[KIRSCH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 387 
An appearance which is fairly common in these sections is shown 
in figure 16. In this it is seen that in some of the thylosal cells, the 
outer walls are very thick, whilst the inner ones are frequently exceed- 
ingly thin. This proves the later appearance of the inner walls, and 
tends to show that cell division can occur in the thyloses at a late stage 

Fig. 16.—Pteris aquilina, stipe, base, Oct. 25th, 1906. Canal filled with thy- 
loses, Th., exhibiting unequal thickening of walls due to late cell 
division. XX 490. 
in development. It is probably owing to this mode of division that the 
thyloses in this region are all comparatively small. 
In figure 17 a thylosal patch is shown which is much larger than 
the normal ones found in this region, although the portion from which 

Fig. 17.—Pteris aquilina,, underground portion of stipe. Showing thyloses, 
P. th., with pitted transverse walls. X 490. 
the section was obtained was of very large diameter and hence exhibited 
active growth when in the formative state. 
Three of the thyloses exhibit thick walls with pits and are thus of 
the same character as the pitted tracheids shown in figure 15 in longi- 
