1901] ANATOMY OF THE OSMUNDACEAE 4la 
According to Van Tieghem’s stelar theory, the last two 
facts can be explained only by considering the central cylinder 
of the Osmundaceae to be “gamostelic.” The centrad exten- 
sions of the phloem opposite the medullary rays could then be 
explained by assuming that steles had united laterally, with the 
disappearance of phloem on the medullary side, but with the 
partial persistence of phloem on the radial planes. This would 
also explain the occurrence of internal phloem, the union of 
internal and external endodermis, and the homology of medul- 
lary and cortical tissue. But from the study of development 
there is not a shred of evidence to prove that there has been a 
union of steles. In fact, such a study shows distinctly that 
there is but one stele in the stem of O. cinnamomea from the 
very first. Van Tieghem’s observations on O. regalis have 
already been quoted (see INrRopUCTION); so we cannot describe 
the cauline vascular system as “gamostelic,”’ if this name 
implies a union of steles. 
There remains yet another interpretation, namely, that the 
vascular system of the stem of O. cimnamomea is a siphonostele 
in which some degeneration from the primitive type has taken 
place. It has been pointed out in a description of the concep- 
tion of the vascular system held by Dr. E. C. Jeffrey that the 
most primitive siphonostele is the amphiphloic siphonostele. 
In this there is an internal phloem and phloeoterma, and in its 
phyllosiphonic form there are wide leaf gaps and branch gaps 
through which jnternal and external phloem, internal and 
external phloeterma, and medulla and cortex connect with each 
other. In O. cinnamomea the gaps in this primitive type have 
closed somewhat, so that medulla and cortex rarely connect 
except through ramular gaps. Also the phloem forms an 
almost unbroken cylinder, and the centrad proliferations opposite 
the medullary rays are the vestigial relics of connection between 
external and internal phloem. The internal phloem has also 
disappeared in greater part. 
With such a conception of the cauline vascular system of 0. 
cinnamomea, the centrad accumulation of sieve tubes opposite 
