164 MR. P. GROOM—CONTRIBUTIONS TO THE 
7. There is a pericambium. 
8. The xylem is well differentiated, but has no vessels. 
9. The phloém is marked by a large increase in the parenchy- 
matous cells and a dwindling of the sieve-tubes. 
10. A pith is present and stores up starch and has a few 
raphide-mucilage sacs. 
11. In the cortex there is sometimes a centrifugal production 
of mechanical cells, whilst in the pith there is simultaneously a 
centripetal manufacture of mechanical cells. 
12. The distribution of the raphide-mucilage sacs appears to 
correspond with that of starch. Usually they are present in 
largest quantities in the mediocortex, and to a less extent in the 
endocortex. But where (specimen B) the distribution of starch is 
reversed, so is that of the raphide-mucilage sacs. In the root-cap 
mueilaginous degeneration is heralded by an abnormal accumula- 
tion of starch. On the other hand, when cells become lignified, the 
mucilage-raphide sacs lessen in number (pith and endocortex). 
Histology of the Rhizome. 
Sections through an old part of the rhizome reveal the following 
structure :— 
1. A regular epidermis with thick cuticularized walls. Here 
and there cells grow out into “ root-hairs.” 
2. The cortex is differentiated more or less in the same style as 
the root. Externally are a few layers of radially flattened cells, 
which towards the interior gradually merge into cells poly- 
gonal in transverse section; and these in turn are succeeded 
by cells with thick lignified walls, which are continuous with 
similar cells in the root. Close to the attachments of a root, 
the outermost cortical cells are sharply marked off from the cells 
lying within, and they contain less starch and often distinct 
mycorhizal hyphe: that is, there is an exocortex. In these regions 
they are succeeded internally by a mediocortex with mycelial 
masses, which are continued from the root. Starch is present 
most abundantly in the cortex near the region of attachment of 
the roots, and is conspicuously less in quantity in the immediate 
neighbourhood of the lateral buds. Large and numerous strings 
of raphide-mucilage cells occur in the cortex. 
There is no distinct endodermis. The vascular bundles are 
large and numerous. They are typically collateral. Compared 
with the root and even the inflorescence-axis, there is a large 
