6 4 
Rev. T. A. Jefferies. 
ground tissue stretches across the whole space, the cells becoming 
larger as the middle of the stem is approached, a few breaking down 
altogether in the centre ; the vascular bundles are scattered in two 
irregular rings towards the margin, the outer ring lying in the band 
of small cells. At the other end of the stalk, just below the 
inflorescence, the section is considerably modified to secure greater 
rigidity (Fig. 5, D ): the vascular bundles are situated as before, 
but here a strong band of mechanical strengthening tissue appears, 
forming an unbroken cylinder deep enough to include the outer 
ring of bundles and the outer halves of the inner ring; this cylinder 
is further stiffened by stereome girders passing to the epidermis 
from the bundles of the outer ring (compare the girdered bundles of 
the leaves), so that the cylinder has the section of an Ionic column ; 
a strong cutinised epidermis further increases its rigidity; the 
spaces corresponding to the flutings of the column are occupied by 
small celled assimilating tissue, to which air is admitted through 
abundant stomata ; the centre is quite hollow for about half the 
diameter of the section. Compared with this region, that part 
which stands immediately above the “joint” is exceedingly weak: 
this seeming anomaly, however, is adundantly compensated by the 
sheaths of the upper series of leaves (Fig, 5, C), which are freely 
supplied with strong girdered bundles and completely enfold the 
weak part of the stem, usually three deep. Thus the apparent 
weakness is really an instance of the careful husbanding of resources, 
and of the disposal of strengthening tissues in a larger circumfer¬ 
ence where they will have greater efficiency as lateral supports. 
The strengthening zone appears in the stem itself as soon as we 
approach the top of the outer leaf-sheath, and reaches its full 
development before the topmost blade bends away from the stalk. 
Later in the season this mechanical tissue is developed in the weak 
part of the stem also so that, when the leaves decay, the stem can 
maintain its erect position without their support. 
The main facts of the stem structure are summarised in Fig. 5, 
which gives a series of diagrammatic sections. They show 
especially the varying amounts and disposition of the mechanical 
tissues at different levels. A, is taken through the thickest 
part of the basal internode ; B, through its narrowest part, a little 
below the “joint C cuts the stem where it is enclosed with the 
leaf-sheaths; and D illustrates the conditions obtaining generally 
between the leaf-sheaths and the inflorescence. 
