18 MR. C. E. JONES ON THE MORPHOLOGY 
inserted on the lower side, are curved upwards. The branches form either short shoots 
which produce secondary and sometimes tertiary branches, seldom grow beyond a few 
centimetres, and develop no roots, or long shoots which reproduce the structure of the 
main axis. The short shoots persist as long as the main axis on which they arise; the 
long shoots not only help to spread out the original plant, but also serve as a means of 
propagation when the main stem dies away behind, and each long shoot then becomes 
an independent plant. The arrangement of the leaves in the adult condition conforms 
to no recognised system: the leaves do not fall on a spiral, but it is possible to group the 
leaves into pseudo-whorls, and alternate pseudo-whorls are similar. This arrangement 
is not always obvious, but due regard must be paid to changes occurring at the insertion 
of branches. A very constant feature, practically for all Lycopods, is the prolongation of 
the leaf-base. As an external feature, and one likely to be preserved in casts and 
impressions, it might prove to be of some importance in the identification of fossil 
remains, even though many of the Conifers ave similarly distinguished. 
Passing from external features to the internal structure of the stem, the arrangement of 
the vascular tissues is similar to that which occurs in roots (Pl. 3. fig. 2 and Pl. 4. fig. 8); 
round the periphery there are situated a varying number of protoxylem-groups, and 
arranged alternately between these a similar number of protophloems. The vascular 
tissue fills up the centre—whether, as in young stages or in ultimate branches, this consists 
of solid xylem, or, as in the main stem and larger branches, opposite xylems are joined up— 
and the general appearance is presented of alternating xylem- and phloem-bands. Around 
the vascular tissue is situated a pericycle varying generally from two to six layers, narrower 
opposite the protoxylems and broader opposite the phloems, and this is again surrounded 
by a 2-3-layered endodermis. The cortex consists of three zones (Pl. 3. fig. 1), a broad 
zone, €, made up of cells with thick lignified walls, which abut upon the endodermis, a 
region of thin-walled cells, ¢,, encircled by a narrow outer zone, c, in which the cells have 
their walls slightly thickened. "The stem is to a very great extent covered by leaf-bases, 
and in these a loose mesophyll separates the outer zone of cortical cells from the epi- 
dermis, but in the stem proper these layers are in contact. The vascular cylinder (PI. 3. 
fig. 2) is somewhat oval and dorsiventrality is obvious in the arrangement of the vascular 
tissues. The parallel bands of the xylem and phloem lie approximately in a horizontal 
plane. The xylem-patches situated dorsaly and ventrally are either heart-shaped, 
crescentic, or in the form of an £. The number of protoxylems is fairly constant 
throughout the axis, and changes in the number are often associated with the development 
ofabranch. The main axis of well-grown plants contains twelve to sixteen or more proto- 
xylems, the long shoots show about as many as the original axis from which they develop, 
the short shoots have fewer (six to four). The protoxylems are very obvious, not so the 
protophloems ; it is easy to trace the protophloems near the apex, more especially 
since they are differentiated early, but in the mature condition they are not so definite; 
the best differentiation between phloem and pericycle was obtained in sections stained 
with hematoxylin and subsequently washed with water. As stated before, the proto- 
phloems and protoxylems alternate, so that the arrangement of these is radial. The 
disposition of the sieve-tubes is definite, as they form a Single row in the centre of the 
