XI LEPTOSPORANGIATJE HETEROSPORE^ 433 



terminal lobes, which at first are not separated (Fig. 253, D). 

 The establishment of the veins follows exactly as in Ferns with 

 a similar venation, and is strictly dichotomous. The stem 

 branches freely in both genera, and the branches arise close to 

 the apex, and below a young leaf somewhat as in Azolla. 



The roots correspond closely to those of the higher 

 homosporous Ferns. The segmentation of the apical cell fol- 

 low^s the same order as in the PolypodiacCcC. Goebel's figure of 

 M. salvatrix ( ( 10), p. 238) differs somewhat from the account 

 given more recently by Andrews ( i ) for M. quadrifolia. The 

 latter observer states that there are no periclinal walls in the 

 root-cap segments, which remain throughout one-layered, and 

 that the separation of the plerome takes place earlier than Goe- 

 bel indicates. Van Tieghem's ((5), p. 535) account of the 

 root of M. Dnimmondii confirms Andrews' observations upon 

 M. quadrifolia. The bundle of the root is diarch, as in the 

 Polypodiaceae, and the lateral roots arise in the same manner. 

 The endodermal cells from which they spring are distinguished 

 from the others by their shorter and broader form, and are very 

 easily recognisable by this as well as from their position. They 

 form tw^o vertical rows exactly opposite the ends of the xylem 

 plate, and the lateral roots therefore are also strictly two-ranked. 

 Narrow lacunae are formed in the cortical tissue of the root, 

 and the cells surrounding these are connected by regular series 

 of short outgrowths, which connect them in a way that recalls 

 very strongly the connecting tubes between conjugating fila- 

 ments of Spirogyra, and produce a similar ladder-like ap- 

 pearance. 



The solid vascular cylinder of the young stem is later usu- 

 ally replaced by a tubular one, but its structure is also con- 

 centric, with phloem completely surrounding the xylem, and it 

 has both an inner and outer endodermis. When the plants are 

 completely submerged the ground tissue is mainly parenchyma, 

 but in the terrestrial forms sclerenchyma may be developed in 

 the cortex of the stem and petiole. The latter is always trav- 

 ersed by a single axial bundle, which in the lamina in Marsilia 

 divides repeatedly near the base of the wedge-shaned leaflets 

 into numerous dichotomous branches. 



Luerssen ((7), p. 601) mentions as special reproductive 

 bodies, tubers found in ]\I. hirsufa. These are irregular side 

 branches covered with imperfectly-developed leaves, and with 

 28 



