. The theory of horticulture. s, the callus, 2 which forms at the end pla-ced in the ground, is thecellular horizontal system,preparing for the receptionof the perpendicular sys-tem, which is to pass down-wards in the form of plants will endureextensive lacerations oftheir surface, and close upsuch wounds with greatfacility. The well knownfact of large inscriptionscut in trees below the bark(which inscriptions wereeffected by removing verybroad spaces of the bark and wood) being coveredover in time by new bark and wood, so as to beno longer visible from the outside, sufficiently pro
. The theory of horticulture. s, the callus, 2 which forms at the end pla-ced in the ground, is thecellular horizontal system,preparing for the receptionof the perpendicular sys-tem, which is to pass down-wards in the form of plants will endureextensive lacerations oftheir surface, and close upsuch wounds with greatfacility. The well knownfact of large inscriptionscut in trees below the bark(which inscriptions wereeffected by removing verybroad spaces of the bark and wood) being coveredover in time by new bark and wood, so as to beno longer visible from the outside, sufficiently provesthis. In such cases, however, the reparation of theinjury takes place chiefly, if not exclusively, by theannual addition of new matter to the lips only of thewound, the effect of which is to reduce the circleannually to a less diameter, till at last the centre isclosed up. 48. In the bark of trees and shrubs two distinctparts are found : the one external and cellular ; andthe other internal, resting upon the wood, and con-. GROWTH BY THE STEM. 27 sisting of woody matter mixed with cellular. Theexternal is the rind or cortical integument, the inter-nal is the liber. These two parts grow independentlyof each other, by their inner faces ; the rind belong-ing exclusively to the horizontal system, the libercomposed of the perpendicular and horizontal systemsintermixed. 49. In all Exogenous plants whose stems acquirean age beyond that of a very few years, the wood isdistinguishable into two parts, heart-wood, and sap-wood or alburnum. The former is more or less cen-tral, and coloured brown or some dark tint; the lat-ter is external, pale yellow, and much softer. Heart-wood was originally alburnum, and altered its naturewith age, in consequence of the solid matter withwhich all its tubes and vessels were choked up ; al-burnum is the youngest wood, with all its communi-cations free and open, no solid matter having hadtime to accumulate within them. The reason whysolid matter collect
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