1go1 | ANATOMY OF THE OSMUNDACEAE 413 
The phloem sheath must have broken into bundles, and the 
endodermis must have looped in between the bundles, and con- 
nected around them on the centrad side. With the formation 
of this astelic type some of the cortex would have been included 
in the medulla, in evidence of which the sclerenchyma in the 
pith would stand as proof. Then next the bundles must have 
fused laterally to produce the gamodesmic type in which there 
is an external and an internal endodermis. Granting that the 
central cylinder could be so plastic in a single species, there are 
left yet to be explained the continuous sheath of phloem, the 
proliferation of sieve tubes opposite the medullary rays, the 
occurrence of isolated tubes in the medullary rays, the occur- 
rence of internal phloem, and the phenomena of the ramular 
gaps. Further, there are no facts in development that point to 
such a series of changes. 
Turning now to the other alternative, namely, the possibility 
that O. cinnamomea has the more primitive form of central cyl- 
inder, it will be granted that by the degeneration of internal 
phloem, endodermis, and medullary sclerenchyma, and by the 
closing of the ramular gaps the central cylinder such as we find 
in O. Claytoniana would result. In proof that such degeneration 
could have taken place, it is to be noted (1) that in O. cinnamomea 
itself, it has been pointed out that the amphiphloic condition is 
localized, that the internal endodermis has already begun to 
degenerate, that medullary sclerenchyma is not a constant fea- 
ture, and that closed steles above the point of branching are not 
at all uncommon; and (2) in further proof, analogous cases of 
degeneration within the same genus are frequent. Thus within 
the genus Equisetum two species such as £. arvense and &. hiemale 
may be chosen, the first long considered medullated monostelic 
and more primitive, the second gamodesmic and considerably 
modified. But a study of development and of nodal por- 
tions of the stem has shown that &. arvense has a reduced cen- 
tral cylinder, the product of degeneration from .. gemedesic 
type, and that therefore &. hiemale is nearer the primitive. Sim- 
ilar cases of degeneration have been pointed out by Van 
