* 
600 TRANSACTIONS OF THE CANADIAN INSTITUTE. [VoL. VI 
plants, both cryptogamous and phzenogamous. On account of the fact 
that the stele or central cylinder in such axes is obviously single and 
undivided, they are designated monostelic. 
According to Van Tieghem the monostelic central cylinder is some- 
times modified by becoming expanded. The stele grows larger, and a- 
certain amount of the stelar parenchyma becomes aggregated in the 
centre to form the medulla or pith. From the pith, strands of paren- 
chyma radiate outwards towards the pericycle and constitute the medul- 
lary rays. These parenchymatous rays divide the xylem and phloém 
into segments, designated meristeles or bundles, which, in his opinion, 
are morphologically very different from the concentric bundles of De 
Bary. The pith and the medullary rays are also of a different morpho- 
logical origin from the extracylindral fundamental tissue of the cortex. 
The whole complex medullated central cylinder of this type is shut off 
from the cortex by the same two layers, vzz., the pericycle and the. 
endodermis, as is the primitive non-medullated concentric fibro-vascular 
strand. This modification of the central cylinder is found in the stems 
of nearly all the Phanerogams, and, in isolated instances, among the 
Cryptogams, ¢.g., the Osmundacez, the genus Botrychium, and certain 
species of Equisetum. 
In the medullated monostelic axis, the endodermis and pericycle, 
according to Van Tieghem, sometimes bend inwards between the meri- 
steles and break into pieces at the bottom of the sinuosities. The frag- 
ments of the thus interrupted pericycle and endodermis unite around the 
inner side of the individual bundles, which they in the first place merely 
subtended. The original medullated monostelic central cylinder is 
considered to become, as a consequence, astelic, since its pith and rays 
are imagined to become continuous with the fundamental tissue outside 
the stele. The astelic type of central cylinder is found in certain amphi- 
bious or limicolous phenogamous orders and among the Cryptogams in 
the genus Ophioglossum and certain species of Equisetum. In these 
cases the separate meristeles, each surrounded by its own pericycle and 
endodermis, may unite so that a more or less complete fibro-vascular 
ring is formed, bounded both externally and internally by a continuous 
pericycle and endodermis. The central cylinder is then said to be 
gamodesmic. For example, in the genus Equisetum, 4. /mosum and 
E. litorale have the individual meristeles completely surrounded by an 
endodermis and pericycle; in the rhizome of £. se/vaticum and in the 
aerial shoots of Z. Azemale, on the other hand, the meristeles are fused to- 
gether and the ring of united meristeles is bounded both outwardly and 
inwardly by a circular endodermis. 
