[xirsca] CERTAIN STRUCTURES IN THE PTERIDOPHYTES 367 
tion is a function of the growth, and thus varies with the rapidity of 
growth. 
In a bud which measured about 4 in. the following was noted in 
transverse sections taken from the tip to the base. 
Sections obtained from the tip for a distance of 1 in. showed the 
tissues to be very immature, the immaturity extending to all parts of 
the vascular bundle except the protoxylem. The latter was well formed 
and presented a mature appearance, except that the cell walls were a 
little thinner than in sections taken lower down. ‘The secondary 
xylem was very thin-walled, and showed a quantity of protoplasmic 
contents, thus being still in a formative stage. In some of the bundles 
the protoxylem elements showed considerable compression, but aside 
from that appeared normal. 
About 14 in. from the tip the structure was very similar to the 
above, except that the secondary xylem had thicker walls and contained 
less protoplasm. In some of the bundles, however, the protoxylem 
showed a considerable variation. Here and there some of the proto- 
xylem elements were seen to be splitting away from each other, the 
separation taking place at the juncture of the cell-walls (see Fig. 1). 

Fig. 1.—Pteris aquilina, bud of stipe. Showing the first break in the proto- 
xylem, Prx., and contents of ruptured wood-parenchyma cells in 
canal, C. X 490. 
This separation is primarily due to the growth and rounding off of the 
secondary xylem elements and gives rise to lacunæ between the constitu- 
ent cells of the protoxylem. Whilst the secondary xylem is still in a 
formative stage, the protoxylem is already fully formed and functional, 
and incapable of further growth; the growing expanding xylem thus 
tears away the protoxylem elements, and thus the more rapid the growth 
cf the region, the more rapidly will the protoxylem elements be torn 
apart. 
That the lacunæ arise schizogenetically is unquestionable, for they 
