Acrogynous J linger mailin' ales. 
271 
are formed, the ventral segments are narrow, the apical cell 
appearing in surface view as an isosceles triangle with a narrow base. 
In the thalloid vegetative portion of the plant-body in Metz- 
geriopsis, and in the leafy shoot of Pleurozia and Arachniopsis, 
however, the apical cell is two-sided, and there are of course only 
two (lateral) sets of segments. 
3 
Fig. 44. Diagram of the apical growing-point, seen from the front. 
X, is the apical cell of the main axis; the last nine segments cut from it are 
numbered ; X,, X 2 , are the apical cells of branches. 
Each lateral segment of the apical cell divides first by a radial 
wall into an upper and a lower cell. This wall may be nearer 
either the upper or the lower side of the segment, and from the 
larger of the two cells an inner cell is next cut off which contributes 
to the formation of the stem, while the two outer cells (upper and 
lower) give rise to the young leaf. Even when the mature leaf 
shows no trace of lobing, the very young leaf is distinctly two-lobed, 
each lobe bearing a mucilage-papilla. In fact, the ultimate form of 
the leaf depends on the further development of the originally similar 
and separate upper and lower lobes of the very young leaf. In 
species with entire leaves, the two lobes grow together ( i.e . are 
carried up by common growth at the base) from the first; whereas 
in forms with divided leaves the lobes grow out independently for a 
shorter or longer time. The growth of the leaf is at first apical, 
segments being cut from the terminal cells by transverse walls. 
When only transverse divisions occur in the two primary cells, 
we get a leaf consisting of two diverging rows of cells (Arachniopsis). 
From these considerations, it is easy to account for the extraordinary 
variety in form of the mature leaf which is met among the 
Acrogynre and which contrasts strongly with the remarkable 
uniformity of the Moss leaf—the leaves of Mosses are never lobed, 
