499 
Mitrospermum compression ( Will .). 
Gymnosperm prothallus. 1 At one side the valves of the shell (which are 
not represented in the drawing) are slightly opened, and the appearance 
suggests that the prothallus is emerging from the seed. It is highly 
probable, however, that this appearance is purely accidental. 
In the case of one seed, 2 in which the nucellus and megaspore are poorly 
preserved, a number of rather opaque, reticulately-marked bodies are seen 
scattered in the shell cavity. They are irregularly oval in shape, and 
measurements of three of the largest showed that the average size was about 
77 M by 58 fjL. In another section 3 similar bodies are seen embedded in the 
ill-preserved sarcotestal wing. It is just possible that these objects may be 
pollen-grains, but it is far more probable that they are foreign bodies. 
III. Histological Details. 
A. The Sclerotesta. 
There is considerable variation in the thickness of the sclerotesta in 
different sections. This probably indicates that under the name Mitro - 
spermmn compression we are including a group of related species. 4 
The cells making up the sclerotesta are somewhat elongated in the 
longitudinal direction, and polygonal in transverse section, the radial 
diameter being the greatest (PI. XXXVIII, Fig. 11). They have thick 
walls which in some cases are distinctly pitted (PI. XXXIX, Fig. 2 5 )- 
The outer cells of the sclerotesta are larger than the inner, which are 
commonly rounder and more thin-walled. The larger cells, when seen in 
transverse section, may be about 100 /x in the radial direction, and less 
than half this width in the tangential direction. 
B. The Sarcotesta. 
The surface of the sclerotesta is irregular, whether seen in transverse 
or longitudinal section, and a variable number of small, roundish, thin- 
walled cells separate it from the large outer cells of the sarcotesta 
(PI. XXXVIII, Figs. 13 and 16, i. c.). These thin-walled cells constitute 
the inner layer of the sarcotesta, and form the main part of the wing tissue 
(PI. XXXVIII, Fig. 12). When, as occasionally happens, the wing is cut 
longitudinally in the principal plane, each of the thin-walled cells is seen to 
be longer than it is broad, and to have a sinuous outline. 
The large outer cells of the sarcotesta (PI. XXXVIII, Figs. 13-16, m. c.) 
usually form a layer one cell thick. Sometimes two or three cells are seen, 
but probably this is, in most cases, due to the obliqueness of the section. 
1 Sokolowa (’ 91 ). 2 M., R. 953 (5). 3 U. C. L, S. 42. 
4 Dr. Scott suggests that a seed met with in his slides 2249 an d 2250 (two sections from the 
same seed), and 2259, may represent a distinct type. The sclerotesta is unusually thick, and the 
distinction between sclerotesta and sarcotesta not very sharp. 
