men) as opposed to "sap-wood" (alburnum) is not attended with any profound anatomical changes; the chief alterations are of the nature of infiltration by foreign chemical substances, and alteration in the physical properties of the cell-walls and in the contents. These changes are somewhat sudden, and the fact that starch ceases to be deposited in this altered wood helps to indicate that the change is one of degradation—the cells of the softer tissues have ceased to be "alive," and the "heart" commences to undergo degradation. At the same time, although we must regard the "heart" as dead, it is very resistant, perhaps owing to the preservative action of infiltrated bodies.
A remarkable phenomenon which may be noticed here is the filling up of the older large vessels with tyloses. These are thin-walled, bladder-like vesicles projecting into the cavity of the vessel from the bordered pits, and are, in fact, due to the protrusion into the cavity of the thin-walled parenchyma cells, which drive the pit membrane in and then swell up. At the planes of contact between various tyloses from opposite points on the wall of the vessel the tyloses are flattened, and the appearance is very like that of a parenchymatous tissue (Fig. 29, d). When young the tyloses are found to contain a nucleus, protoplasm, and cell-sap, and they are known to form division membranes and divide like cells of the pith or cortex; later on they lose their contents and form a sort of packing in the by this time functionless vessel.
