374 ROYAL SOCIETY OF CANADA 
In some of the canals the process of lysigenous degeneration is well 
illustrated, for, in these, strips of parenchyma cells attached to proto- 
xylem cells are being torn away from the tissue to which they belong 
(Photo 5). The cells thus separated float off into the cavity, and since 
they are not as resistive to decay as the protoxylem elements, they 
gradually degenerate and disappear. In this manner the canals are 
greatly enlarged. 

Fig. 3.—Pteris aquilina, Stipe, ant. Coll. May 29th, 1899. Longit. Showing 
thyloses, Th., in Canal, C., and remains of protoxylem, A. tr., Sp. tr. ; 
W. Par, Wood Parenchyma X 367.5. 
In longitudinal sections the localisation of the thyloses at this 
stage is well shown; some parts of the canal being blocked completelv 
whilst others are not. In most cases the thyloses fill about half the 
cavity, the greater part of the remainder being occupied by the remains 
of the disorganised spiral and annular tracheids of the protoxylem. 
The thyloses are frequently seen coming fram the wood parenchyma 
cells (Fig. 3), and forming a fairly regular tissue whose cells exhibit 
polyhedral shape due to lateral pressure arising from contact. The 
original wood parenchyma cells themselves are often seen to have divided | 
by means of transverse walls, so that instead of being several times 

