Sphcerocarpales. 8 7 
invests the archegonium ; like that of the antheridium, it consists 
of a single layer of cells. 
The embryogeny is of the Marchantiales type, the first (basal) 
wall being transverse, and the lower cell giving rise to the foot of 
the sporogonium, which is separated from the spherical capsule by 
a short neck-like seta. The archesporial cells at first contain 
chloroplasts and starch-grains, but after a time these are replaced 
by oil-drops in certain cells, and these cells give rise to the spores. 
The spore mother-cells are spherical and do not become lobed before 
the tetrad division; in this respect Sphcerocarpus differs from the 
Jungermanniales and resembles the Marchantiales. The sterile 
cells retain their starch, and according to Leclerc du Sablon (21) 
they resemble the spore mother-cells in the division of the nucleus 
into four; they become ovoid in form and are used up by the 
developing spores. The calyptra, formed from the venter of the 
archegonium, is ruptured by the increase in volume of the ripening 
capsule, and the spores are set free by the decay of the capsule wall 
and of the envelope. In S. terrestris the four spores of the tetrad 
remain united within a common outer spore-coat, and as has been 
shown by Douin (11) and by Strasburger (32), of the four resulting 
plants, two are commonly male and two female. In 5. Donnellii, 
however, the four spores separate when mature, and in 5. cristatns 
the spores separate long before the capsule ripens. 
Geothallus, discovered and described by Campbell (5, 6), is a 
Californian genus closely allied to Sphcerocarpus, from which it 
differs in being a much larger plant and in the definite organisation 
of leaves. Moreover, the plant is perennial, and at the beginning 
of tbe dry season the younger part of the axis is converted into a 
tuber in which food is stored up, the apex persisting and resuming 
growth with the return of favourable conditions. The sexual organs 
and sporogonium are very similar to those of Sphcerocarpus , and the 
chief interest of the genus centres in the differentiation of the plant 
body into axis and leaves. A leaf is formed from each lateral 
segment cut from the apical cell. The leaves are at first placed 
vertically, but later become shifted so as to be “succubous” (i.e., 
the hinder edge of each leaf overlaps the anterior edge of the leaf 
behind it) or horizontal. This marks a great advance upon 
Sphcerocarpus, in which the leaves are simply wings of the thallus 
interrupted by the notches in which the growing-points are situated. 
In Geothallus, leaf-like outgrowths also arise from the upper surface 
of the axis, and sometimes these are continuous with the lateral 
