1898-99.] MORPHOLOGY OF THE CENTRAL CYLINDER IN THE ANGIOSPERMS. 599 
LHE “MORPHOLOGY, OF.THE CENTRAL. CYLINDER 
IN THE ANGIOSPERMS. 
By EDWARD C. JEFFREY, B.A. (ToR.), Ph.D. (HARV.) 
(Read May 6th, 1899.) 
The prevailing views in regard to the morphology of the vascular 
apparatus of the higher Cryptogams and the Phzenogams are due to the 
studies of Van Tieghem and Strasburger. Both these writers have 
abandoned the conception of fibro-vascular strands, which is embodied in 
the classic comparative anatomy of De Bary. In this treatise, the axial 
organs, primary and secondary, of the higher plants are regarded as 
composed of more or less modified fundamental tissue of a parenchy- 
matous nature, bounded externally by the integumentary tissues of the 
epidermis and traversed by conductive strands typically composed of 
vessels and sieve-tubes. These fibro-vascular strands may consist of 
clusters of xylem and phloém placed side by side, and in this case the 
strands are called collateral fibro-vascular bundles. In another type the 
phloém surrounds the xylem, and the bundle is then described as con- 
centric. The latter type, by the localization of the phloém at intervals 
on the outside of the xylem, becomes the radial bundle. 
' As a result of his study of the anatomy of the stem of the phenoga- 
mous order, Primulacee, Van Tieghem’ published in 1886 an import- 
ant memoir on the morphology of fibro-vascular strands, which has 
formed the basis of most of the subsequent anatomical work in this direc- 
tion. Although the general conclusions reached are founded on the study 
of the Primulacez alone, Van Tieghem extends them to all vascular plants, 
including the vascular Cryptogams. The primitive type of axis, accord- 
ing to Van Tieghem, contains but a single concentric fibro-vascular 
strand, which is limited externally by a specialized layer, the pericycle. 
The innermost stratum of the fundamental tissue, immediately adjoining 
the pericycle, is also differentiated from the outer cortical tissue and is 
named the endodermis. The fibro-vascular axis so delimited is called 
the central cylinder or stele. This sort of fibro-vascular axis is found in 
the stem of many Cryptogams and in the root of nearly all vascular 
1. Sur la Polystélie. Ann, de Sci. Nat. Bot., 7 sér., tom. 3. 
