It is in the mode of arrangement of the vascular tissue that the difference between the stems of monocotyledonous and dicotyledonous plants depends, and which has given rise to the use of the words "endogenous," as applicable to the stem-growth of monocotyledonous plants, and "exogenous," as applicable to the stem-growth of dicotyledonous plants. In the endogenous structure they are disposed throughout the cellular tissue of the stem, without any arrangement of pith, medullary rays, or bark, though of course there is the epidermis. The structure of exogenous stems requires a longer description, and then it will be only an imperfect one. Commencing at the centre of the stem axis is the pith or medulla, formed of cellular tissue; then surrounding it is the medullary sheath, formed of spiral vessels; from the pith towards the bark, periodically continued by the new growth, are flattened cells, called pith rays or medullary rays, - they apparently keep up the connection with the pith and the new growth, which would otherwise be shut off by the heart-wood, and also strengthen the stem fabric; then come layers of cellular tissue, called ligneous tissue or woody fibre (the older portion of cellular tissue hardened by deposit, and through which little or no sap can flow), called heart-wood or duramen, the use of which to the stem is to give it mechanical strength; next come the younger layers of cellular tissue outside the duramen, called the sap-wood or alburnum.

It is through the cells of this tissue that sap rising from the roots passes to the leaves; and as each new growth or formation of cells is made exterior to the alburnum periodically - not necessarily yearly - there will be a new layer of growth, which will indicate one period of growth, whether of one year or less. In tropical climates, if the trees shed their leaves three times in a year, or five times in two years, we should expect to find as many layers of wood; the innermost tissue of the alburnum becomes in its turn solidified and hardened by deposit, and in its turn becomes heart-wood or duramen. Then comes the cambium, or the cambium layer, variously described in its earlier stages as "semi-fluid mucilage," " viscid secretion," " cambial or formative tissue," "opaque fluid," or " mucilaginous semi-fluid matter; but it is cellular tissue of delicate texture filled with assimilated granular matter in a state of vital activity forming new cells; and in this tissue the primary vascular tissue, or bundles, as they are sometimes called, originate: here all the changes of the new growth take place, and with it the leaves seem to be in connection: it is found even at the point of the stem (punctum vegetationis) under the bud.

Part of it goes to form the new woody fibre of the alburnum, and to continue the medullary rays in the direction of the bark; other part of it goes to add to the internal bark or liber; and other part separates the alburnum and the bark. The bark may be said to consist of three principal layers of cell-tissue - the inner one of fibro-vascular tissue, called the endophlceum or liber, the bast layer; the middle layer of cells, differing from the external layer, called the mesophloeum; and the external or corky layer, epiphloeum; and then the epidermis. The pith wood and medullary rays may be seen in the herbaceous stems of an exogenous plant; and there are medullary rays, but no pith, in the roots of exogenous plants. No hard and fast rules can be laid down by man which will always explain the works of nature, and accordingly there will be found among the exogenous plants exceptions to our explanations of the formation and arrangement of its tissue. In the Calycantheae (Caroline Allspice family) there are some species which form woody columns in the bark, independent of the central woody formation; and in some climbing plants a greater thickness of the medullary rays than usual will be found.

In dicotyledonous plants the division of the stem into nodes, with internodes or spaces between, is taken advantage of by the gardener to increase the number of plants: this he does either by making cuttings of a portion of the stem at a leaf-bud just below a node, which, when placed in soil and warmth, will soon emit roots; or by layering the stem or branch to the earth, fixing it at a node to the ground, and making an incision with a knife into the node on the under side, and so forming a layer which, will soon emit roots. It is thought that the presence of vascular tissue at the node is the cause of the rooting faculty of the cutting or layer. A cutting made of the internode of the stem only, without the node, will not grow; nor will any cutting, however made (with or without a node), of a monocotyledonous plant grow.

In complete union with the stem is the leaf, which in land-plants is formed of parenchyma, traversed by veins of fibro-vascular tissue, furnished at its upper surface with a covering or epidermis, then with a layer of cells flattened by the pressure of the atmosphere, then a layer of cells filled with chlorophyll granules, and then air-cells, and again an epidermis filled with stomata or breathing pores forming the under surface of the leaf. The leaves of aquatic plants are formed differently - the distribution of veins in the leaves is called venation, and for the most part they are reticulated in the dicotyledonous plants, and parallel in monocotyledonous plants; but some families, such as Smilaceae, have not unfrequently reticulated leaves. They are usually described according to their position, such as opposite, or alternate; or to their mode of insertion, as by a stalk, or as sessile, or as sheathed, or whether simple or compound (the descriptions of the forms and names given to leaves are too long for insertion here), or whether they have any appendages or not; and the edges of leaves, called circumscription, have been found to possess capital characteristics for their descriptions; sometimes they are annual; and the trees bearing leaves subject to the process of annually dropping or shedding their leaves are called deciduous trees.

Sometimes the leaves continue longer than a year, and the trees are then called evergreen or persistent - that is, not falling off. Some leaves - not, however, strictly leaves - have on the centre midrib the flower, such as Ruscus aculeatus. There is a popular notion that every leaf originates in a cell of the stem, and that with increase of cells comes increase of foliage; and that each leaf has a corresponding root, and that with increase of foliage there must be increase of roots. It is difficult to trace the origin of these ideas.

The leaf containing in its cells chlorophyll, exercises a great power in the life of the plant. By the performance of some hidden function, due to cell-life in the presence of sunlight, the plant obtains its carbon, and the process is said to be as follows: during the action of sunlight upon the leaf of a plant, the green chlorophyll granules perform some function, not yet clearly shown what, by which carbon dioxide (carbonic acid), absorbed by the leaf from the atmosphere, is decomposed, and the carbon is retained, probably in conjunction with hydrogen in the water of the cell, some oxygen is set free, and some retained; it is rather a function of biology than of chemistry; a dead leaf has not this power. It may be here stated that it has been said, and probably correctly, that the hydrogen present in plant-life is more probably derived from the water in the cells containing chlorophyll than from ammonia.