ed by buds as commodiously as by roots. Those of one tree may be engrafted on the, bark of another of the same species, or one nearly akin, by which, as is well known, valuable varieties are multiplied. Fig. 8.

It is remarkable that nature should permit such devastation and waste as is made by many insects, whose caterpillars or grubs feed on the buds of trees. Several species of fir are infested with their appropriate insects, which, literally speaking, devour their vitals, and should seem to be capable in one season of destroying a whole forest. Yet these are only instruments in the hand of Providence, which, like many others, thongh formidable in appearance, are never allowed to transgress their due bounds.

OF THE STEMS AND STALKS OF PLANTS. Botanists reckon seven kinds of stems or stalks of plants.

1. Caulis, a stem, fig. 9, properly so called, bears both leaves and flowers, as the trunks and branches, ef all trees and shrubs, as well as of many herbaceous plants besides. By its means the organs of plants are raised to a commodious height above the ground, and presented in various directions to the atmosphere and light. In germination, it always takes a contrary direction to the root. As it advances in growth, it is either able to support itself, or twines round, or adheres to other bodies. Some stems creep on the ground, and take root here and there, by which the plant is inereased. The stem is either simple, as in the lily, or branched as in the generality of plants. When regularly and repeatedly divided, with a flower springing from each division, it is called caulis dichotomus, a forked stem. Though generally leafy or scaly, a stem may be naked in plants destitute of leaves altogether, as the creeping cereus, and the genus Staphelia, Climbing stems are of several kinds; as radicans, clinging to any other body for support by means of fibres which do not imbibe nourish ment; scandens, climbing by means of spiral tendrils like the vine and passion-flower; volubilis, twining round any thing that comes in its way by its own spiral form, either from left to right, according to the apparent motion of the sun, like the honeysuckle, or from right to left, like the convolvolus and French bean; nor can any art or force make a twining stem turn contrary to its natural direction. In the manner of their growth and branching stems are very various, being either straight, irregularly spread

ing, or zigzag, either alternately branched or oppositely; two-ranked, when the branches spread in two horizontal directions, or brachiate, four-ranked, when they spread in four directions, crossing each other alternately in pairs. Caulis determinate ramosus, an abruptly branched stem, belongs particularly to the heaths, the rhododendron, &c. and is a term invented by Linnæus to express their peculiar mode of growth; each of their branches, after terminating in flowers, throws out a number of fresh ascending shoots from just below the flowering part. The Indian fig has a remarkable jointed stem, whose ovate portions look like leaves; possibly the scales with which they are covered may be equivalent to leaves.

The shape of a stem is either round or two-edged, as in the everlasting pea, or with three, four, or more angles. Square stems are extremely common, and such generally bear opposite leaves. Several stems are winged, the angles being extended into leafy borders, as in thistles.

The surface of the stem is either smooth, rough, warty, viscid, bristly, hairy, downy, woolly, hoary, or glaucous. It is either striated with fine parallel lines or more deeply furrowed; sometimes it is spotted with a purplish hue.

The inner part of the stem is either solid, in which case its centre is occupied with pith; or hollow, and lined with a white shining membrane, of which the hemlock is an example. When the stem is wanting, a plant is called acaulis, as is the case with the daisy and primrose. The nature of the stem agrees in many respects with the caudex, or body of the root, at least in trees and shrubs; for such are capable of being propagated by cuttings of their stem or branches, which, when planted, throw out roots. This is not the case, however, with annual stems. Linnææns calls the stems of trees roots above-ground. It is frequently indifferent which end of a cutting is planted in the earth; and the extremity of a branch bent down to the ground in most cases readily takes root, which circumstances confirm his idea.

The stem of several plants is subject to a disease, whence it becomes as it were compound or clustered, forming a broad flat figure, crowded with leaves or flowers at the extremity, and sometimes besprinkled with them at the sides. We have seen it in the ash, holly, broom, nasturtium, wallflower, toad-flax, &c. A kind of pea is frequently cultivated in Norfolk with red and

white flowers, and a tender eatable pod, called the top-knot pea, in which this variety of stem is regularly propagated by seed.

2. Culmus, a straw, or culm, fig. 10, is the peculiar stem of grasses, rushes, and such like plants. It bears both leaves and flowers, and in that respect comes under the denomination of a caulis; but is readily known by its habit, though difficulties attend its definition. In most grasses, corn, &c. it is jointed in a manner peculiar to itself, and then cannot be mistaken; but in common rushes, and some few grasses, it is destitute of joints. When these parts are bent, it is called geniculate, and such joints readily take root.

3. Scapus, a stalk, fig. 11, springs immediately from the root, bearing flowers and fruit, but not leaves, as in the primrose and cowslip. It is either simple or branched, naked or scaly. In the cyclamen it becomes spiral after flowering, and buries the seeds in the ground. Dr. Smith has found, contrary to the opinion of Linnæus, that a plant may sometimes be increased by its scapus, as in lachenalia tricolor, which occasionally bears bulbs on its stalk.

4. Pedunculus, the flower-stalk, fig. 12, springs from the stem or branches, bearing flowers and fruit, but not leaves. Pedicellus is a partial flower-stalk, or, in other words, the ultimate subdivision of a general one. The most common situation of a flowerstalk is axillary, originating from between a leaf and the stem, or between a branch and the latter. It is rarely opposite to a leaf, as in some species of geranium, and still more rarely intermediate between two leaves, as in some kinds of solanum. It is either terminal or latera!: solitary, clustered, or scattered; simple or branched. According to the various modes in which it is subdivided several kinds of inflorescence are distinguished, to be mentioned hereafter. Sessile flowers are such as have no stalk. The flower-stalk is occasionally naked, or furnished with bracteas. Very rarely it bears tendrils.

5. Petiolus, the foot-stalk, fig. 13, is applied exclusively to the stalk of a leaf, and is either simple, as in all simple leaves, or compound, as in the greater part of compound ones. Sometimes it bears tendrils. It is generally channelled on the upper side, and more or less dilated at the base; in one or two instances the flower-stalk grows out of it, as in turnera. Leaves that have no foot-stalk whatever are called sessile,

The sap-vessels are for the most part very conspicuous in foot stalks, and their spiral coats are easily observed. 6. Frons, a frond. This term, which properly means a bough, is technically applied by Linnæus to express the stem, leaf, and fructification being united, that is, the leaf bears the flowers and fruit. The term is only used in the class Cryptogamia. Ferns which bear seeds on the back of their leaf are genuine instances of this, and it is applied to lichens, &c. Plate II. fig. 14.

7. Stipes, stipe, is the stem of a frond, fig. 15, or the stalk of a fungus, as in the common eatable mushroom. In the former instance it is very generally clothed with scales of a peculiar chaffy texture; in the latter it is very often invested by a ring formed of the membrane which had previonsly covered their fructification.

OF THE LEAVES.

The leaf, folium, fig. 16 and 17, is a very general organ of vegetables, yet not absolutely necessary to all plants, for the stems and stalks occasionally perform its functions. What those functions are we shall in a compendious manner explain. Leaves are generally so formed as to present a large surface to the atmosphere; when they are of any other hue than green, they are said in botanical language to be coloured. Their duration is for the most part annual, but in some trees and shrubs they survive two or more seasons, and such plants being always in leaf are denominated evergreens. The internal surface of a leaf is highly vascular and pulpy, and is clothed with a cuticle very various in different plants, but its pores are always so constructed as to admit of the requisite evaporation or absorption of moisture, as well as to admit and give out air. Light also acts through this cuticle in a definite manner. That air and moisture and light have considerable, and even the most important effects, upon the leaves of plants, has long been known to those who have studied the subject; that heat and cold affect them is familiar to every one. The expe riments of Hales, Bonnet, and others, have thrown much light upon the absorption and perspiration of leaves, while those of Priestley and Ingenhouz have explained their ef fects upon the atmosphere, and the manner in which air and light particularly act upon them. Leaves have a natural tendency to present their upper surface to the light, and turn that surface towards it in whatever dis rection it is presented to them. When

trees in leaf are nailed to a wall, and the position of their leaves is consequently disturbed, they soon recover their natural direction. Light evidently acts as a wholesome stimulus to their upper surfaces, and as a hurtful one to the under. When the latter is forcibly presented for a long period to its rays, destruction is the consequence. Leaves seem to require occasional repose from the action of light on their upper surface; for, when it is withdrawn from them, many leaves close or fold themselves together, as if in a state of relaxation, and spread themselves forth again at the returning beams of the morning. This is more especially the case with winged leaves, as those of the pea kind. Those of the white acacia, robinia pseudo-acacia, have been remarked by Bonnet to be over-excited by the sun of a very hot day, and to fold their upper sides together, in a manner directly contrary to their nocturnal posture. The effect of moisture upon leaves every one must have observed. By absorption from the atmosphere, they are refreshed, and by evaporation, especially when separated from their stalks, they soon fade and wither. Aquatic vegetables, whose leaves are immersed in the water, both absorb and perspire with peculiar facility. Anatomical investigations have shewn that the nutritious juices, imbibed from the earth, and become sap, are carried by appropriate vessels into the substance of the leaves. Mr. Knight, in his papers in the Philosophical Transactions, has demonstrated that these juices are returned from each leaf, not into the wood again, but into the bark. Hence a new and curious theory of vegetation has been established. It appears that the sap is carried into the leaves for the purpose of being acted upon by air and light, with the assistance of heat and moisture. By all these agents a most material change is wrought in its component parts and qualities, differing widely according to the diversity of the species. Thus the resinons, oily, mucilaginous, saccharine, bitter, acid, or alkaline secretions are elaborated. heedless observer of a leaf is little aware of the wonderful operations constantly going on in its delicate substance, nor can the most enlightened philosopher explain more than a very small part of the chemical processes of which it is the immediate agent. It is scarcely necessary to observe how materially plants differ in the flavour and qualities of their leaves, all which must depend in a great measure on the operation of the

The

leaf itself, for the common sap of plants, from which all their secretions are made, differs very little in plants whose qualities are very unlike to each other; those qualities depending upon the secreted fluids elaborated principally by the leaves.

The green colour of the organs in question is easily proved to be almost entirely owing to the action of light. Plants which grow in the dark are of a sickly white, which is the case with any parts artificially or accidentally covered with earth, as in cultivated cellery or asparagus, whose stems and leafstalks are purposely managed in this way to render their flavour and appearance more delicate. Such blanched parts soon become green on exposure to light. Leaves are subject to a sort of disease by which they become partially spotted or streaked with white or yellow. In this state they are termed variegated, and occasionally contribute to the ornament of our gardens. The whiteness frequently extends to the leaf-stalk, and sometimes to the branch, as may be seen in the variegated elder. Such varieties are propagated by cuttings, layers, or roots, but not by seed. They appear to be somewhat more tender than the plant in its natural state. One variety of the holly has, in addition to a yellow variegation, a beautiful tinge of purple, but this is a rare instance. In the amaranthus tricolor the leaves are naturally adorned with most beautiful and splendid colours, and in some other species of the same genus, with more uniform and less vivid tints.

The irritable nature of some leaves is remarkable, not but that all leaves may truly be said to possess irritability with respect to light. The phenomena however to which we now allude are of the most striking kind. The sensitive plant, mimosa pudica, common in hot-houses, when touched by any extraneous body, folds up its leaves one after another, while their foot-stalks droop as if dying. After a while they recover themselves again. Each leaf of the dionæa muscipula, or Venus's fly-trap, is furnished with a pair of toothed lobes, which, when touched near the base, fold themselves together and imprison any insect that may be in their way. It is presumed that the air evolved by the body of the dead insect may be wholesome to the plant, for leaves are known to purify air impregnated with carbonic acid gas, produced from the breathing of animals or the burning of a candle. sarracenia, of which several species from America are now cultivated in our more

The