Every member of the vegetable form, from the minutest root to the most fragile spray, has its epidermis, cellular integument, bark, woody fibre, and medullary matter; these are most apparent in the stem and branches.

The first of these, the epidermis, is analogous to the human cuticle, or scarf skin, being the external envelope of the whole surface. It is commonly transparent and smooth, sometimes hairy; in other instances hard and rugged, occasionally so abounding with silica, or flint, as to be employed as a polisher for wood, and even brass. In every instance it is a network of fibres, the meshes of which are filled with a fine membrane. The epidermis appears to be designed as a preservative from the injurious effects of the atmosphere, to regulate the quantity of gaseous matter and moisture respired, and as a shield from the attacks of animals, etc. It is certainly devoid of sensation. The texture of the membrane between the meshes varies much in different species of plants. In very succulent plants it is so contrived that it readily allows the absorption of moisture, but prevents perspiration. Such plants are, consequently, well qualified to inhabit hot climates and dry soils. Neither is it at all impossible that it possesses the quality of allowing the passage of some gases, and rejecting others, as the bladder of animals permits water to pass through its texture, but is impervious to alcohol.

In old trees it cracks, and in many cases becomes obliterated, the dead layers of bark performing its offices. Its growth is slower than that of other parts, and its powers of expansion, though great, occasionally cannot equal the rapid enlargement of the parts it encloses and defends. This is very frequently the case with the stem?and branches of the Cherry; the tree is then said by gardeners to be hidebound, and is relieved by making longitudinal incisions. It is still more apparent in the fruit of the Cherry and Plum: when rain falls abundantly during their state of ripeness, their pulp swells so rapidly, that in an hour or two the epidermis of every ripe drupe upon a tree will be cracked-Gardeners are very prone to scrape with no gentle hand the bark of their fruit trees; whereas every care should be taken not to wound its surface unnecessarily, and never to reduce its thickness until all danger of severe frosts is passed.

The epidermis regulates the evaporation from a plant, and preserves it in some degree from the detrimental sudden changes of temperature. The Birch (Betulus alba), has more films of epidermis than any other European tree; and it ascends to greater heights in the Alps, and approaches nearer to the frozen zone than other trees of the same climates.

It is quite certain that stems and branches can imbibe nourishment through their epidermis. If a branch be cut off, and a wetted towel be wrapped round the bark, yet without touching either the cut end or the leaves, that branch will retain its foliage verdant much longer than another branch similarly cut off, but not enfolded by a wetted towel. So all gardeners know, that enclosing the stems of newly-transplanted large trees with moss or hay-bands, and keeping these moist, is an efficient mode of enabling them to bear the removal. A branch, or a whole tree, may be killed by painting over its entire epidermis with gas tar, - showing either that the admission and emission of gases and moisture being prevented, or that creosote or other poisonous matter is absorbed from the tar, death is the consequence.

We could give many similar results of experience, but will only add further that Mr. Hales states, as the result of many experiments, "that the air enters very slowly at the bark of young shoots and branches, but much more freely through old bark; and in different kinds of trees it has very different degrees of more or less free entrance." - (Vegetable Statics, i., 160).

Knowing these facts, and knowing also the benefit a tree derives from keeping its epidermis freed from lichens, we have never doubted that its clean and health)7 state is of as much importance to a plant as is a clean and healthy skin to an animal.

The roots exercise a kind of discriminating power in admitting to the circulation of the plant the various substances which are present in the soil. The vessels of the stem exhibit an analogous power of admitting or rejecting the solutions of different substances into which they may be immersed. Thus Boucherie states that, when the trunks of several trees of the same species are cut off above the roots, and the lower extremities are immediately plunged into solutions of different substances, - some of these solutions will quickly ascend into, and penetrate the entire substance of, the tree immersed in them, while others will not be admitted at all, or with extreme slowness only, by the vessels of the stems to which they are respectively presented. On the other hand, that which is rejected by one species of tree will be readily admitted by another. Whether this partial stoppage of certain substances, or total refusal to admit them, is a mere contractile effort on the part of the vessels, or is the result of a chemical change of the substance itself, or of the fibre or sap with which it comes into contact, by which change their exclusion is effected or resisted, does not as yet clearly appear.

That it does not depend upon the lightness and porosity of the wood, as might be supposed, is shown by the observation that the Poplar is less easily penetrated in this way than the Beech, and the Willow than the Pear-tree, the Maple, or the Plane, - Johnston's Lectures on Agricultural Chemistry.