40 



proceed horizontally from the centre of the 

 treej often run to a great extent vertically^ 

 and are never cured. 



I think that the medullary rays may be 

 seen to prolong themselves into the latest 

 layers of bark if the stem of a living oak 

 tree is cut across ; at least there is a white 

 line across these layers opposite the end 

 of each medullary ray. If these lines are 

 prolongations of the medullary rays they 

 probably play a prominent part in the 

 deposit of the new layer of wood and 

 bark ; at least these prolongations must 

 of course remain for ever, and the new 

 growth, both of wood and bark, must be 

 deposited between them. These white 

 lines across the new layers of bark may, 

 however, be only organs of elaboration or 

 of respiration, or conduits for absorption 

 and exudation. If they are not in unison 

 with the medullary rays, at least the 

 commencement of each white line cor- 

 responds exactly with the termination of 



