4.6 Scott and Brebner . — On the Secondary 
the phloem towards the nearest surface of the leaf. Occa- 
sionally concentric (amphivasal) bundles are found in the 
middle of the mesophyll. The bundles forming the ring each 
have a stout strand of sclerenchyma outside the phloem. 
Isolated strands or plates of sclerenchyma are also present in 
the leaf, especially at its edges. The central part of the 
mesophyll is colourless. 
The leaf-base, which completely embraces the stem, has of 
course a different structure, and is in fact bifacial. We only 
found stomata on the outer (morphologically lower) surface, 
to which also the assimilating tissue is limited. The xylem of 
the bundles is here directed towards the upper surface. The 
sclerenchyma is mainly towards the upper surface, where it 
forms a continuous hypodermal layer. 
The leaf-base has about twenty bundles altogether. The 
two largest are both median, lying one behind the other in the 
same radial plane. The other bundles are mostly of fairly 
uniform size, but become gradually smaller in the posterior 
direction. There are a few much smaller scattered bundles, 
usually placed further to the exterior than the main ones. 
The leaf-traces on entering the stem curve in towards the 
middle of the cylinder, and then very gradually pass outward 
again, fusion between the bundles taking place towards their 
lower ends. In other words, the course of the bundles belongs 
to the familiar Palm-type. The larger bundles penetrate 
most deeply into the cylinder. The upper median bundle on 
entering the stem turns sharply upwards, and then as sharply 
down again, to take the usual course into the cylinder. We 
found that the number of vascular bundles in the transverse 
section of the primary cylinder of the stem averages about 
seven times the number of bundles in a leaf-base. Hence we 
may infer that on the average the bundles pursue a separate 
course through about seven internodes. It is probable, how- 
ever, that this varies greatly, even among the bundles from the 
same leaf. 
If we examine a transverse section of the flat stem, in 
a region where living leaves are still present, we find the 
