YASCULAEIA. 425 



leaves arranged in four nearly vertical rows, and generally of two 

 forms. In Isoetes the stem is perennial and conn-like, having the 

 internodes suppressed. The leaves are spirally arranged in the 

 form of a rosette, broadly inserted. The corm-like stem emits 

 adventitious roots on the underside, but, owing to the dense rosette 

 of leaves, is invisible from above. The leaves are larger than in 

 Selaginella, and expanded into a kind of sheath at the base ; they 

 are traversed by four longitudinal air-canals. In both Selayinella 

 and Isoetes the leaves are simple, unbranched, pointed, and 

 traversed by one vascular bundle. Sporangia, which are placed 

 on the leaves in Isoetes, and on short stalks in their axils or above 

 on the stems in Selaginella, produce two kinds of spores macro- 

 spores and microspores. In Isoetes the macrospores are numerous, 

 in Selaginella usually 4, sometimes 2 or 8 ; in both the microspores 

 are numerous. Sexual Generation : The macrospores produce 

 female, and the microspores male prothallia. Illustrative Genera : 

 Isoetes, L. ; Selaginella, Spring. 



Structure and Life-history. In the Selaginellacese, as has been said, 

 the microspores produce the male prothallia. The germination of the 

 microspore is effected by its dividing into a few cells, of which one is 

 sterile and the others produce spermatozoids. The macrospores produce 

 the female prothallia by developing within themselves a mass of cells, 

 while the wall of the endospore begins to increase in thickness and sepa- 

 rates into lavers. Owing to the cell increase within, the exospore bursts, 

 and after some time the wall of the endospore leaves uncovered that part 

 of the protliallium the function of which it is to produce the archegonia. 

 Fertilization by means of the spermatozoids takes place in the manner 

 common to all* Vascular Cryptogams, and the asexual generation arises 

 from the embryonic cell. 



Affinities, &c. From the point of view of its vegetative structure this 

 Order is closely related to the Lycopodiacese, while it differs from it in 

 the nature of its sexual reproduction, since in the Lycopodiaceae the 

 spores which produce the female and male prothallia are of the same size 

 and form, and in the Selaginellaceae macrospores produce female, and 

 microspores male prothallia. The Order agrees, on the other hand, more 

 closely with the following Order (Rhizocarpese) in the production of 

 macrospores and microspores, but differs more widely from it in its 

 vegetative structure. 



Distribution, Qualities, &c. SelaginellcB are delicate, and usually found 

 climbing or creeping over low objects in damp and warm places. The 

 Isoetes grow in the mud at the bottom of pools. The corm is perennial 

 and of a woody structure when old. They are more generally diffused 

 in the northern hemisphere, whereas the SelagineUce occur in greater 

 abundance in warm climates. Neither have any recognized medicinal 

 or economic properties. Many species of Selaginella are cultivated in 

 greenhouses for the elegance of their foliage. 



