in the Ge7ins Primula, 
3 * 
and flattened. The central parenchyma is, in the median 
stele, generally marked off as a distinct medulla, but in the 
smaller steles it is either scanty or absent. Both in P .japonica 
and in P. denticulata the whole stele is surrounded by an 
exceptionally well-marked endodermis enclosing a collenchy- 
matous pericycle, the latter consisting of about three or four 
layers towards the lower surface of the leaf, and rather less 
towards the upper. 
In order to account for the appearance of these steles in 
the petiole, I will now describe the changes that the leaf-trace 
undergoes during its course. In these two plants the leaf- 
trace invariably arises from one of the steles in the stem 
as a single large meristele. It is at first curved into the 
shape of a horseshoe, open towards the interior of the stem, 
and is completely surrounded by the endodermis, which is 
folded round the corners of the horseshoe, following the 
concavity of the inner side. Within the endodermis lies the 
pericycle, and the concavity between the latter and the xylem 
is occupied by a few conjunctive cells continuous with the 
pith of the stem-stele. As the meristele passes through 
the cortex of the stem it branches so as to form several 
strands, the extreme lateral branches remaining meristeles 
throughout the petiole. As the three median strands (to 
which alone I now refer) are followed in their outward course, 
the endodermis is seen to straighten out across the gap 
between the arms of the horseshoe, gradually falling less 
and less into its concavity (Fig. 5). At the same time, the 
vascular tissue at each corner gradually extends itself across 
the gap so as to meet that approaching from the other side, 
and thus to complete the circle, so that finally a complete 
stele is the result. Again, towards the tip of the lamina the 
steles gradually lose their internal bundles in an inverse but 
perfectly analogous manner, becoming meristeles once more. 
Although I do not wish to lay too much stress upon the 
value or necessity of a strict continuity of tissues, yet it 
should be noted that these steles are essentially different 
from the single central stele described by Van Tieghem in 
