The structure of these vegetable absorbents consists of a spiral line, and not of a vessel interrupted with valves like the animal lymphatics, since on breaking almost any tender leaf and drawing out some of the fibres which adhere longest this spiral structure becomes visible even to the naked eye, and distinctly so by the use of a common lens. See Grew, Plate 51.
In such a structure it is easy to conceive how a vermicular or peristaltic motion of the vessel beginning at the lowest part of it, each spiral ring successively contracting itself till it fills up the tube, must forcibly push forwards its contents, as from the roots of vines in the bleeding season; and if this vermicular motion should begin at the upper end of the vessel it is as easy to see how it must carry its contained fluid in a contrary direction. The retrograde motion of the vegetable absorbent vessels is shewn by cutting a forked branch from a tree, and immersing a part of one of the forks in water, which will for many days prevent the other from withering; or it is shewn by planting a willow branch with the wrong end upwards. This structure in some degree obtains in the esophagus or throat of cows, who by similar means convey their food first downwards and afterward upwards by a retrograde motion of the annular muscles or cartilages for the purpose of a second mastication of it.
II. The fluids thus drank up by the vegetable absorbent vessels from the earth, or from the atmosphere, or from their own cells and interfaces, are carried to the foot-stalk of every leaf, where the absorbents belonging to each leaf unite into branches, forming so many pulmonary arteries, and are thence dispersed to the extremities of the leaf, as may be seen in cutting away slice after slice the footstalk of a horse- chesnut in September before the leaf falls. There is then a compleat circulation in the leaf; a pulmonary vein receiving the blood from the extremities of each artery on the upper side of the leaf, and joining again in the footstalk of the leaf these veins produce so many arteries, or aortas, which disperse the new blood over the new bark, elongating its vessels, or producing its secretions; but as a reservoir of blood could not be wanted by a vegetable bud which takes in its nutriment at all times, I imagine there is no venous system, no veins properly so called, which receive the blood which was to spare, and return it into the pulmonary or arterial system.
The want of a system of veins was countenanced by the following experiment; I cut off several stems of tall spurge, (Euphorbia helioscopia) in autumn, about the centre of the plant, and observed tenfold the quantity of milky juice ooze from the upper than from the lower extremity, which could hardly have happened if there had been a venous system of vessels to return the blood from the roots to the leaves.
Thus the vegetable circulation, complete in the lungs, but probably in the other part of the system deficient in respect to a system of returning veins, is carried forwards without a heart, like the circulation through the livers of animals where the blood brought from the intestines and mesentery by one vein is dispersed through the liver by the vena portarum, which assumes the office of an artery. See Note XXXVII.
At the same time so minute are the vessels in the intertexture of the barks of plants, which belong to each individual bud, that a general circulation may possibly exist, though we have not yet been able to discover the venous part of it.
There is however another part of the circulation of vegetable juices visible to the naked eye, and that is in the corol or petals of flowers, in which a part of the blood of the plant is exposed to the influence of the air and light in the same manner as in the foliage, as will be mentioned more at large in Notes XXXVII and XXXIX.
These circulations of their respective fluids seem to be carried on in the vessels of plants precisely as in animal bodies by their irritability to the stimulus of their adapted fluids, and not by any mechanical or chemical attraction, for their absorbent vessels propel the juice upwards, which they drink up from the earth, with great violence; I suppose with much greater than is exerted by the lacteals of animals, probably owing to the greater minuteness of these vessels in vegetables and the greater rigidity of their coats. Dr. Hales in the spring season cut off a vine near the ground, and by fixing tubes on the remaining stump of it, found the sap to rise twenty-one feet in the tube by the propulsive power of these absorbents of the roots of it. Veget. Stat. p. 102. Such a power can not be produced by capillary attraction, as that could only raise a fluid nearly to the upper edge of the attracting cylinder, but not enable it to flow over that edge, and much less to rise 21 feet above it. What then can this power be owing to? Doubtless to the living activity of the absorbent vessels, and to their increased vivacity from the influence of the warmth of the spring succeeding the winter's cold, and their thence greater susceptibility to irritation from the juices which they absorb, resembling in all circumstances the action of the living vessels of animals.
NOTE XXXVII—VEGETABLE RESPIRATION.
While spread in air the leaves respiring play.