787. Tendency of the flow. The fluids which are thus taken into the system by absorption can not remain inactive and stagnant. As their inward flow is regular and constant in its season, so must be their upward and outward flow, in a course more or less direct, toward the parts where they find an outlet or a permanent fixture.

788. In those Cryptogams which are composed of cellular tissue alone the circulation of the sap consists only of a uniform diffusion from cell to cell throughout the mass, as through a sponge.

789. In the higher plants, the different tissues perform appropriate offices in the circulation, some conducting upward, some downward, some conveying the crude sap, some latex, and some air.

790. Air-vessels. Spiral vessels and others of the trachenchyma are generally filled with air, and take no part in the circulation of fluids, except in the spring, when the whole system is gorged with sap. The intercellular passages, also, generally circulate air alone.

791. The moving force. From the roots the newly absorbed fluid flows upward through the stems and branches, toward the buds, leaves, and flowers, being probably drawn thither into them by the exhalation and consequent exhaustion there going on.

792. Through what tissue. The tissue of the stem and branches through which the ascending sap loves chiefly to travel is the pleuren-chyma - those long cells of the wood fiber, whether arranged in broad layers, as in the Exogens, or scattered in slender bundles, as in the En-dogens.

793. Through which layers. And when the stem grows old, the sap ceases to traverse the inner layers, - the duramen, where its passage becomes obstructed by thickened cell walls, and frequents only the outer newer layers, - the alburnum, next adjoining the liber.

794. The crude sap. The fluid which thus flows upward seeking the leaves consists largely of water, is colorless, and is called the crude sap. It contains in solution minute quantities of gases and mineral salts, imbibed by the roots, together with dextrine and sugar (no starch) which it dissolved out of the cells on its way. This is the fluid which flows so abundantly from incisions made in trees in early spring.

795. The overflow of the sap depends upon the excess of absorption over exhalation. After the decay of the leaves in autumn, and the consequent cessation of exhalation, the rootlets, being deep in the ground, below the influence of frost, continue their action for a time, and an accumulation of sap in the system, even in the air-vessels and spaces, takes place. Also in early spring, before the leaves are developed, this action recommences, and the plant becomes gorged with sap, which will burst forth from incisions, as in the sugar maple, or sometimes spontaneously, as in the grape. As soon as the buds expand into leaves and flowers, the overflow ceases.

796. The true sap. Throughout its whole course to the leaves the sap gains in density by solution. There arrived, it loses by exhalation a large part of its water, gains additional carbon, and undergoes other important chemical changes (hereafter to be noticed), and becomes the true sap, dense and rich, both in nutritive matter for the immediate growth and in special products for the future nourishment of the plant.

797. Returning, the true sap distributes its treasures in due and exact proportion as needed to every organ. Its course lies in the tissues of the bark, cellular and woody, first distributed over the under surface of the leaves, thence by the leaf stalks into the liber, and so pervading all, down to the extremities of the roots.

798. On its passage it makes deposits of food, first in the cells, of the pith at the base of every incipient bud; then in the cambium region a copious store; next in the medullary rays a due portion, some carried outward for the supply of the cortical layer, and some inward for solidifying the wood; and lastly, the residue, often the richest legacy of all, falls to the root, and fills every branch and fiber, however vast its extent. This last deposit is that which is first met and dissolved by the rising tide of fluid in the following spring.

799. Growth progresses downward. Since the flowing of the true elaborated sap is downward, it scarce admits of a doubt that the progress of the growth is also downward, from the leaves to the roots. And on no other supposition can we account for the results of the following

800. Experiment. Girdle an exogenous tree by removing an entire ring of its bark. It will flourish still during one growing season, and form a new layer of wood and bark everywhere above the wound, as before, but not at all below. The next season the tree will die. Why? Because the true sap returning can not descend to nourish the roots.

801. Exp. If a ligature be bound firmly around a stem (sc. of silver-leaf poplar) its growth is checked below, while the part just above will exhibit, after a year or two, a circular swelling evidently caused by the interruption of the descending sap.

802. Exp. If a chip be cut from the trunk, the wound heals evidently from the upper side.

803. Exp. Cut off the top of a branch just below a leaf. The upper remaining internode will perish. It has no leaf above it to send down its food.

804. Exp. Girdle carefully the stem of a potato-plant. No tubers will be formed below. And, again, girdle a fruit tree, and the fruit will for once be increased in amount.

805. IN A FEW INSTANCES TREES HAVE SURVIVED THE GIRDLING process. In such cases the medullary rays complete the broken currents. The descending sap, on arriving at the ring, flows inwardly by the medullary rays, making a circuit, and appears again in the bark below the interruption.

806. Rotation. Beside this general circulation of fluids rising and falling from extremity to extremity, there is also a special circulation going on pretty constantly in each new cell, called rotation.

807. Rotation is a flowing of the protoplasm in slender and devious currents on the inner surface of the primordial utricle, rendered perceptible by the opaque particles floating in it. The cytoblast also partakes of the movement. It is well observed in the hairs of Tradescantia, leaves of Vallesneria, and especially in the stems of Chara, where the current expands into an entire revolving layer of protoplasm. It is a vital movement.