SPORES OF SELAGINELLA 



These features are readily observed in longitudinal sections 

 through the cones, which also show the presence of a 

 small outgrowth (the ligule, Fig. 181, /.) from the upper 

 surface of each sporophyll, between its upturned tip and the 

 sporangium. Such ligules, though most conspicuous on the 



sporophylls, oc- 

 cur also on all 

 the vegetative 

 leaves, but 

 their function 

 -i nL is altogether 

 obscure. A 

 \ ligule is not 

 met with in the 

 genus Lycopo- 

 dium, but ap- 

 pears to have 

 been character- 

 istic of most of 

 the extinct re- 

 presentatives of 

 the family, 

 which also pos- 

 sessed two 

 kinds of spores. 

 The young 



FIG. 181. Selaginella umbrosa. A, Longitudinal 

 section through part of a cone, showing micro- 

 sporangia (mi.) above and megasporangia (mg.) spora-ngia of 

 below. E, Small part of megasporangium en- Selaginellahave 

 larged to show the stalk and wall. I., ligule. ,_, 



B, Single megaspore enlarged. C, Tetrad of 



microspores on the same scale of magnification ered wall, en- 

 as B. D, Tetrad of microspores enlarged. closing a large 



number of spore 



mother-cells, and, up to this stage, all are alike. In the micro- 

 sporangia each mother-cell gives rise to four small spores, but 

 in the megasporangia only one develops further, enlarging 

 rapidly at the expense of the others, and dividing to form the 

 single tetrad (Fig. 181, A). An American species (5. rupestris) 

 exhibits an even greater reduction, since occasionally only one 

 of the four megaspores reaches maturity, and, in this and 



