an archaic type of Seed from the Palaeozoic Rocks . 81 
to a slightly higher level than the upper half. The probable position of 
this section is plotted in the diagram (Text-fig. 3, p. 80, M.H. 369 ), the 
right-hand side of the diagram corresponding to the upper side of the 
photograph as it is orientated in PI. V, Fig. 1. 
This separation of the integument at the seed-base would appear to be 
of practically universal occurrence in the specimens of Physostoma. The 
phenomenon is perhaps analogous to the splitting often shown by cut 
flower-stalks standing in water, which depends on the unequal swelling of 
the different tissues (cf. PI. VI, Fig. 21, /). 
(2) Below the plane of splitting is the zone of secretory sacs (£.£.), 
consisting of a delicate parenchyma in which are scattered great numbers 
of thin-walled secretory cells, with black, structureless contents. These sacs 
generally show stretching in the tangential direction. Their longest dimen- 
sion, however, coincides with the axis of the seed. They are not distributed 
homogeneously throughout their zone, but have a maximum occurrence 
midway between the radii occupied by the bundles. At these points the 
sacs are about five deep, whilst opposite the bundles the number falls to 
about two. 
(3) Within the secretory zone, and bordering the central cavity, is the 
conspicuous tapetal zone, -i millim. across, of thick- walled elements with 
marked radial flattening (PI. V, Fig. 1, tp.). Towards the inner limit the 
cell-cavities are barely visible in consequence of the running together of the 
thick, black membranes ; further out, where the layer abuts on the zone of 
secretory sacs, the large, tangentially elongated elements become conspicuous. 
The second section of the series (M.H. 370, PI. V, Fig. 2) has a mean 
diameter of millims., whilst the internal cavity has expanded to 
i-J millims. 
The three principal zones are recognizable, though their aggregate 
width shows a slight diminution when compared with the previous section. 
Of the three, it is the middle (secretory) zone that shows the most con- 
spicuous thinning. The ribbing at the surface is very characteristic. Ten 
ribs are present lying in the same radii as the ten vascular strands. 
Separation between the outer and middle seed-zones is restricted to the 
immediate neighbourhood of the xylem-strands, so that each rib overlies 
a canal of crescent-shaped or semi-circular section. These canals evidently 
owe their origin to the breaking down of a delicate tissue that lay just on 
the peripheral side of the xylem-strands. Between the bundles, the tissues 
of the integument are in continuity with those of the secretory zone, and at 
these spots the deeper-seated layers of the integument — and occasionally 
the more superficial ones as well — show a radial seriation of the cells 
(PI. V, Fig. 5, r.fi). In general, it may be remarked that the cells of the 
outer zone that underlie the furrows show less radial expansion than do 
those below the ridges. 
G 
