234 BCIENCE OF AGRICULTURE. Part II. 



it is not also effected by die epidermis of the other parts of the plank We can scarcely 

 suppose it to be effected by the dry and indurate epidermis of the bark of aged trunks, 

 of which die original organisation is obliterated ; nor by that of the larger and more aged 

 branches. But it has been thought that there are even some of the soft and succulent parts 

 of the plant by which it cannot be effected, because no pores are visible in their epidermis. 

 Decandolle found no pons in the epidermis of fleshy fruits, such as pears, peaches, and 

 gooseberries; nor in that >>t' roots, or scales of bulbs; nor in any part not exposed to 

 the influence of air and light. It is known, however, that fruits will not ripen, and that 

 roots will not thrive, if wholly deprived of air ; and hence it is probable that they inhale 

 it by their epidermis, though the pores by which it enters should not he visible. In the 

 root, indeed, it may possibly enter in combination with the moisture of the soil ; but in 

 the Other parts of the plant it enters no doubt in the state of gas. Herbs, therefore, and 

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

 Bpbere by means of the pores of their epidermis, and thus the plant effects the intro- 

 susception of its food. 



15M. Ascent of the sap. The means by which the plant effects the introsusception 

 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 ; which, 

 Ik lore 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. But 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 w ill 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 favourable to vegetation as 

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

 wholly suspend the movement of the sap. Balms 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 suf- 

 ficient 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 rigour of the 

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

 wholly suspended. 



1540. 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 

 m 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 off. 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 w ith 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 siphon, so contrived that the mercury might be made to 

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



