480 DR. MARIE STOPES ON BENNETTITES SCOTTII, 
the radius, and a zone of phloem approaching in extent that of the xylem. 
The radii of vascular tissue are closely ranked, and are separated by wide 
medullary-ray cells into the double or triple strands typical of old Bennettites 
wood. But by seetion /, the highest in which the woody cylinder is well 
marked before it dies out in the stem apex, the wood is reduced to about five 
elements on each radius. 
The protowylems lie in radial continuity with the secondary wood, and 
there are no detached groups of small tracheids such as occur in some of the 
Bennettitalean stems (cf. Cycadeoidea Wielandi, Wieland, 1906, or Colym- 
betes, Stopes, 1915). 
The elements of the secondary wood are squarish in outline, but vary con- 
siderably in the way common in Bennettitalean wood, the average size is 
about 15x 20 u to 25x20 p. The tracheids in the higher sections are 
noticeably smaller than in the lower ones where the wood zone is thicker. 
The tracheids seem to have only the barred thickening characteristic of the 
family. 
In a number of places the cambium is well preserved, as one or two thin- 
walled, radially narrow elements between the xylem and phloem. This is 
particularly g ood in slide n. 
The pith is oval, about 8x 1-1 mm. in diameter, and is composed of a 
mass of soft-walled tissue through which run numerous ** gum-canals," as is 
characteristic of the family. There seem to be no isolated groups of tracheids 
or transfusion tissue in any part of it. 
A feature of particular interest is seen in the lowest section of the series 
(r, text-fig. 1), viz. a centrally placed mass of tissue quite unusual in Bennet- 
titalean piths. Roughly this mass is 4L em. in diameter, circular. and 
placed in the centre of the oval of the pith. In the section above this, g. the 
differentiated tissue-mass is not present, but when the section is viewed with 
the naked eye some distance from a black background, a central circular 
area is apparent, about *5 em. in diameter and distinctly different in colour 
from the rest of the pith. Under the microscope this area is not delimited 
in any way, but in the central region the cells seem larger and clear, and a 
little more irregular than the rest of the pith, sufficiently different perhaps 
to account for “the microscopically obvious difference in colour. A few of 
these cells seem to be stone cells with thiek walls. 
The circle of tissue on slide r can be seen in fig. 4, Pl. 19, where at s not 
quite the whole circle is shown. — It contains four definite zones : (1) an outer 
circle of very large, rather irregular cells with excessively thick walls ; 
between the groups of these large stone cells, small, narrow cells lie rather 
as do medullary rays in a bast zone ; there is, however. no conclusive proof 
that the circle is a vascular cylinder of any sort, and this appearance is 
probably fictitious. (2) Within this ring is a zone of two or three cells 
thick of narrow, thick-walled, dark cells, remarkably like a cambium. 
