460 VASCULAR CRYPTOGAMS. 



apical cell of the stem forms two rows of segments, a right and a left. Sori either male 

 or female : each sorus contained in a unilocular sporocarp. Spores invested by hardened 

 frothy mucilage (Massulae, Epispore). The prothallium derived from the macrospore 

 is well developed and bears several archegonia. Azolla has roots, Salnjinia has none. 



Family 2. MarsiliacesB, Plants creeping on moist earth, or floating to some extent 

 in water. The three-sided apical cell of the stem forms two dorso-lateral rows, and 

 one ventral row of segments. Each sorus includes both macro- and microsporangia, 

 and two or more sori are contained in the multilocular sporocarp. The spores are in- 

 vested by hardened mucilage — epispore — which presents a radially prismatic structure, 

 and is to some extent capable of swelling-up. The prothallium of the macrospore bears 

 a single archegonium. This family includes the genera Marsilia and Pilularia. 



CLASS IX. 



DlCHOTOME^ ^. 



Hitherto the Lycopodieae, Psilotum, the Selaginellae, the Isoetese, etc., have 

 been included together in the group of the Lycopodiaceae, and rightly so, for 

 these genera exhibit not only in their habit, but also in their morphology, a degree of 

 relationship which makes it impossible to separate any one of them from the others 

 with the view of erecting it into a new class or of including it in one of the other 

 two classes of Vascular Cryptogams. Recent investigations have, however, shown 

 that this class includes two well-defined subdivisions which must be kept distinct. 

 To one of these two orders belong the genus Lycopodium and its immediate allies, 

 and it must therefore necessarily take the name of Lycopodiaceae; consequently 

 a new name has to be found for the whole class, and I select that of Dichotome^, 

 because it brings into prominence one of the most obvious of the characteristics 

 of these plants, that the branching presents the appearance of being the result of 

 dichotomy, although, as a matter of fact, the branching of the stem is monopodia!. 

 It must be remembered, however, that these two modes of branching gradually pass 

 one into the other. These plants are as remarkable among vascular plants for their 

 evident tendency to branch dichotomously as are the Equisetaceae for their whorls of 

 leaves. That dichotomy is indeed a typical peculiarity of these plants is proved by 

 the fact that their roots are the only ones at present known which branch 

 dichotomously. 



The dichotomous habit of the branching is by no means the only characteristic 

 common to the members of this class, as a comparison with the Equisetaceae and 

 Filicineae at once shows. They possess but Htde in comm^on with the Equisetaceae 

 except perhaps the relatively slight development of the leaves, although that is 

 brought about in this class in a very different way, for the similarity in the mode of 

 development of the sporangia in the two classes does not merit consideration since, 

 as we have seen in the Filicineae, this is a characteristic which is critical only for the 



^ [Inasmuch as dichotomous branching is not universal even among the Lycopodieae, many 

 authors prefer to designate this group of plants as Lycopodinse.] 



