292 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



macrosporangia contain usually four, more rarely two or eight macrospores. In the 

 division of the Articulatae only the lowest sporangium on a spike is a macrosporangium, 

 in the others there are several macrosporangia. The sporangia differ from those of 

 Lycopodium in their origin and in their separation into microsporangia and macro- 

 sporangia, but agree with them almost entirely in their development (Fig. 236). 

 The sporangium is formed from a group of superficial cells at the growing point 

 of the stem lying immediately above the cells, from which the leaf beneath each 

 sporangium is produced. The archesporium is formed in exactly the same way 

 as in Lycopodium (Fig. 236). The sporogenous group of cells which is formed 

 from the archesporium is surrounded by a layer of tapetal cells which are elongated 

 in the radial direction (Fig. 237). Up to this point the process is the same in both 



A 



FIG. 236. Selaginella. Longitudinal section 

 through a young sporangium and a part of the leaf 

 beneath it with the ligule /, t primary tapetal cell. 

 The archesporium is dotted. 



FIG. 235. Selaginetla inaequalifolia. A fertile branch half 

 the nat. size. B its summit in longitudinal section, with micro- 

 sporangia to the left and macrosporangia to the right. 



kinds of sporangia. The sporogenous cells soon become isolated and rounded off, 

 and in the microsporangia each of them divides after previous indication of bipar- 

 tition (Fig. 237 E, e,f] into four spores disposed tetrahedrally, and this disposition 

 is retained till the spores are mature (Fig. 237 E, g, /;). In the macrosporangia on 

 the other hand one of the mother-cells grows more vigorously than the rest, divides 

 and produces four macrospores, while the other mother-cells remain undivided but 

 still maintain themselves for some time, at least in inacquaUfolia, by the side of 

 the vigorously growing macrospores. The macrospores remain, till they are 



