Johnson — Tyloses in the Bracken Fern. 103 



producing in some cases a pseudo-parenchyma in it. Tyloses may 

 in some cases be purely pathological, due to injury or the age of 

 the tissue in which they occur. In some instances, as, e.g., in some 

 eases of leaf-fall, they may arise in connexion with the healing of 

 the wound, may block up the vessel, and stop the transpiration 

 current through it ; or in some cases they may, only partly filling 

 the cavity of the vessel, withdraw food-materials from it for the 

 living parenchyma in the xylem. 



EXPLANATION OF PLATE VI. 



Fig. 1. — Low-power micro-photograph, showing one of the xylem 

 tracheides, t, filled by tyloses. 



Fig. 2. — A small portion of fig. 1, more highly magnified, showing, 

 in fair outline, a nucleated tylose cell at t. 



K2 



