Lycopodium ' complanatum and Lycopodium clavatum . 223 
however, be due to an insufficient quantity of this substance being present 
to be detected by these tests, as the sieve-tubes are very narrow. 
The cells immediately surrounding the protoxylem are of cortical 
origin in the lower region of the stem, and also in the upper parts in those 
plantlets which have not appeared above the ground. The following 
regions can be distinguished external to the stele (see PI. XXII, Fig. 14) : 
(1) an irregular ring (pc) of thin-walled, fairly large cells (pericambium), 
which abut on the xylem and phloem regions. Some parts of the walls 
of these cells show signs of lignification, so that it is often difficult to draw 
a distinct boundary between this and (2) the protective sheath (en), which 
immediately surrounds it, and which also consists of an irregular band of 
cells, one, two and occasionally three cells in thickness, the walls of which 
are not thickened but strongly lignified. Outside the protective sheath 
are (3) thick-walled cortical elements (; ic ), closely packed cells, the walls 
of which show lignification at the corners in the neighbourhood of the 
protective sheath-cells. Proceeding outwards, the cells become larger and 
thinner-walled, gradually merging into (4) a much more loosely arranged 
layer of cells (nc) ) in which large intercellular spaces occur. It is the 
presence of these large intercellular spaces in the outer cortex, and also 
the greater differentiation of cells forming the cortex and epidermis, which 
distinguish the basal region of the stem from that of the root in the 
neighbourhood of the foot, where the stelar structure is very similar. 
Bounding the cortex externally is (5) the epidermis (ep), the cells of which 
vary in size, especially in the underground parts; their outer walls Are 
thickened, and in the aerial parts numerous stomata occur. 
(b) Changes occurring in the vascular structure . The arrangement of 
the xylem and phloem elements is subject to many changes, especially in 
the lower region of the stem, where the various obstacles met with in the 
soil have resulted in the stem taking a very irregular course. The strands 
of xylem are constantly uniting, then separating, or by extension of the 
protoxylem elements at the periphery of the stele a further subdivision 
may take place. There is, however, in the young sporophyte a tendency 
towards the formation of a solid core of xylem in the centre of the stele, 
and in the upper parts of the stem a more constant arrangement is 
maintained. Here a tri- or tetrarch condition occurs, large metaxylem 
elements in the centre uniting two groups of protoxylem, sometimes 
a third, and occasionally a fourth group. When all the groups are united 
the triarch condition resembles that occurring in the smaller branches of 
P silo turn and in yo'ung stems of Sphenophyllum in which secondary 
thickening has not begun. , 
Diagram 3, Figs. 1-10, shows the changes which occur in the arrange- 
ment of xylem and phloem in the younger sporophyte of L. complanatum , 
proceeding upwards from the point where the stele of the first root joins 
