1898-99.] MORPHOLOGY OF THE CENTRAL CYLINDER IN THE ANGIOSPERMS. 601 
Instead of expanding and becoming medullated monostelic, or pass- 
ing from this modification into the astelic condition, the fibro-vascular 
axis, may, according to Van Tieghem, undergo successive bifurcations 
and thus become polystelic. This type of central cylinder is very preva- 
‘lent among the vascular cryptogams, and occurs also in certain species 
of Primula and Gunnera among the Phanerogams. In a cross section 
of an older stem of this type, numerous concentric fibro-vascular strands 
are to be seen imbedded in a matrix of fundamental tissue. The origin- 
ally separate steles of the polystelic axis, may unite, according to Van 
Tieghem, to form a concentric annular stele, which is described as gamo- 
stelic. 
More recently, Van Tieghem? has returned to the subject of the central 
cylinder of the Equisetacee. In his essay on Polystely, he describes 
the central cylinder of 4. arvense, E. pratense, etc., as medullated 
monostelic; 4. /zosum and &. /ttorale as astelic, and the aerial shoot of 
E.. hiemaleas gamodesmic. Confirming an earlier research of Pfitzer,* 
he recognizes that in the species originally described by him as medul- 
lated monostelic, viz., E. arvense and E. pratense, etc., there are, in the 
region of the nodes, and at the basis of the smaller branches, well 
marked indications of an internal endodermis, which disappears in the 
internodes, only to recur in the successive nodes. He concludes that it 
was primitively present throughout the entire length of the stem, and 
has become in these species vestigial, persisting only at the nodes. He 
expresses the opinion that the astelic type of central cylinder, as found 
in £. “mosum, is the primitive one, and that by fusion there resulted the 
gamodesmic type found in the aerial shoots of &. Azemale, and the 
subterranean stem of £. szl/vatecum. Forms like EL. arvense, E. pratense, 
and £. scerpoides, are also gamodesmic, although the fact is obscured by 
the partial degeneracy of the internal endodermis. Asa result of this 
investigation, it is apparent that the medullated monostelic type of 
central cylinder does not exist among the Equisetacee, and that those 
species which at first sight appear to possess a stelar system of this type 
are really degenerate astelic gamodesmic. The writer has recently 
studied the development of the young stem in the Equisetacez,‘ and has 
shown that the stelar system in the young axis of this group, is 
primitively gamodesmic, possessing a well-marked, continuous, internal 
endodermis. The modification, in which the bundles are surrounded by 
individual endodermal sheaths, appears quite late in the development of 
2. Remarks sur la Struct. de la Tige des Préles. Journ, de Bot., 4, 1890, p. 365. 
3. U. d. Schutzscheide d. Deutsch, Equiset., Jahrb. f. wiss. Bot. 6, 
4. Mem. Bost. Soc. Nat. Hist. Vol. 5, No. 5, p. 171. 
