AND ANATOMY OF THE STEM OF LYCOPODIUM. 19 
phloem-strips. Outside the vascular tissue the endodermis is sharply differentiated from 
the pericycle and stains very similarly to the xylem; the inner cortex, and to a less 
degree the outer cortex, forms the mechanical tissue, while the median zone serves for 
water-storage. ‘The cells of the median zone run obliquely, at about an angle of 20° with 
the vertical as seen in longitudinal section. 
The leaf-traces pass down into the stem, entering near the upper insertion of the leaf, 
bend inwards at a slight angle with the vertical, and shortly connect on to a protoxylem. 
As there is a single leaf-trace it might be expected that the number of leaves on the stem 
or branch would furnish an indication of the number of protoxylems, and the leaf-traces 
would in turn join up to the various protoxylems. This is not found to be the case. 
Where the number of leaves on the smaller branches is reduced to four or six, then it is 
usual to find a corresponding number of protoxylems ; but where the number of proto- 
xylems increases beyond six, then there is no apparent proportion between the two. Pl. 4. 
fig. 9 gives the general appearance of a stem with thirteen protoxylems, and in the figure 
the points at which the successive leaf-traces connect up to the central cylinder are repre- 
sented, the insertion of the traces having been followed through six nodes. The positions 
of the protoxylems vary slightly, and their number may change, especially towards the 
ventral side; but taking the protoxylems, either lateral or dorsal, which remain nearly 
constant in position, it will be observed that in the six whorls of leaves some protoxylems 
connect with as many as six leaf-traces, others only with four. 
LYCOPODIUM ANNOTINUM, Linn. 
This species in its structure approaches L. clavatum very nearly. The main axis 
creeps along the ground, producing roots ventrally and branches laterally. "The leaves 
on the main shoots can be resolved into alternating pseudo-whorls, and a common 
arrangement consists of alternations of six and seven leaves. The roots and branches are 
fewer in number than in Z. clavatum, there is not the same marked difference between 
short and long shoots, and the number of short shoots is smaller. On branches of the 
second and higher orders the leaves are arranged in alternate whorls of four, and this 
applies also to branches of the first order which do not form runner-shoots. All parts 
of the plant, except the strobilar branches, are plagiotropie. The internal structure of 
the stem, so far as the vascular cylinder (Pl. 4. fig. 10) is concerned, is essentially similar 
to that of L. clavatum. The number of protoxylems in the specimens examined did not 
exceed twelve and the branches showed six to eight. The cortex is not, however, differ- 
entiated into three zones, but practically the whole consists of thick-walled tissue; the 
leaf-bases are decurrent, so that the stem is almost entirely invested, and, concomitantly 
with the absence of the aqueous tissue, the epidermis is more heavily cuticularised. The 
transverse section shows a cavity between the cortical tissue and the mesophyll of the 
decurrent leaf-base. This cavity extends from the point where the leaf joins the stem 
and continues through the swollen leaf-base, but does not continue into the free portion 
of the leaf. The position of the cavity c, is indieated in Pl. 4. fig. 11, and between 
the epidermis and the cavity there are three or four layers of parenchymatous cells 
(fig. 12). The probable function of the space is for aeration, since it contains no 
