234 SCIENCE OF'^ AGRICULTURE. Part II. 



be intermediately conveyed to some vlscus proper to give it elaboration, or immediately 

 distributed throughout the whole body of the plant. Our present object, therefore, is 

 that of tracing out the progress of its distribution or ascent. The sap is in motion in 

 one direction or other, if not all the year, at least at occasional periods, as the bleeding of 

 plants in spring and autumn sufficiently illustrates. The plant always bleeds most freely 

 about the time of the opening of the bud ; for in proportion as the leaves expand the sap 

 flows less copiously, and when they are fully expanded it entirely ceases. I5ut this sus- 

 pension is only temporary, for the plant may be made to bleed again in the end of the 

 autumn, at least under certain conditions. If an incision is now made into the body 

 of the tree, after the occurrence of a short but sharp frost, when the heat of the sun or 

 mildness of the air begins to produce a thaw, the sap will again flow. It will flow 

 even where the tree has been but partially thawed, which sometimes happens on the south 

 side of a tree, when the heat of the sun is strong and the wind northerly. At the seasons 

 now specified, therefore, the sap is evidently in motion ; but the plant will not bleed 

 at any other season of the year. It has been the opinion of some phytologists, that the 

 motion of the sap is wholly suspended durfng the winter. But though the great cold of 

 winter, as well as the great heat of summer, is by no means s 'favorable to vegetation as 

 the milder though more changeable temperature of spring and autumn, yet it does not 

 wholly suspend the movement of the sap. Palms may be made to bleed at any season of 

 the year. And although this is not the case with plants in general, yet there is proof suffi- 

 cient that the colds of winter do not, even in this climate, entirely prevent the sap from 

 flowing. Buds exhibit a gradual developement of parts throughout the whole of the 

 winter, as may be seen by dissecting them at different periods. So also do roots. Ever- 

 greens retain their leaves ; and many of them, such as the arbutus, laurustinus, and the 

 beautiful tribe of the mosses, protrude also their blossoms, even in spite of the rigor of the 

 season. But all this could not possibly be accomplished, if the motion of the sap were 

 wholly suspended. 



1514. Thus the sap is in perpetual motion with a more accelerated or more diminished 

 velocity throughout the whole of the year ; but still there is no decided indication, exhibited 

 in the mere circumstance of the plant's bleeding, of the direction in which the sap is 

 moving at the time ; for the result might be the same whether it was passing from the 

 root to the branches, or from the branches to the root. But as the great influx of the 

 sap is effected by means of the pores of the epidermis of the root, it follows that its mo- 

 tion must, at least in the first place, be that of ascent ; and such is its direction at the 

 season of the plant's bleeding, as may be proved by the following experiment : If the 

 bore or incision that has been made in the trunk is minutely inspected while the plant yet 

 bleeds, the sap will be found to issue almost wholly from the inferior side. If several 

 bores are made in the same trunk, one above another, the sap will begin to flow first from 

 the lower bore, and then from those above it. If a branch of a vine be lopped, the sap 

 will issue copiously from the section terminating the part that remains yet attached to the 

 plant ; but not from the section terminating the part that has been lopped oflT. This 

 proves indubitably that the direction of the sap's motion, during the season of the plant's 

 bleeding, is that of ascent. But if the sap flows so copiously during the season of bleed- 

 ing, it follows that it must ascend with a very considerable force ; which force has accord- 

 ingly been made the subject of calculation. To the stem of a vine cut off about two feet 

 and a half from the ground. Hales fixed a mercurial gauge which he luted with mastic ; 

 the gauge was in the form of a syphon, so contrived that the mercury might be made to 

 rise in proportion to the pressure of the ascending sap. The mercury rose accordingly, 

 and reached, at its maximum, to a height of thirty-eight inches. But this was equivalent 

 to a column of water of the height of forty-three feet three and one-third inches ; demon- 

 strating a force in the motion of the sap that, without the evidence of experiment, would 

 have seemed altogether incredible. 



1515. Thus the sap in ascending from the lower to the upper extremity of the plant is 

 propelled with a very considerable force, at least in the bleeding season. But is the as- 

 cending sap propelled indiscriminately throughout the whole of the tubular apparatus, or 

 is it confined in its course, to any particular channel ? Before the anatomy of plants had 

 been studied with much accuracy, there was a considerable diversity of opinion on the 

 subject. Some thought it ascended by the bark ; others thought that it ascended by the 

 bark, wood, and pith indiscriminately ; and others thought it ascended between the bark 

 and wood. The first opinion was maintained and supported by Malpighi ; and Grew 

 considers that the sap ascends by the bark, wood, and pith, indiscriminately. Du Hamel 

 stript several trees of their bark entirely, which ci0ntinued, notwithstanding, to live for' 

 many years, protruding new leaves and new branches as before. Knight stript the trunk 

 of a number of young crab-trees of a ring of bark half an inch in breadth, but the leaves 

 were protruded, and the branches elongated, as if the operation had not been performed. 

 Du Petit Thouars removed the central wood and pith from the stems of several young 

 sycamore trees, leaving the upper part to be supported only by four pillars of bark : in 



