HO TEE OAK. 



men) as opposed to " sap-wood " (alburnum) is not at- 

 tended with any profound anatomical changes ; the chief 

 alterations are of the nature of infiltration by foreign 

 chemical substances, and alteration in the physical prop- 

 erties 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 in- 

 dicate 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 preserv- 

 ative 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 bor- 

 dered 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 parenchyma- 

 tous 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. 



