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- 
iess vessel. 
