86 BRUCE—STRUCTURE AND FUNCTION 
numerous intercellular spaces. Just underneath the vascular 
bundle of the mid-rib is a group of large cells which do not seem 
to have protoplasmic contents. The vascular bundle is sur- 
rounded by an endodermis of polygonal and rounded cells. 
On the upper side of the vascular bundle and above the endo- 
dermis, and separated from the epidermis by a single layer of 
chlorenchyma, is a crescent-shaped group of sclerenchyma ; and 
between the endodermis and the large loose tissue underneath 
the vascular bundle is another similar group. The xylem is 
situated in the centre of the bundle, and at the two lower corners 
there is a single phloem-group. These two phloem-groups are 
quite separate from one another, and are not united to each 
other in the middle line under the xylem. 
The above description of the anatomy of the leaf applies to 
a section about its base or middle. Towards the apex the 
structure changes. At the base it is dorsiventral and swollen 
near the mid-rib, with the sides of the lamina nearly horizontal 
(Fig. 11). Upward the laminar sides become smaller (Fig. 12), 
more raised from the horizontal (Fig. 13) and thicker, until the 
outline of the leaf on transverse section appears like a thick 
flattened U (Fig 14). Close to the apex the laminar sides ~ 
disappear, so that the outline becomes circular. The vascular 
bundle is here situated in the centre, surrounded by a ring of 
chlorenchyma, outside of which is the epidermis in the outline of 
a circle (Fig. 1 5). The large loose cells situated underneath 
the vascular bundle at the base of the leaf are less developed 
towards the apex, where they entirely disappear. At the base 
they are separated from the epidermis by a single layer of small 
cells which do not bear chlorophyll. About the upper third of 
the leaf they become fewer in number and smaller in size, and 
form a small group separated from the upper epidermis by two 
layers of chlorenchyma. At the apex they gradually merge into 
the bundle-sheath. This circular apical part of the leaf gradu- 
ally passes into the terminal tentacle, so that it is quite 
impossible to say where the leaf ends and the pedicel of the 
tentacle begins. 
The structure of the marginal vascular bundles is similar to 
that of the large central one, only on a smaller scale. 
