Barber . — The Structure of P achy theca. II. 149 
Surrounding each tube, and fitting into the interstices be- 
tween the neighbouring ones, is a set of indistinct sections of 
much smaller tubes, placed with such regularity that, at the 
centre of the preparation, each large tube is surrounded by 
a group of exactly six smaller ones. The smaller tubes have 
all the appearance of vegetable cells cut transversely, and in 
size they approach more nearly to the cell-filaments described 
in the medulla and cortex, and as possibly passing between 
the oval bodies, in Figs. B ], 2, 5, 6, &c. 
The appearance of smaller tubes in the clear matrix, al- 
though frequently very indistinct, is characteristic of all parts 
of the preparation ; in fact, wherever the yellow matrix is 
well seen, the sections of the smaller tubes may be sought. 
This clear yellow matrix thus presents the appearance of 
a mineral concretion, which has collected around, and follows 
in all its branchings, a framework of algal filaments ; and it is 
only in certain parts of the preparation that the form and 
size of the original cell-rows can be followed. 
Slide 3 (Fig. 3) contains the remaining two sections cut 
from the same specimen. These appear to be cut obliquely, that 
is, halfway between radially and tangentially, and parallel to the 
section on Slide 1. I shall refer to one of these sections only. 
A glance at the figures will substantiate the conclusions 
already arrived at as to the identity of the browner spaces 
with the oval bodies and their outward prolongations. Indeed, 
the general resemblance between parts of this section and 
Figs. B 7 and H 2 is easily recognisable. Further, between 
the sections of the obliquely cut larger tubes may occasionally 
be detected the sections of smaller cell-filaments (Fig. 3). 
From the study of these three slides, it appears probable 
that the cellular structure, apparent under a low power, is to 
be considered as a mineral framework, between the parts of 
which the original plant cells are to be sought. The beauty 
and regularity of the striated walls of the larger tubes lead 
one to suggest that they are made up of numerous parallel 
crystals such as are frequently to be found at the boundaries 
of crystalline concretions ; and such an assumption is con- 
