500 Agnes Arber .— On the Structure of the Palaeozoic Seed 
The cells may be empty, or filled with carbonized contents. They vary in 
size, and may, in extreme cases, exceed 500 /x in length, but less than half that 
length is more usual. In transverse sections they measure about ico/tx in a 
direction tangential to the seed surface, and 50 /x in the direction at right 
angles to this. In cases, however, where they are particularly large, they may 
measure as much as 200-280 /x in the tangential direction. 1 These measure¬ 
ments are taken from the part of the sarcotesta which clothes the shell. 
The outer large cells of the wing are generally somewhat smaller. The 
irregularity in size of these cells in different specimens,or even indifferent parts 
of the same specimen, is very striking (cf. PI. XXXVIII, Figs. 14 and 15). 
In certain cases, in which only a few of these cells are preserved, they are 
much swollen, and stand up freely from the surface of the shell. Altogether 
the appearance and behaviour of this layer suggests that the cells may have 
been mucilage cells, analogous to those met with in the surface layers of the 
testa of many modern seeds, and that the irregularity in size may have been 
due to the varying amount of water which the mucilage had taken up 
before fossilization. Mucilage cells are markedly characteristic of the 
testas of Palaeozoic seeds of the Lagenostoma group ( Lagenostoma , 
Co 7 iostoma , and Physostoma). 2 
Although the mucilage cells of Mitrospermum compressum usually 
appear to form the outermost layer of the sarcotesta, in more than one case 
there is a tissue external to them, consisting of small cells radially arranged 
(PI. XXXVIII, Fig. 13,0. cl). The fact that this layer is usually absent may 
be accounted for by the nature of the mucilage cells beneath. The swelling 
of the latter would cause an inelastic outer layer to crack and peel off. This 
would be somewhat analogous to the case of Conostoma oblongum , mentioned 
by Oliver, 2 in which * the common outside membrane of the very conspicuous 
palisade-cells of the testa is sometimes found “blown off”, as though a 
number of these cells had emitted a quantity of mucilage \ 
C. The Seed-Base. 
Sections cut in the principal plane do not show such a sharp distinction 
between the elements of the sclerotesta and sarcotesta in the region below the 
two branch bundles (PL XXXIX, Fig. 26 , b) as in the rest of the seed. 3 The 
sclerotesta here tends to become thinner-walled. The histological distinction 
between the two layers of the integument seems, however, to be variable 
in this part of the seed, for, though almost lost in certain sections, it is clear 
in others. Many cells with carbonized contents whose appearance suggests 
secretory cells are found in the seed-base (PI. XXXIX, Fig. 30). There 
are occasional indications of an absciss layer in the hilary region. 4 
1 Royal Holloway College Collection, 351 (9), and U. C. L., S. 50. 
- Oliver (’ 09 ), p. no. 3 Cf. Trigonocarpus. Scott and Maslen (’ 07 ), p. joi. 
4 e. g. U. C. L., S. 65. 
