1898-99.] MORPHOLOGY OF THE CENTRAL CYLINDER IN THE ANGIOSPERMS. 633 
smallest morphological problems, all interpretations of the greater 
questions of morphology must ever remain more or less hypothetical. 
The persistent recurrence of a tubular stelar system throughout the 
various groups of vascular plants suggests that the tubular arrangement 
of the primitive skeletal tissues is often an advantage. The nature of 
the advantage is apparently not far to seek. One important function 
of plant axes is to afford support to their appendicular organs, and we 
may regard the tubulization of the stele as an adaptation for this 
purpose. Where the mechanical function is taken on by the extra- 
stelar tissues, siphonostely is often absent or degenerate. Further, 
those organs of plants which are normally supported by the soil, viz., 
the roots, are not primitively siphonostelic at all, and in the com- 
paratively rare cases where their central cylinder is medullated, its 
parenchyma is derived from the pith of the stem. The truth of the 
latter statement will be more apparent when the development of the 
stem in certain Filicales has been described. It would consequently 
appear that the tubular fibro-vascular cylinder, which is so characteristic 
of the cauline axis of the Angiosperms, is the result of the operation of 
mechanical causes, and that the anatomical peculiarities which distinguish 
the primary central cylinder of the root from the stelar tube of the 
shoot are primitive features, retained undisturbed by the mechanical 
influences acting on the stem. 
SUMMARY OF RESULTS AND CONCLUSIONS. 
1. The polystelic type of Van Tieghem is not characterized by the 
repeated bifurcation of the epicotyledonary stele, but there is primitively 
in the young stem of this type a tubular concentric stele with foliar 
gaps subtending the points of exit of the leaf-traces. 
2. The astelic type of Van Tieghem does not result from the separa- 
tion of the epicotyledonary stele into its constituent bundles ; for in the 
young so-called astelic axis, there are no bundles present at all, but a 
collateral stelar tube with foliar gaps subtending the leaf-traces, through 
which the internal and external phlceotermal sheaths communicate. 
3. The medullated monostelic type of Van Tieghem does not originate, 
as he states, by the dilatation of the epicotyledonary stele and the forma- 
tion of an zztraste/ar pith ; for the writer’s observations show that in favor- 
able cases, both among the Dicotyledons and the Monocotyledons, the so- 
called medullated monostelic central cylinder of the older stem may be 
seen to be derived from the so-called astelic condition of the young 
axis, by the degeneration of the internal phlceoterma. 
