480 



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 ape 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, «, ^). 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, E, 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 Isoeles, it has been shown 

 FIG. 338- A nearly ripe macrosporangium of Seia- by Tchistiakoff, by Hcgelmaier, aud by Goe- 

 fs"roUnS:rd'£1Sr'''°"^''^'''""''^'"^^ 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 swelling 

 in the fovea. As in the Selaginellese, 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 



