6 



STRUCTUKAL BOTANY 



Kraussiana and S. helvetica (see Fig. 3), in having four 

 rows of leaves, two large and two small, in the vegetative 

 region, while in the spikes all the leaves are alike. In 

 another group, however, to which our native species (S. 

 spinosa) belongs (see Fig. 5), the leaves are all similar, 

 and are arranged spirally, both on the ordinary stem and 

 on the spike. In certain foreign species again, the case 

 is just opposite, for the leaves of the spike, like those of 

 the vegetative stem, are of two kinds. 



R EEPKODUCTIVE ORGANS 



The true reproduct- 

 ive organs of Selaginella 

 are the sporangia, con- 

 taining the spores. 

 Each sporangium is a 

 stalked sac, reaching a 

 diameter of about a 

 millimetre (2*5 inch), 

 and is borne in the 

 axil of one of the 

 leaves of the cone (see 

 Figs. 4 and 5). 



The sporangia are 

 of two kinds : the one 

 kind (the microsporan- 

 gium) contains very 

 numerous small spores 1 

 (microstores), compar- 

 able in size to pollen- 

 grains. The other 



FIG. 4. Selaginella helvetica ; part of 

 longitudinal section through spike, 

 showing two sporophylls. ma, niega- 

 sporangium dehiscing ; three out of four 

 megaspores visible ; note abortive 

 mother - cells ; mi, microsporangium 

 with numerous microspores. Magnified 

 about 15 diameters. (After DodeKPort. ) 



1 The word spore is applied to any single cell which becomes isolated 

 from the parent plant for reproductive purposes ; cf. Part I. p. 113. 



