166 



SCIENCE OF GARDENING. 



Part II. 



soft parts of woody plants, absorb moisture and inhale gases from the soil or atmosphere 

 by means of the pores of their epidennis, and thus the plant effects the intro-susception 

 of its food. 



741. Ascent of the sap. The means by which the plant effects the intro-susception of 

 its food, is chiefly that of absorption by the root. But the fluids existing in the soil when 

 absorbed by the root, are designated by the appellation of sap or lymph j which, before 

 it can be rendered subservient to the purposes of vegetable nutrition, must either be 

 intermediately conveyed to some viscus 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. jSut 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 during the winter. But though the 

 great cold of winter, as well as the great heat of summer, is by no means so favorable 

 to vegetation as the milder though more changeable temperature of spring and autumn, 

 j'et 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 sufficient that the colds of winter do not, even in this climate, entirely 

 prevent the sap from flowing. Buds exhibit a gradual developement of parts through- 

 out the whole of the winter, as may be seen by dissecting them "at different periods. So 

 also do roots. Evergreens 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. 



742. Thus the sap is in perj^etual motion loilli a more accelerated or more diminisJied 

 velocity throvghout the whole of the year ; but still there is no decided indication, exhi- 

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

 is moving at the time ; for tl^e 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 motion 

 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 tlie 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 off. This proves in- 

 dubitably that the direction of tlie sap's motion, during the season of tlie plant's bleeding, 

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

 follows that it must ascend with a very considerable force ; which force has accordingly 

 been made the subject of calculation. To tlie 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 

 guage 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, as its maximum, to a height of thirty-eight inches. But this was equivalent to 

 a column of water of tlie height of forty-three feet three and one-third inches ; demonstrat- 

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

 seemed altogether incredible. 



743. Thus the sap in ascending from the lower to the ujrper extremity of the plant isjiropelled 

 with a very considerable force, at least in the bleeding season. But is the ascending sap pro- 

 pelled 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. Tlic 

 first opinion was maintained and supported by Malpighi ; and Grew considers that tlie 



