endogenes they run parallel to each other, and are simply connected by transverse veins. When the petiole becomes lengthened so as to curl up, it is called a tendril, and many curious forms, such as that of the Pitcher Plant, are but expansions of this portion of the leaf. The limb of a leaf presents three distinct parts; the superior and inferior surfaces, and the mesophyllum, or substance contained between the nervures. Both the surfaces are ordinarily furnished with stomata, the under side much more abundantly than the upper; but in leaves which rest by their under surface on the water, this relation is reversed, their upper surface (that which is exposed to the air) being alone furnished with stomata. In like manner, leaves which are constantly immersed have no stomata. The nervures of the superior surface are supposed to be the channels by which the juices are conveyed from the stem to the limb; those of the lower surface conduct them back to the bark. we attempt to twist a leaf so that the naturally superior surface shall be undermost, it endeavours to regain its original situation; and if the force used, prevent it from doing so, the leaf quickly perishes.

If

34. Stipules. This name has been given to small leafy organs, whose only essential character is their lateral position at the base of the leaf. They are occasionally changed into true leaves, and one of them is sometimes wanting; they vary exceedingly in appearance.

35. Leaf Buds are those vital points, surrounded by scales, which are usually found in the axils of the leaves, and from whose growth a branch is formed.*

* "Leaf buds are always formed from the cellular portion of the stem or branches, on which the function of extending the growth of the individual seems especially imposed. They may be distinctly traced, in young branches, to the pith; and where

The scales, as the vegetation proceeds, are replaced by leaves. When leaf buds are found under ground, and become swollen and large, like the crocus, &c., they are called bulbs or corms (31). In both cases young bulbs are produced in the axils of the scales, and feed on the old bulb. Some of the latter tribe raise themselves out of the earth by a very curious process. "In some Gladioli," says Professor Lindley, "an old corm produces the new one always at its point; the latter is then seated on the remains of its parent, and being in like manner devoured by its own offspring, becomes the base of the third generation." Leaf buds are divided into regular and adventitious, the former being always found in the axil of the leaves, none of which, in fact, are ever really without them, though in some cases they are undeveloped; so that the arrangement of the branches of a plant would always be the same as that of its leaves, were it not that the buds are very unequally matured: and this regularity is found to exist in reality through every part of a plant, although from the obliteration of some portions, and the non-development of others, it cannot always be traced throughout. "It has been distinctly proved, that while roots are prolongations of the vertical or woody system, leaf buds universally originate in the horizontal or cellular system."

36. The nutritive organs of cellular plants are far less defined than those of the vascular tribes, and it even appears as if the whole mass of the former were composed of one homogeneous substance, capable of taking diverse forms, and fulfilling different functions, without being separated into distinct or

this has dried up, they may be seen to arise from the medullary rays." (Carpenter's Veg. Phys., p. 197.)

gans. They are analogous in many cases to those of vascular plants, but never consist of vessels. They vary so much in the different species of the cellular tribes, such as the Mosses, Hepaticæ, Lichens, &c., that it would be impossible to describe them here without entering into details far exceeding the limits of this work.

37. On considering the phenomena of vegetable nutrition, one fundamental principle meets us at the outset ; viz., that no aliment ever penetrates the plant, unless water serve it for a vehicle. Without water there is no vegetation. The first thing then to inquire is how it enters into the system. The habitual and vital absorption of water is performed by the spongioles of the roots (8), although under certain conditions, such as rain, heavy dew, artificial watering, &c., the surfaces of the leaves have also the power of imbibing it. Plants being utterly without locomotion, and unable to seek their own food, it follows that their nourishment must be so abundant in nature as to be almost universally within reach, and so easy of absorption as to offer no resistance to their comparatively feeble powers of action. These necessary conditions are beautifully fulfilled by the spongioles and by the nature of water. The spon

gioles make no selection of healthy material for the nourishment of the plant; and the absorption of fluid through their medium appears to be regulated merely by the readiness with which certain solid substances held in solution can be received along with the waThus the action of the spongioles separates a portion of the water from a solution of gum arabic, leaving the gum behind in the remaining solution, in an increased state of saturation; but sulphate of copper in solution-one of the substances most injurious to vegetation-is rapidly absorbed. Dr. Car

ter.

penter, however, mentions an exception to this, in the power which some plants exert of taking up certain mineral substances which seem peculiarly requisite for them. He says, “if a grain of wheat and a pea be grown in the same soil, the former will obtain for itself all the silex, or flinty matter, which the water can dissolve; and it is the deposition of this in the stem which gives to all the grasses so much firmness. On the other hand, the pea will reject this, and will take up whatever calcareous substances (or those formed of lime and its compounds), the water of the soil contains, these being rejected by the wheat." (Carpenter's Veg. Physiol., p. 89.) On this subject Professor Daubeny has made many curious experiments.

38. Plants, then, absorb water by their roots; but is it pure water only they require? Modern chemistry has decided this question in the negative. Water in its absolute purity, such as we obtain it by distillation, does not exist in nature: if exposed to the influence of the atmosphere it holds some of it in solution; if it is in contact with the soil it will imbibe saline, or organic particles, and thus the water which reaches plants is always more or less charged with other substances.

39. When water, accompanied by the soluble matter it contains, has entered the spongioles, it becomes a part of the juices of the living plant, is propelled forward with great force, and receives the name of sap. This sap rises in the plant, and probably in its course furnishes the air with which the vessels are filled. The rapidity with which the sap rises has been proved by several curious experiments. Hales introduced the root of a vigorous pear tree into a glass tube hermetically sealed at the top, with a lute quite impervious itself to air; this tube was filled

with water, and placed in a cup of mercury; in six minutes the mercury had risen eight inches in the tube, to replace the water that had been absorbed. From other experiments on the force with which the sap rises, Hale drew the conclusion that it is five times greater than that with which the blood is thrown into the crural artery of a horse. "If a piece of bladder be tied over the surface of a vine stump when the sap is rapidly rising, it soon becomes tightly distended, and will ultimately burst. These effects manifestly bespeak an action very different from the ordinary results of capillarity, and indicate the pressure of a powerful force, a "vis à tergo," residing in the lowest extremities of the roots, by which the propulsion of the sap is regulated. Although these results so closely resemble those of endosmosis (20), there still exists a difficulty in con necting the two phenomena; for whilst we may admit the possibility of an interchange between the contents of the vesicles composing the spongioles, and the water in the soil which surrounds them, by the ordinary operation of endosmosis, it is difficult to explain how the sap may be propelled forward so violently as it appears to be, in the open channels through the centre of the stem, which contain crude sap of nearly the same specific gravity as water itself. It would be further necessary to account for the manner in which a continued supply of fresh material is obtained for carrying on the endosmosis, which must otherwise soon cease when the fluid within has become much diluted. We shall find, however, that a constant supply of fresh material is actually provided by the direct action of the vital force, during a subsequent period, in the function of nutrition; and hence it is not impossible, though it has not been proved, that both the propulsion as well