aS 
1898-99.] |. MORPHOLOGY OF THE CENTRAL CYLINDER IN THE ANGIOSPERMS. 617 
ranean stem, the internal phlceoterma is less apparent, and at the base 
of the aerial shoots, both external and internal phlceotermal layers 
cease to be recognizable as such. This disappearance takes place 
pari passu with the appearance of the sclerified sheaths about the 
individual bundles. It is highly probable that the outer layer of 
sclerified cells on the external and internal faces of the bundle-sheath 
represents the phlceoterma, and that the rest of the fibrous tissue is to 
be regarded as sclerified pericycle. It is not easy to come to an opinion 
in regard to the position of the phlceoterma on the flanks of the 
bundles, but since the larger so-called bundles are completely sur- 
rounded by a phlceotermal sheath in the hypogzous stem, it may be 
safely assumed that the larger bundles of the aerial shoots are to be 
considered as morphologically bounded by a similar membrane. The 
smaller bundles in the hypogzeous stem are often united with the 
larger ones by their pericycle; as a consequence, they are surrounded 
by the same phlceotermal sheath. It seems probable that they are to 
be regarded as similarly united with the larger bundles in the epigzous 
shoots. 
The development of 2. sceleratus has been studied by the writer, and 
does not differ essentially from that described in R. acris, in respect to 
the structure of the young central cylinder and the distribution of the 
phlceotermal sheath. 
The young stem of &. repens also resembles closely that of R. acris 
in every respect, both as regards development and the distribution of 
phlceoterma. In the older stem of this species, the phlceotermal sheath 
disappears completely in the internodes of its characteristic runners ; it 
becomes distinct again, however, in the rooting nodes, both internally 
and externally. 
R. abortivus is described by Marié* as having an external phlco- 
terma. The development of this species is, accordingly, of special 
interest. The conformation of the young fibro-vascular tube does not 
differ in any important feature from that of A. acrzs, and consequently 
need not be described. The distribution of the phlceoterma in the 
young stem of this species is, however, of considerable morphological 
interest. In the lower region of the epicotyledonary central cylinder, 
an internal phlceoterma is present, which communicates with the 
phlceotermal layer outside the fibro-vascular tube, through the foliar 
gaps, in a manner quite similar to that obtaining in XR. acrts, as 
described above. The internal phlceoterma is somewhat persistent, 
41. Op. Cit., p. 82. 
