plants, are filled with fluids when young, but in old branches, the fluids disappear, and the cells are filled with air. In general herbs and shrubs have a greater proportion of pith than trees. It is also more abundant in young than old vegetables; it extends from the root to the summit of the trunk or stem of the plant.

The medullary* rays are lines which diverge from the pith towards the circumference; they are fibrous textures interwoven in the wood, the alburnum, and the different layers of the bark. The new buds seem to originate from the points at which they terminate. The pith has been compared to the spinal marrow in animals; it appears to be an important part of the vegetable substance, though its offices are perhaps less understood than those of the other parts. The letter e, Fig. 118, represents the medullary rays as proceeding from the pith and terminating in the cellular integument.

You are not to expect that every stem or branch of a dicotyledonous plant will present all the various parts which we have described as constituting the vegetable body; neither when they exist are they always distinct, for they often pass into each other in such a manner as render it difficult to define their boundaries. Many species of plants, have no distinct layers of bark, and in many others there is such a similarity between the alburnum and perfect wood, as to render it difficult to distinguish them.

Growth of a Dicotyledonous Plant.

Let us now review the most important circumstance in the growth of a woody plant. Before germination, the substance of the plume or ascending part of the embryo, exhibits a delicate and regular cellular texture; where the liber and medullary rays are to be formed, traces of cambium appear

When the germination commences, the vascular system begins to organize around the pith, and the medullary rays to form; the extremities of these rays exhibit cellular texture, which is soon converted into libers. (See ƒ, Fig. 118, which shows the extremities of the medullary rays, and the points where the liber is formed.) While this change is taking place, the cambium, which may be considered a fluid cellular mass, flowing between the bark and the wood, hardens into a new layer of liber, and a new layer of alburnum-the latter is at length changed to this; each year a new layer succeeds, and thus the growth of the vegetable goes on until death completes its term of existence.

Each layer of wood is generally the product of one year's growth; but it is only near the base of the trunk, that the number of layers of wood is a criterion of the age of the tree; for in trees where one hundred layers may be counted near the base, no more than one can be found at the extremity of the branches. These layers, then, do not extend through the length of the tree; but while the base exhibits all the layers which have been formed, the extremity of the branches contains under the bark only the continuation of an annual layer. The age of branches may be determined by the number of layers of wood at the base of each branch.

We will now consider the manner in which the tree increases in

So called from medulla, marrow, a name often given to the pith.

Medullary rays-Pith, to what compared ?-Various parts not always distinct in different plants-Appearance of a dicotyledonous plant before germination, or while in embryo-Change at the commencement of germination-Process in the formation of perfect wood-Number of layers of wood near the base of the trunk, a criterion of the age of a tree-How may the age of branches be determined?

height. A seed germinates; the plume rises; the cambium, in developing, gradually becomes less capable of extension; at length, when it is converted into wood, its circulation ceases. The layer of wood then exhibits the form of an elongated cone; at the summit of the cone a bud is formed, from which a new shoot issues; a new layer of alburnum organizes upon the surface of the cone; this, in turn, becomes perfect wood, covering the layer first formed; and thus the tree goes on increasing in height and in diameter. The ter minal bud is formed each successive year. After a hundred years of vegetation, a hundred cones might be found boxed within each other in the manner first described; the spaces comprised between the summits of the cones would show the succession and elongation of the annual shoots.

As the wood is formed by the conversion of cambium into alburnum, so from the same liquid the inner layers of bark are formed to renew the waste occasioned by the destruction of the epidermis. While the wood is growing externally, that is, at an increasing distance from the centre, the bark is forming internally, and the new layers are pressing outward.

Growth of Monocotyledonous Plants.

The growth of trunks, as hitherto considered, has relation only to woody plants; but between plants which grow from seeds with one cotyledon, and such as grow from seeds with two cotyledons, there is a great difference as to the mode of organization and growth.

The first kind of plants are called monocotyledonous; the second dicotyledonous. Their stems, on account of their different modes of growth, have been distinguished into endogenous, signifying to grow inwardly; and exogenous, signifying to grow outwardly. The discovery of the different modes of growth in these two great divisions of plants, is of recent origin, and constitutes an important era in vegetable physiology.

The stems of monocotyledonous or endogenous plants have seldom a bark distinct from the other texture; they have no liber, or alburnum disposed in concentric layers; they have no medullary rays; and their pith, instead of being confined to the centre of the stem, extends almost to the circumference.

The wood is divided into fibres running longitudinally through the stem, (see Fig. 119, where the dots represent the fibres ;) each of these fibres seems to vegetate separately; they are ranged around a central support, and are so disposed that the oldest are crowded outwardly by the development of new fibres in the centre of the stem; this pressure causes the external layers to be very close and compact. This mode of increase, little favourable to growth in diameter, produces long and straight stems, nearly uniform in size throughout their whole extent; as the palms and sugar-canes of the tropics, and the Indian corn of our climate. Most of these plants present us with roots of ne fibrous kind.

Describe the manner in which the tree increases in height?-Difference in the growth of wood and bark-Remarks on the different organization of plants-Monocotyledonous plants-Why called endogenous ?-Exogenous plants--Describe the stem of monocotyledonous or endogenous plants-Describe the stem of a monocotyledouous plant.

Fig. 120, at A, represents a section of the stipe or stem of a palmtree; at B, is the same magnified; a, b, shows a part of the stipe in which the woody fibres are most dense and hard; b, c, shows the fibres less numerous, less compact, and less hard; c, d, includes the woody fibres, tender and scattered; the orifices of tubes which have disappeared are seen at c, a. In the part c, d, the cellular tissue occupies a greater space than at c, b, and much more than at b, a, where the woody fibre, or vascular texture, predominates. The fibres at e, are of new formation; at f, they are older, and at g, still more ancient; thus the development of the wood in this plant proceeds inversely to that of dicotyledonous plants.

Endogenous plants continue to increase in height, long after they cease to grow in diameter; the stem is gradually extended upward by new terminal shoots, which are formed annually.

The epidermis is formed of the foot-stalks of leaves, which an nually sprout from the rim of a new layer of wood; the leaves fall ing in autumn, their foot-stalks become indurated, and remain upon the outer surface of the plant.

We have now taken a brief view of the most important facts and principles which constitute the science of vegetable anatomy and physiology. Although the vegetable structure is much less complicated than the animal, there are many analogies between thein; and many parts of the former have been named, and various phenomena explained, by a reference to names and principles common to animal anatomy and physiology. You cannot therefore expect, at the first glance, to comprehend explanations which presuppose some knowledge of those intricate subjects. By attention to the vegetable structure, you will, doubtless, be induced to think more upon the wonderful mechanism of your own material frames; upon the analogy, and yet infinite difference, between yourselves and the lilies of the field. In considering these things, we are led to exclaim, in the language of the Psalmist, "Oh Lord, how manifold are thy works, in wisdom hast thou made them all!" The human body is nourished by the same elements as the grass which perisheth; the flowers have a much more refined corporeal substance than you, but how much more precious are you in the sight of the Almighty!

Do you ask, why you are of more value "than the lilies of the field," or even than " many sparrows?" It is the very principle

What is Fig. 120 designed to illustrate?-How is the Epidermis formed ?-Reflec tions on the analogies between the vegetable and animal substances.

within you which enables you to make this inquiry, that renders you thus precious;-it is your soul that raises you above the inaninate and brute creation. Though the body is sister to the worm and weed, the soul may aspire to the fellowship of angels. Oh, then, let me entreat you, suffer not your chief thought to be given to the decoration of the perishable part, the mere temporary dwelling-place of the immortal mind! but seek to prepare this mind for admission nto “the glorious company of the spirits of the just made perfect."

LECTURE XX.

PHYSIOLOGICAL VIEWS-CHEMICAL COMPOSITION OF PLANTS-PROXIMATE PRIN CIPLES CHEMICAL ANALYSIS OF THE SAP.

We have, according to our method of arrangement, considered the anatomy of the vegetable in connexion with its physiology: that is, when treating upon each particular organ, we have remarked upon its uses in the life and growth of the whole plant. We have treated of the germination of the seed, the minute vessels which constitute the vegetable fabric, with the fluids which circulate through these vessels; we have considered them as constituting, in various ways, three essential parts of woody plants, the bark, wood, and pith. We have inquired into the manner in which these separate parts are formed, and observed the great distinction in the growth of the stems of monocotyledonous and dicotyledonous plants.

Yet, although we have attempted to show how plants grow, it is no easy thing to explain how they live. The great principle which operates in organic life, appears not to have been laid open to the eye of man. But by a careful observation of facts, we can learn all that it is important for us to know, in order to cultivate plants successfully; their habits, food, and the causes of their diseases and death.

The physician who spends a long and laborious life in the study of the human frame, can give only the result of his observation. He finds a certain article efficacious in the relief of a particular disease; but he knows not why this should be so; or if he is able to give some reasons, he is ultimately arrested in his speculations by a barrier which he cannot pass. Thus he knows that soda or pearlash corrects acidity in the stomach; ask the reason of this, and he tells you that these are alkalies, substances which neutralize acids, and thus render them harmless; inquire still further, why alkalies do thus affect acids, and the physician is as ignorant as yourselves.

Before closing our view of the vegetable structure, we will, by the aid of chemistry, examine the elements which compose it.

The growth of vegetables, and the increase of their weight, show that they imbibe some external substances, which are incorporated int

their own substance. This constitutes nutrition, and distingu hes living substances from dead matter. A stone does not receie nourishment, although it may increase by an external accumulation of matter. Vegetable substances, analyzed by a chemical process, have been found to contain carbon, oxygen, hydrogen, and sometimes nitrogen, sulphur, silex, the oxide of iron, soda, magnesia, and chalk."* These different substances are by the root, stems, and leaves of the plant, derived from the earth, air, and water.

* Mirbel, "Elemens de Botanique."

Recapitulation-A difference between the knowledge of facts, and of their cause Substances which compose plants.

Proximate Principles.

Vegetation produces chemical combinations, which are distir guished by the name of proximate principles. Although the proximate principles of plants are very numerous, but few of them are well known; they are the result of the action of the vital forces of plants, and are, therefore, important subjects of investigation to those who pursue the study of physiological botany to any great extent. Carbon, oxygen, hydrogen, and nitrogen, are the most important of the ultimate elements of plants, and the constituent parts of their proximate principles. These principles may be divided into two classes.

I. Those principles which are composed of carbon, hydrogen, and oxygen, without any nitrogen.

II. Such as contain, besides the substances belonging to the other class, some nitrogen. There are few of this class.

The FIRST CLASS of proximate principles is divided into three orders. 1st. Principles which have more oxygen than sufficient to form water.

2d. Principles in which oxygen and hydrogen exist in the exact proportion to form water.

3d. Principles where hydrogen is in excess.

The 1st order includes vegetable acids; as,

Acetic acid, or pure vinegar; this is generally produced by fermen tation from wine, cider, and some other liquids; it is also found in a pure state in the Campeachy wood, and the sap of the elm.

Malic acid may be extracted from green apples and the barberrv. Oxalic acid is found in several species of sorrel, belonging to the genera Oxalis and Rumex.

Tartaric acid is obtained from the tamarind and the cranberry this acid, combined with potash, forms what is commonly called cream of tartar.

Citric acid is found in the lemon; it is mixed with the malic acid in the gooseberry, the cherry, and the strawberry.

Quinic acid is obtained from the Peruvian Bark, (Cinchona.) Gallic acid is obtained from the oak, and the sumach; it is highly astringent.

Benzoic acid is found in the Laurus benzoin, and in the Vanilla this is highly aromatic; it is thought to give the agreeable odour common to balms.

Prussic acid; this acid gives out a strong odour like bitter al monds; it is an active poison; it is obtained from peach-meats and blossoms, from bitter almonds, &c.

The 2d order includes gum, sugar, &c.

The Gums. Of these there are many kinds; they have neither taste nor smell; dissolved in water, they form a mucilage more or less thick. The principal gums are,

Gum Arabic, which flows from the plant MIMOSA nilotica ;*

Common Gums, such as issue from the peach-tree, the cherry tree, and many others.

Sugar is a substance which dissolves in water, and has a sweet taste; it is obtained from the sugar-cane, the sugar-maple, from the stalks of Indian-corn, pumpkins, beets, and sweet apples. All vegetables which have a sweet taste, may be made to yield sugar.

By some writers called ACACIA Arabica.

Proximate Principles-What are the most important ultimate elements of plants?— Proximate principles divided into two classes-First class divided into three crdersFirst order-Second order-Third order.