[KIRSCH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 391 
sonably be drawn from this appearance, is that the thyloses are capable 
of arising at various times in the course of the season’s growth, their 
development, in all probability, being determined by local conditions. 

Fig. 22.—Same section as fig. 21 at a lower focus. Showing canal, C., with 
thylose, Th., growing into it. X 490. 

Fig. 23.—Pteris aquilina, rhizome. Showing thick-walled thylose,°Th., with 
disorganised protoxylem elements,, A.tr., Sp.tr., showing through. 
X 367.5. 
Figure 23 shows a thick-walled, lignified, thylose from the same 
region, which seems to contain remains of protoxylem elements; these 
elements are, however, undoubtedly in a space below the cross-wall of 
