98 Oliver. — On Physostoma elegans , Williamson , 
greater part of the seed-body. There is a marked thinning out of the sacs 
between, and a corresponding accumulation beneath, the bundles. The 
secretory zone thus forms for the most part a thin mantle with thicker ribs 
or cushions underlying the bundles (PI. V, Fig. 5, s.z.). These relations hold 
till the zone is reached at which the integumental arms break away from 
the nucellus. As the arms separate, their tissues become quite free 
of secretory sacs (PI. VI, Fig. 13). The zone of sacs is continued in the 
nucellus right round the tip of the embryo-sac, though it cannot as a rule be 
separated from the tapetum, with which, in this region, it forms the black, 
structureless crust, so characteristic of the generality of specimens (PI. V, 
Fig. 8). Occasional specimens show, however, that several layers of sacs 
are present immediately outside the tapetum. The narrow zone which tapers 
up to the pollen-chamber (the £ plinth ’) is clothed by a small-celled 
epidermis which separates at a slightly higher level from the tissue within, 
to form the wall of the pollen-chamber. 
9. The Tapettim. 
Within the zone of secretory sacs the megaspore-cavity is limited in 
most specimens by a broad, black, structureless crust which reaches a width 
of 100 ix. This layer, which lines the embryo-sac throughout, is so intimately 
consolidated with the secretory layer in the free apex of the nucellus, that 
the two together form the structureless, carbonized cone which serves as the 
hollow, dome-shaped core upon which the pollen-chamber is seated (Fig. 17). 
In other specimens, however, the black layer is often frayed out on the 
inner concave side, so as to present the appearance of a number of black 
rods inserted at right angles to the direction of the layer as a whole, whilst 
in yet others this condition co-exists with a definite cellular structure on the 
side abutting on the secretory layer (PI. VI, Fig. 23, tp 3 , and PI. VII, Fig. 28). 
Transitional cases such as these lead up to the condition found but 
rarely and only in the best preserved specimens, of which Professor Bertrand’s 
series affords an admirable example. Here the zone referred to consists of 
some six or more radially compressed layers, of which the innermost show 
the maximum compression, whilst the outermost — that abutting on the 
secretory zone — has resisted, or not been subjected to, compression (see 
PL V, Figs. 2 and 5, //., PI. VII, Fig. 24). 
This zone when preserved is composed of rather large cells with 
blackened walls but without contents. Though the successive layers have 
undergone displacement in the process of compression, it is probable that 
they stood in radial files, as suggested by Professor Bertrand’s section, 
M. H. 370 (PI. V, Figs. 2 and 5) and several others. The U. C. L. sections 
R, 71 a, and R, 74 a (PI. VI, Figs. 14 and 23, and PI. VII, Fig. 24) afford 
corroboration to this view. 
Sections such as these prove that both the usual structureless condition 
