of the humidity of the 

 atmosphere cannot be conceived to operate as a propelling cause, though it may easily be conceived to 

 operate as afltbrding a facility to the ascent of the sap in one way or other ; which under certain circum- 

 stances is capable of most extraordinary acceleration, but particularly in that state of the atmosphere 

 which forbodes or precedes a storm. In such a state a stalk of wheat was observed by Du Hamel to grow 

 three inches in three days ; a stalk of barley six inches, and a .shoot of a vine almost two feet; but this 

 is a state that occurs but seldom, and cannot be of much service in the general propulsion of the sap. 

 On this intricate but important sxibject Linnseus appears to have embraced the opinion of Du Hamel, or 

 an opinion very nearly allied to it ; but does not seem to have strengthened it by any new accession of 

 argument ; so that none of the hitherto alleged causes can be regarded as adequate to the production of 

 the effect. 



1521. IrHtaMlity. Perhaps the only cause that has ever been suggested as appearing to be at all ade- 

 quate to the production of the efTect, is that alleged by Saussure. According to Saussure the cause of the 

 ap's ascent is to be found in a peculiar species of irritability inherent in the saj)-vessels themselves, and 

 dependent 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 sap by 

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

 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 bv any other adequate means ; then the 

 first cylinder being excited by the stimulus of the sap, begins gradually to contract, and to propel the con- 



