minate and regular rupture, such as takes place when fruits, arrived at maturity, burst open (the beechmast, for instance); the lines which mark the direction these separations will take are often rather prominent, and may be observed before the ripening of the part-the term suture has been applied to them. 14. Two grand classes are obvious on considering the foregoing organography, viz., Cellular and Vascular plants. The first being wholly composed of cellular tissue, the last of both cellular tissue and vessels. Vascular plants may again be divided into two principal kinds-those whose vessels and cells extend longitudinally, and whose growth takes place towards the centre of the stem; which from this circumstance have been termed Endogenous:* and, secondly, those which have vessels or bundles of elongated cells, taking either a longitudinal or transverse direction, and in which the growth is always towards the circumference of the stem-these are called Exogenous.† 15. Having shown what the general structure of plants is composed of, without reference to those particular organs on which their growth, nourishment, and reproduction depend, it may here be desirable to give some idea of the properties inherent in vegetable tissue, before the organs, which are modifications of that tissue, and of course partake of its properties, are more especially noticed. Organized beings are, like all other bodies, subject to the laws of physical and chemical action; we must therefore inquire, in the first place, concerning every fact of their existence; whether it is merely a consequence of those laws, or whether that conse * From evdov, within, and yiyvoμai, to produce. γιγνομαι quence be modified by the structure and condition of their organization. The first case will come under the class of simple chemical or physical facts; the other will range itself among those which are denominated properties of tissue; that is, properties which are not indeed strictly vital, but which arise from the peculiar structure of living bodies. Other facts, which we cannot include under either of the above heads, are the direct consequences of that mysterious state called life. The distinction of these three classes is the basis of all true physiology. 16. Vegetable tissue possesses three properties which deserve attention, viz., Extensibility; Elasticity; and the power of imbibing moisture. 17. Extensibility. All organic tissues have in a greater or less degree the power of extending themselves even in the act of growth. This property is greater in proportion as the tissue has received fewer solid deposits, diminishes as it becomes older, and at a certain period ceases altogether. If we watch the development of a branch, we shall find that its cuticle stretches during a considerable period, after which it breaks, and is replaced by an epidermis : the same thing occurs in all cases in which we can follow the growth of any organ; and if plants appear to increase indefinitely, it is because fresh organs are perpetually added to the former ones, and the older parts fall sooner or later into that inert state in which they are no longer capable of extension. 18. Elasticity of vegetable tissue is that property by which each membrane is enabled to resume its proper position when deranged by any external force. It implies a certain degree of rigidity, and is consequently less sensible when the tissue, having received but few deposits, is still in a semi-fluid state, than when it is of older growth. This property is worthy of remark, because it occasions certain movements, which might be mistaken for vital action. It is very variable in intensity. Every one must have observed that a branch, if bent out of its natural course, returns to it of itself; but in certain cases this is not so the dracocephalum-moldavicum has pedicels which may be turned from their natural direction, and will remain in that which has been forced on them. The plant, on account of this deviation from the ordinary law of elasticity, has been called cataleptic. The elastic movements of plants are sometimes determined by an arrangement of the organs, which, once deranged, although spontaneously, have nevertheless no power to return to their original state thus the four stamens of the parietaria tribes have their filaments turned inwards before flowering; but as this process advances, and the filaments enlarge, a moment arrives when they no longer adhere together, but burst open with considerable force: this is facilitated by the tubercles which are formed in the inside of the filament; the anthers, shaken by this sudden movement, scatter their pollen, the filaments die, and the phenomenon can never be repeated. All these effects are consequent on the manner in which the parts are arranged, which indeed is connected with the life of the plant, but must not be confounded with those movements which are really dependent on vital action. 19. The power of imbibing moisture exists in both organic and inorganic substances; thus deliquescent salts, as they are called, are so eminently hygrometric, that their own particles are in the end dissolved in the water they have imbibed. The effect cannot be carried to this extent in organized bodies, being limited by their nature; thus hair, whalebone, &c., though capable of being employed to indicate the state of comparative dryness or dampness of the air, from their power of attracting moisture to a certain extent, are nevertheless, under ordinary circumstances, insoluble in water. It is the same with several vegetable productions, which can, consequently, be similarly employed. Vegetable tissue is in general more hygrometric in proportion as it is less loaded with extraneous substances: the woody fibre is, in this respect, very different from the bark; this latter being scarcely hygrometric, while the woody fibre imbibes moisture with great facility. This absorption of water occasions an enlargement of the woody portion, which thus presses itself, as it were, against the bark, and it is in consequence of this pressure, that the gums contained in and under the bark of certain trees are forced outwards, as in the cherry, plum, &c. Senebier has greatly exaggerated the effects of this power in attempting to account by its agency for the ascent of the sap, and for some of the most important phenomena of vegetation. The fact, that the sap ascends in plants which live in water, and that it does not rise in dead plants, might alone prove his theory to be erroneous. 20." Connected with the hygroscopicity of vegetable membrane, we may here mention a property* of all membrane, which has probably a considerable influence in the econoiny both of animal and vegetable life. When a membrane is viewed under the highest powers of the microscope, it appears to possess a perfectly homogeneous texture, without pores of any kind; and yet water, milk, and other fluids, placed under certain circumstances, are capable of passing through it with considerable facility. The * This property is called Endosmosis. conditions required for producing this effect are these: -Any two fluids which exert a mutual affinity towards each other, being placed on opposite sides of a membrane, their immediate intermixture will commence, each of them passing through the substance of the membrane. If, for instance, a little treacle be enclosed in a piece of bladder, and this immersed in water, a portion of the treacle will soon be found to have exuded, while a still larger quantity of water will have penetrated into the bladder; and this action will continue until the fluids have acquired the same density. The remarkable circumstance attending this phenomenon is the fact of the lighter fluid having penetrated the membrane with greater velocity than the denser fluid." (Henslow's Principles of Botany, p. 159-60.) 21. Vegetable existence has been supposed to possess three vital properties, so termed from their analogy with the powers similarly named in the animal economy; viz., 1. Excitability. 2, Irritability, and 3. Sensibility: by the first is understood that peculiar state of the vegetable tissue, which enables it to resist decomposition by water much more energetically while living than after death, and which also renders it capable of supporting the action of air and heat during life, in a manner totally different from that in which their agency affects it afterwards. Many phenomena common to all plants concur to prove that this difference is inexplicable without the admission of vital excitability; such are the rapid mounting of the sap in the living plant, compared with the slow absorption of water in the lifeless tissue; the influence of light on the ascent of the sap, &c. 22. The quality to which the term Irritability has been applied by some physiologists, is that by |