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 Fitnnria (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. 



