336 
tracheids in the parenchyma. Duval-Jouve (14) and other 
authors found separate tracheids in the parenchyma of leaves 
(Nepenthes, Salicornia) ; while the occurrence of a Caspary layer 
in a secondary cork tissue is known (Tieghem, 1891). In 
the second place the ‘‘’outer”” Caspary layer in the ferns seems 
to be formed from the same meristematic cell as the pericycle 
and the phloem! Many inaccuracies have been committed in 
the use of the word endodermis. English and American authors 
seem to mean by this word the German ,,Schutzscheide‘ : 
while the other form of endodermis, the ,,Stärkescheide‘ 
occurs much more frequently. It is probably due to this fact 
that often the endodermis is said to be absent. 
Too much importance has been attached to the questions 
concerning this endodermis and the internal pith. As Bower 
has pointed out, the internal pith may be caused by abundant 
nutrition. L 
Statement of Problem. 
The question of the pith is of secondary importance as we 
are by no means certain that the stelar theory will hold in its 
rigid form. In fact, embryologic evidence, gained in the last 
ten years, seems to suggest the phytonic explanation rather 
than the stelar. Our general statements, however, will be con- 
fned to the Eusporangiate ferns, because these forms are the 
best known in embryological detail. The work of Camp- 
bell (9), Jeffrey (19), Lyon (24), and Farmer and Hill (15) 
gives us a fairly complete picture of the general development 
of the group. Ît seemed desirable, however, in connection 
with the views expressed in this chapter to investigate the for- 
mation of the young vascular structure in one particular genus 
of this group; Boirychium. The unusual facilities of Dr. Camp- 
bells laboratory, both as to material and advice, enabled me 
to carry out this research. 
