230 



PTERIDOPHYTA. 



other Archegoniatre, but are quite embedded in the prothallium 

 (Figs. 235 cr, 236). 



The asexual generation varies very much in the different 

 orders. 



Order!. Isoetaceae (Quill- worts). The only known genus, 

 Isoetes (Quill-wort), has an extremely short, tuberous, unbranclted 

 stem with very short internodes (Fig. 237). The STEM is 

 vemai'kable as being- the only one among the Vascular Cryptogams 

 which increases in thickness (see page 202). The meristematic 



237 

 FIG. 237. Isoetts lacustris (slightly diminished): st the stem ; r roots ; b leaves. 



FIG. 238. Isoetes lacustris. Longitudinal section through the base of the leaf with a 

 microsporangium. The edge of the groove, in which the microspangium is placed, is 

 continued as a thin covering which envelopes the sporangium. The inferior edge of the 

 ligular groove (L) forms a lip ( J) ; f, sterile cell-rows (trabeculte) which divide the 

 sporangium into compartments; I vascular bundle. 



FIG. 239. Selaginella inaqualifolia. Cone in longitudinal section; microsporangia are 

 seen on the left side, macrosporangia on the right (most frequently each with four 

 macrospores). 



cells are situated round the axial cylinder, and form, especially, 

 parenchymatous tissue in two or three directions, giving rise to 

 - M grooves in which the dichotomously-branched ROOTS are 

 produced. The LKAVKS are arranged spirally in a close rosette. 



