276 
F. Cavers. 
Each of the ventral segments cut from the apical cell divides 
first by a tangential wall; the inner cell adds to the stem tissue, 
the further development of the outer cell varies considerably. In 
the simplest case (e.g., Lejeunea calcarea) each of these outer cells 
simply gives rise to a single row of cells on the underside of the 
stem, but as a rule a radial division occurs giving two cells placed 
side by side, then each of these produces a mucilage papilla, and an 
underleaf is formed. The underleaves show hardly any of the 
modifications exhibited by the side-leaves, as might be expected 
from their different position. The side-leaves are turned towards 
the light, the underleaves are usually appressed to the substratum 
and cut off from light, hence they are usually smaller and often 
absent altogether. Frequently, however, the shoot grows more or 
less erect and the underleaves may be equal in size to the side- 
leaves, the shoot showing practically radial symmetry. 
In striking contrast to the varied external morphology of the 
shoot in the Acrogynae, we find practically an entire absence of 
histological differentiation in the stem and leaves. The tissue of 
the stem is generally quite uniform; though the inner cells may 
have thinner walls than the outer cells, we rarely find anything 
approaching even the simple type of conducting strand found in so 
many Mosses, much less the elaborate differentiation seen in the 
Polytrichaceae, etc. In some of the Lejeuneas and Frullanias. 
certain cells of the leaf are much larger than the others, these 
“ ocelli ” being either scattered or arranged in a median line running 
up the leaf. In a few forms (e.g., Herberta, Diplophyllum) there is 
a “ midrib ” consisting of a band of elongated cells, which as shown 
by Tansley (83) serve for conduction. The leaf is rarely more than 
one cell thick, though in some cases (species of Gottschea, etc.) it is 
several-layered at the base and for some distance upwards. The 
rhizoids are typically unicellular and unbranched, or slightly 
branched at the ends, though Goebel (33) notes in Gottschea the 
occurrence of numerous septa in the distal branched portion of the 
rhizoids. 
Various features of interest in the vegetative organs of the 
Acrogynae are described by Goebel (32). For an account of asexual 
reproduction as exhibited in this group, reference may be made to 
a paper published in this .Journal (5). 
The distribution of the sexual organs varies greatly, but the 
commonest condition is that in which the antheridia and archegonia 
are borne on the same plant but in separate groups or “ flowers.” 
