97 
an archaic type of Seed from the Palaeozoic Rocks. 
and are very prone to break down without leaving any traces (PI. VII, Fig. 
2 6). As a matter of fact, tracheal elements are very rarely seen in the 
tentacles at all, and never, so far as our experience goes, in the more distal 
parts. By far the most striking feature about the vascular system is its 
extreme delicacy and liability to break down. From the phylogenetic 
point of view, the maintenance of the separateness of the bundles to a point 
very close to the seed-base is of interest. Taken in connexion with the ribs 
and tentacles of the integument, it is reasonable to regard this peculiarity 
in the arrangement of the vascular supply as relatively archaic. For if the 
integument originated as a whorl of separate outgrowths beneath the 
nucellus, these outgrowths would have, primitively, each its separate vascular 
supply. The point is further discussed at p. 105. 
8. The Zone of Secretory Sacs. 
This layer would seem to have been the most delicate region of the 
seed, and the details of its structure can be studied only in the best 
preserved specimens. Lying immediately within the ring of vascular strands, 
and abutting upon the tapetum within, this zone stretches from the chalaza 
to the apex of the megaspore cavity. It is broadest at the seed-base where 
it may reach 120 /x across (Pigs. 1 and 14), whilst over the body of the seed 
it hardly exceeds 60-70 /x beneath the ridges (Fig. 2). At the apex it 
closely invests the megaspore cavity, and appears never to run out into the 
free arms or tentacles of the integument. For reasons given at p. 106, the 
secretory zone is regarded as representing the nucellus. 
Histologically, the zone consists of delicate thin-walled cells, flattened 
in the tangential plane, the dark sacs mingled with clear parenchyma. On 
the transverse section the number of sacs on any radius ranges from one or 
two to five or six. Radial sections of the seed-wall show that the sacs often 
run in longitudinal seriation. The sacs are thin oblong cells of tabular form 
with the following average dimensions : length 60 jx ; radial diameter, 
8-10 fji ; tangential diameter, 50-60 /x. They contain a black, structureless 
secretion, recalling that of the similar sacs in Lyginodendron. 
As already stated, the zone of sacs lies within the vascular system of 
the seed. At the chalaza the entire funnel-shaped space — limited above by 
the embryo- sac, and on the flanks by the divergent vascular strands — is 
occupied by crowded sacs (Fig. 19, s.s.). The edge of this funnel is continued 
so as to enclose the embryo-sac, and for a short distance the secretory zone 
is seen at its maximum width (Fig. 14, s.z.). The principal accumulations 
of sacs in this region lie between, and usually project somewhat beyond, the 
vascular strands. At these spots the sacs may be counted as many as six 
deep (PI. V, Fig. 1), whilst beneath the strands they are not more than two 
deep. These relations only obtain, however, to a distance of about a millim. 
from the seed-base, giving place gradually to the arrangement typical of the 
H 
