110 THE OAK 



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 phy- 

 sical 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 preser- 

 vative 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 function- 

 less vessel. 



