DEVELOPMENT OF THE SPOROGONIUM IN THE MOSSES. 279 
walls are formed only in the most peripheral cells, that is only in 
the external layer of cells of the sporogonium. A reference to 
Pl. XII. figs. 37-40, will make the account of the development 
of the exomeristem clear. 
From the fact of divisions taking place as just described, it is 
easy to see in a transverse section from which cell in the central 
part of the exomeristem several radial rows are derived, since 
cell-division takes place much as in the cork cambium of the 
Phanerogamia; in transverse section the cells of the exome- 
ristem are not rectangular, for of the tangential walls the peri- 
pheral is greater than the more central (figs. 37, 38, 39, 40). 
At about the stage represented in fig. 39 intercellular spaces 
are begiuning to be formed between the innermost cells of the 
exomeristem. 
In the development of the growing-point, I may briefly refer 
to the order in which the radial walls are:formed at the growing- 
point of the sporophyte of the Muscine® and of the lower-Pteri- 
dophyta. A reference to Pl. XII. figs. 44, 45, 46—representing 
transverse sections through the growing-point of the sporophyte of 
one of the Hepatics, one of the Musci, and of Equisetum—makes 
it evident that the principle, according to which at any rate the 
earliest divisions appear, is the same in each case ; although in 
Equisetum there is a complication arising from the tact that 
the apical cell is three-sided, and not two-sided as in the 
Muscines. 
It isin the seta only that the small-celled tissue, formed by the 
frequent divisions taking place in the endomeristem, gives rise to 
the structure known as the central strand. The sheath of the 
central strand consists of the innermost layer of cells of the exo- 
meristem (figs. 41, 42). Ata stage somewhat later than that 
shown by fig. 42, the tissue of the endomeristem begins to be 
distinguishable into an outer zone of thick-walled cells con- 
taining a quantity of protoplasm, while the axial strands of cells 
do not increase in thickness, and the quantity of protoplasm 
diminishes. un 
With regard to the respective methods of cell-division and 
growth in the exomeristem and endomeristem, the exomeristem 
grows from a peripheral zone, so that the cells at the periphery 
are the youngest ; and as the cells at the centre cease dividing, 
they may be looked upon as the oldest. Therefore the sheath or 
endodermis of the central strand consists of the oldest cells of 
