82 
F. Cavers. 
tiales and Jungermanniales. In the Marchantiales the plant-body 
is invariably a thallus of simple external form. The growing-point 
consists of a group of initial cells ; when apical branching is about 
to occur, the group broadens out, the middle region grows forward 
and its cells stop dividing, and we get two growing-points separated 
by a “ middle lobe.” The apical branching is usually “ dichoto¬ 
mous,” owing to equal or sub-equal growth of the two new growing- 
points. Apart from the erect gametophores of the higher forms, 
the branches of the thallus grow solely in the horizontal plane, 
whether they are formed by “ dichotomy ” of the growing-point or 
arise laterally from the underside of the “ midrib,” and the margins 
are entire. Throughout the group, however, the thallus tissue is 
differentiated (except in the genera Monoclea and Dinuortiera) into 
an upper zone of green tissue with air chambers, opening by pores 
on the surface, and a lower zone of compact colourless tissue 
which serves for food-storage, frequently contains fungal hyphae, 
and is sometimes traversed by sclerotic fibres or by mucilage ducts. 
The lower surface bears scales and two kinds of rhizoids ; of these 
some spring from the bases and the axils of the scales (which are 
typically arranged in two longitudinal rows), are provided with 
short peglike internal thickenings of the cell-wall (tuberculate 
rhizoids), and run back parallel with the underside of the thallus, 
while the others have smooth walls, have no relation to the scales, 
and grow directly downwards into the substratum. The sexual 
organs are either scattered over the thallus and separately em¬ 
bedded in it, or are collected into groups (receptacles) which are 
either sessile or stalked. The antheridium consists at an early 
stage of a row of cells, owing to repeated transverse divisions 
taking place in the mother-cell anterior to the formation of the 
first vertical walls. The archegonium neck consists of six rows of 
cells. The embryo sporophyte typically shows an octant stage ; 
the mature capsule has (except in Monoclea) either no seta or a 
very short one; the capsule wall consists of a single layer of cells, 
and it opens in various ways but probably never splits into four 
equal valves. The spore mother-cell does not (except in Targionia 
and Monoclea) become four-lobed. 
In the Jungermanniales, the plant-body is, in the vast majority 
of cases, differentiated into a cylindrical stem and two or three 
rows of leaves (one cell in thickness, with rare exceptions). When 
it is thalloid, it never has air chambers, but it shows a great variety 
of form and mode of branching, with a strong tendency to the 
