[kIrscH] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 375 
longer than broad, they are almost isodiametric and thus present the 
same appearance as the thyloses which originated from them. The thy- 
loses are very thin-walled and contain fine granular protoplasm. 
A stipe collected at the same time as the above (May 29th), but 
which measured only 2.5 em. in length, was sectioned in order to note 
‘any differences due to the obviously slower growth. The whole stipe 
proved to be in a very formative state, and all the tissues of the vascular 
bundles had very thin walls, which, at the tip, were practically invisible, 

Fig. 4.—Pteris aquilina. stipe, ant. Coll. May 29th, 1899. Showing forma- 
tion of canal, C., and protoxylem, A.tr., floating away. X 490. 
so that the cells in this region were only defined by the clear spaces 
between the protoplasmic masses. The secondary xylem cells did not 
exhibit definitely thickened walls at any part of the stipe except at the 
base, and were frequently ruptured. All the cells except the protoxylem 
were rich in protoplasmic contents. 

Fig. 5.—Pteris aquilina. Stipe, same as figure 4. Showing beginning of 
canal, C. X 490. 
At the tip the protoxylem was not very well formed, but it still 
showed breaks, and broken off protoxylem elements were to be seen float- 
ing in the canal (Fig. 4). The canals were not very large, this being 
due to the slow growth. 
About 1 cm. below the tip the protoxylem elements assumed a more 
definite shape, and were splitting away from each other (Fig. 5), form- 
