Acrogynous Jungermanniales. 283 
(1) The Isotachis type, in which the symmetry of the shoot remains 
unchanged, and the sac-forming meristem arises as a ring-like 
outgrowth of the stem tissue around the archegonial group (Fig. 50, 
VII. to IX.). (2) The Tylimanthus type, in which the sac-forming 
meristem is at first a solid mass of tissue, which grows at right 
angles to the long axis of the stem itself, and which invests and is 
penetrated by the embryo (Fig. 51, I., II.). (3) The Acrobolbus 
type, in which the meristem arises as a hollow ventral outgrowth 
from the archegonial branch (Fig. 51, III. to V.). 
The origin of the Isotachis type of marsupium, in which the 
latter forms a direct linear continuation of the archegonium-bearing 
axis, may be traced in certain species described by systematists as 
having the perianth and involucre “fused” or concrescent. The 
“ fusion ” is brought about by the growth of a ring-like zone of 
tissue at the base of the perianth and involucre, both of which are 
carried up on a collar or tube (Fig. 50, II.) This process reaches 
different stages in different genera, e.g., Nardia, Marsupella, 
Southbya, and is most striking in Isotachis. Here the archegonial 
branch grows erect, and the stem-tissue grows up to form a long 
thick envelope which carries up with it the involucral and neigh¬ 
bouring leaves, as shown by Goebel (33). We thus get in Isotachis 
what looks like a perianth and has usually been described 
as one, but is really a marsupium. In the marsupia of the 
Tylimanthus and Acrobolbus types, the solid or tubular tissue-zone 
which produces the marsupium grows downwards, making a right 
angle with the axis of the plant and usually becoming attached by 
rhizoids to the substratum, into which it may burrow to some depth. 
The beginnings of these different types of marsupium may be 
traced among certain species of Nardia. The case of N. geoscypha 
is specially interesting. Here the plant may either grow erect or 
horizontally. In the former case, the cup-like outgrowth which 
after fertilisation carries up the perianth and involucre, as already 
described, grows upwards and the developing sporogonium is 
parallel with the axis of the plant. In creeping plants, however, 
the ventral tissue of the stem grows so as to form a solid mass 
below the archegonial group ; this swelling projects below and bears 
rhizoids, and its tissue contains food for the nutrition of the 
developing sporogonium, the absorbing organ (foot) of which 
becomes buried in the swelling (Fig. 50, III.) Thus in this species 
we have the beginnings of both the Isotachis and the Tylimanthus 
types of marsupium. The ventral form of incipient marsupium is 
