278 BRYOPHYTA 
sides, so as to cut off two rows of segments successively from the terminal, 
two-sided initial cell (Fig. 135 A and B). The apical growth is, however, 
not very long continued, and gives place later to intercalary activity. The 
hypobasal cell, as also sometimes the lowermost segments, undergo less 
regular sub-divisions, but the upper segments sub-divide with greater 
regularity—though still with some differences of detail—in such a way 
that a definite result is arrived at, viz., the formation of a central tract 
of tissue (endothecium), consisting of four rows of cells, and a peripheral 
series (amphithecium), consisting of more numerous cells, which soon 
divide both radially and periclinally to form a thick wall (Fig. 135 c). 
It is important, from a theoretical point of view, to note that the endothe- 
cium thus established, though less definite in the lowest of the epibasal 
segments, extends upwards throughout the length of the capsule to its 
apex: it is not merely a local development in that part which is ultimately 
to be the fertile region, but it is a continuous and definite column of 
tissue, occupying the centre of the spindle-shaped sporogonium. It may 
be a question what is the morphological importance of a tract thus defined 
by embryonic segmentation. In Chapter XIV. the relation of the leading 
anatomical regions of axis and root to the apical segmentation has been 
discussed, and it was seen that there is no obligatory correspondence 
between early segmentation and the definition of mature tissue-tracts: for 
it has been found that, in parts of such complicated outline as the leaf- 
bearing shoot, the correspondence between early segmentation and mature 
structure is not strictly maintained. But it is the fact that in parts of 
such simple outline as roots there is a definite correspondence of that 
nature, and this is particularly clear in certain Pteridophytes. The case 
of the simple spindle-shaped sporogonium of a Moss is comparable, in its 
form as well as in the early segmentation of its central tract, with such 
roots ; and there seems good reason to regard the endothecium accordingly 
as being in fact a morphologically definite region throughout its length. 
The most important function of the endothecium is that it is the exclusive 
source of spore-formation; but as a matter of fact, it is only a relatively 
small extent of it which carries this into effect, the rest remaining sterile, 
performs other duties. 
The archesporium originates from a restricted region of the endothecium 
some distance back from the apex of the sporogonium, and a very con- 
siderable distance from its base: the sterile region of the capsule at the 
distal end forms the calyptra and peristome: the much longer sterile 
region at the base forms the apophysis and the seta. These regions may 
vary in their proportion to the fertile region in different types of Mosses: 
a fair average is that seen in Funaria (Fig. 136 a). The origin of the 
archesporium is by periclinal division cutting off a single layer of cells 
from the periphery of the endothecium: this ultimately divides up into 
several layers of minute cubical cells, all of which undergo the tetrad- 
division in the usual way, and produce spores. 
