1G8 SCIENCE OF GARDENING. PART II. 



such bundles. Now the colored fluid was found in each case to have passed through 

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

 out 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 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 pro- 

 longation of the central vessels, which did not however appear to be accompanied 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 alburnum, 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 con- 

 nected 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 con- 

 veyed 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 transverse direction. 



747. 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 fer- 

 mentation. 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 ac- 

 count 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. 



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

 count for the phenomenon from theagency 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 in- 

 fluence 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 ve- 

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

 temperature 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 affording 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 forebodes 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 subject Linnams 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 c-ftect. 



749. Irritability. Perhaps the only cause that has ever been suggested as appearing to be at all adequate 

 to the production of the effect, is that alleged by Saujsure. According to Saussure the cause of the sap's 



