48o 
VASCULAR CRYPTOGAMS. 
beneath the sporangium without giving off a branch to it. By repeated divisions of 
the primary cells a mass of tissue is formed which is differentiated into an outer layer 
of cells, the wall of the sporangium, and an internal group of cells, one of which is 
the archesporium ; the tapetum (Fig. 337, <r) is formed toward the free surface of the 
sporangium, from cells which are cut off from the archesporium, and it is completed 
toward the base by cells which are cut off by tangential walls from the cells which 
surround the archesporium. The cells forming the wall also undergo division by 
walls parallel to the surface, and thus the wall of the sporangium comes to consist of 
two layers (Fig. 337, b). The mother-cells of the spores are produced by repeated 
divisions of the archesporial cells. These cells soon become isolated and round 
themselves off, and, in the case of the microsporangia, they all divide, after an indicated 
division into two, into four tetrahedrally-placed spores which retain their relative 
positions until they reach maturity (Fig. 337, g, h). In the macrosporangia, on 
the other hand, one of the mother-cells grows more strongly than the rest ; it divides 
and gives rise to the four macrospores, all the 
other mother-cells remaining undivided but 
continuing to exist (at least in Selaginella 
incEqualifolid) for a considerable time. The 
macrospores are arranged, in consequence 
of the mode of division of the mother-cells, 
as the corners of a tetrahedron, an arrange- 
ment which persists until they are set free. 
Very commonly weakly macrospores are to be 
found in otherwise normal spikes of sporangia. 
The tapetum persists until the spores are ripe, 
whilst in the case of Ferns it is absorbed 
during the formation of the spores. 
[In the case of Isoetes, it has been shown 
by Tchistiakoff, by Hegelmaier, and by Goe- 
bel, that the sporangium arises from a group 
of cells at the base of the leaf, this group 
including cells belonging to the three superficial layers of their tissue. In con- 
sequence of cell-division and growth the sporangium soon appears as a swelHng 
in the fovea. As in the Selaginelleae, the fibro-vascular bundle of the leaf runs 
beneath the sporangium without giving off a branch to it. The most deeply-placed 
cells of the group form the short thick stalk of the sporangium ; the superficial layer 
forms its wall ; the intermediate layer constitutes the archesporium, from which the 
mother-cells of the spores as well as the trabeculae are derived. In the micro- 
sporangium the archesporial cells elongate and are divided by walls parallel to the 
free surface of the sporangium, and thus rows of cells are formed. Of these rows 
some undergo no further change, and these form the trabeculae. In the others 
either single cells or groups of cells increase in size and become divided by both 
transverse and longitudinal walls ; these divisions produce a tapetal layer at an early 
stage which surrounds each group of spore-mother-cells. The differentiation of the 
macrosporangia proceeds in much the same manner, but here the sporogenous 
cells of the archesporium only undergo such divisions as are necessary for the 
Fig. 338.— a nearly ripe macrosporang-ium of Sela- 
gmella incequalifolia ; the fourth spore which lies behind 
is not indicated (X loo). 
