154 Barber . — The Structure of P achy theca. II. 
The tubes at the centre are not, however, always united 
into a tissue, but frequently leave intercellular spaces be- 
tween them ; and these latter are filled up by crystals of 
the same white substance as that of which the mass of the 
section is composed. The walls are usually composed of two 
layers, similar to the ‘ shells ’ of thickened cell-walls ; and the 
outer layer is of the same thickness as the striated walls of 
Slide 2 , without, however, their brown-yellow colour, or well- 
marked striae. 
The tubes near the periphery are obliquely cut, and 
present much the same characters as the central ones. In 
this part of the section, however, the outer shell, bordering 
upon the intercellular spaces, is occasionally of a darker 
colour, and presents numerous parallel striae ; it is, in these 
parts, identical, in appearance and position, with the thick 
striated walls already referred to. 
The brown, granular contents of the tubes in this prepara- 
tion are evidently, from their disposition and size, the cell- 
filaments of the plant ; and the white substance forming the 
tubes is the mineral concretion around them. There is no 
appearance of structure, whether of transverse or longitudinal 
walls, in the cells. 
Slide 9 bears a section cut through the centre of the 
organism, which presents much the same state of preservation 
as section 8 just described. 
The cell-filaments are represented by granules without any 
transverse or longitudinal walls. In the medulla there are 
occasionally well-defined, thick, white tubes around these 
granular cores ; but in the greater part of the section, in- 
cluding the cortex, the region of oval bodies and most of the 
medulla, the white mineral has fused to form a translucent 
basis in which the course of the cell-filaments may be de- 
termined fairly accurately. 
It follows, from what has been said concerning the nature 
of the oval bodies, that they are not present in the best 
preserved portions of this section. They have hitherto formed 
interspaces between the cell-containing tubes, and here there 
