104 



THE ANATOMY OF WOODY PLANTS 



structures originate under various conditions, but are more usually 

 found in connection with those changes which precede the trans- 

 formation of sapwood, or alburnum, into heartwood, or duramen. 

 As the living cells in proximity to the vessels (whether they belong 

 to the radial or vertical systems of storage tissues appears not to 



FIG. 81. Tyloses in oak and locust. Explanation in the text 



be of importance) are about to die, they push processes into the 

 adjacent vessels by the bulging and growth of the cellulose mem- 

 branes of the pits connecting them with the vascular structures. 

 These ingrowths are occupied by protoplasm and, sometimes at 

 any rate, by nuclei as well. When abundantly developed, they 

 completely close the lumen of the vessel and render impossible the 

 passage of water. Fig. 43, on page 57, has made clear the contrast 

 resulting from this phenomenon in the sapwood and the heartwood 



