Tissues in Certain Monocotyledons . 47 
following structure (see Fig. 9). The middle part of the stem 
is occupied by a well-marked central cylinder, of lenticular 
section, which presents the ordinary characters of monocoty- 
ledonous structure. The scattered vascular bundles (which 
number 140 or more) are of extremely unequal size. The 
ground-tissue of the cylinder is thin-walled in its inner part, 
but becomes sclerotic towards the exterior. The cortex is 
conspicuously thicker at the ends than at the sides of the 
section, so that the stem as a whole is more strongly flat- 
tened than is the stele. The cortex is traversed by leaf- 
trace bundles. They are collateral here, as they are in the 
leaf, the xylem only partly embracing the phloem. As 
soon as the bundle enters the cylinder, however, it .becomes 
concentric. 
If we now consider the structure of the primary cylinder 
rather more in detail, the first point to be noticed is that 
the vascular bundles differ among themselves in structure as 
well as in dimensions. Only a few of the larger bundles have 
any definite group of protoxylem. Of all the bundles in the 
cylinder perhaps one-eighth possess protoxylem (see Fig. 12, 
px). When present it occupies the usual position on the 
proximal 1 side of the strand. The large and small bundles 
are scattered irregularly throughout the cylinder; the larger, 
however, are more frequent towards the middle, the smaller 
towards the outside, to which part the smallest of all are 
limited. Several large bundles are always grouped near the 
centre, sometimes forming a ring around a central point of 
the ground-tissue which might be called pith. Of these 
inner bundles some, but not all, have protoxylem. 
The bundles with protoxylem are those which are dif- 
ferentiated earliest, namely the upper parts of the principal 
traces. The lower portions of the latter and the finer 
1 The terms inner and outer are confusing in this connection as it is often 
doubtful whether they refer to the individual vascular bundle, or to the stem as 
a whole. We therefore propose to use the word proximal for that side of the 
bundle (or other structure) which is turned towards the centre of the axis, distal 
for that side which is remote from the centre. 
