Fa 
1898 99.] MORPHOLOGY OF THE CENTRAL CYLINDER IN THE ANGIOSPERMS. 603 
Van Tieghem) in which the internal endodermis has become more or less 
obsolete. It has further been rendered probable by the interesting in- 
vestigations of Poirault® that the apparently medullated monostelic 
central cylinder of the stem of the gleicheniaceous genus, Platyzoma, 
also possesses a pith derived from the extrastelar fundamental tissue. 
The only remaining example cited by Van Tieghem in his classic 
essay on Polystely, of a Cryptogam possessing a medullated monostelic 
central cylinder is that offered by the Osmundacee. The writer hopes 
to show in a memoir on the anatomy of the Pteridophyta, which will 
appear shortly, that here too, the pith is in reality an included portion of 
the extrastelar fundamental tissue. 
The medullated monostelic type of central cylinder may accordingly 
be regarded as of very doubtful occurrence among the Cryptogams, and 
in those cases where it is apparently present, it is derived from a modi- 
fication of Van Tieghem’s gamodesmic type. There would thus appear 
to be very slight evidence for regarding the medullated central cylinder, 
where it occurs among the Cryptogams, as derived from the dilatation 
of an originally pithless stele. It is a well-established principle with 
morphologists to attempt always the explanation of the structure of the 
- higher plants by the more easily understood corresponding features of 
the lower groups. It seems to be in harmony with this method to eluci- 
date the obscurities of the morphological interpretation of the central 
cylinder in the Angiosperms by the facts derived from the study of the 
anatomy and development of the Pteridophyta and Gymnosperms. 
The writer will shortly publish a memoir describing his studies on the 
latter groups. In the present essay, he proposes to examine in a 
general way, certain features of the structure and development of the 
Angiosperms, which in the light of the investigations mentioned above, 
seem to afford to some extent an elucidation of the morphology and 
phylogeny of the higher Phenogams. 
PROBLEMS. 
The questions to be treated in the present memoir are briefly as follows: 
Has Van Tieghem correctly described the mode of origin of his poly- 
stelic type of central cylinder? Is his astelic type essentially different 
from the polystelic? Does the medullated monostelic central cylinder 
ever arise by the dilatation of a primitively pithless stele? What are the 
salient anatomical features of the central cylinder of the Angiosperms ? 
8. Op. Cit., p. 182. 
