DIV. I 



PTERIDOPHYTA 



517 



After a period of rest the sporocarps germinate in water. In Pilularia the 

 tissue surrounding the sori swells, bursts the hard coat, and emerges as a 

 mucilaginous mass ; this contains the sporangia from which, by further swelling 

 of the walls, the spores become free. The development of the prothalli and 

 fertilisation take place in the mucilaginous mass that persists for some days. The 

 sporocarp of Marsilia, on the other 

 hand, opens as two valves. A car- 

 tilaginous cord of tissue lying 

 within the ventral suture of the 

 sporocarp swells greatly, and split- 

 ting the ventral suture emerges 

 bearing with it the sori, enclosed 

 by membranous investments (Fig. 

 482). 



From the microspore a reduced 

 male prothallus is developed within 

 the spore -membrane. This when 

 mature contains two antheridia, 

 each with 16 spermatozoids, and 

 liberates these as cork-screw-like, 

 spirally-wound, motile spermato- 

 zoids bearing numerous cilia (Fig. 

 483). 



The thick -walled macrospore 

 has, as in the case of Salvinia, 

 denser protoplasm at the summit. 

 This is cut' off from the large cell 

 enclosed in the spore-coat by a wall, 

 and develops into a small green 

 saddle-shaped prothallus composed 

 of a few cells. This only forms a 

 thus 



FIG. 484. Marsilia vestita. A, Macrospore with the 

 nucleus at the summit in the protoplasm from 

 which the female prothallus shown in B is derived ; 

 o, egg -cell of the archegonium, with the ventral- 

 canal-cell and neck-canal-cell above it; k, nucleus 

 of the large cell enclosed in the spore-membrane. 



C, Young embryo in the archegonium showing the 

 first divisions ; 1, basal wall ; 2, quadrant walls. 



D, Later stage ; to, young root ; b, first leaf ; st, stem ; 

 /, foot. (A x (60 ;' B x 360; C x 525; D x 260- 

 After D. CAMPBELL.) 



single archegonium and is 

 greatly reduced (Fig. 484). 



The embryogeny follows the 



type of the Leptosporangiate Ferns, the egg-cell dividing first by a longitudinally - 

 placed basal wall and then by transverse walls into quadrants ; these then divide 

 to give the octants. The first leaf and the root arise from the two upper pairs of 

 octants ; the lower pairs give rise to the foot and the stem-apex (Fig. 484 C, D). 



The prothallus grows for a time enclosing the embryo, and forms a few rhizoids 

 from its lower cells. If fertilisation does not take place, a somewhat longer-lived 

 prothallus results, which does not, however, form further archegonia. 



An apogamous formation of the embryo has been shown to exist in certain 

 Australian species of Marsilia belonging to the group of M. Drummondii ( 123 ). 



CLASS II 

 Equisetinae (Horse-tails) 



Order 1. Equisetaceae 



92 - 112 - 115 - l24 



The Equisetaceae include only the one genus Equisetum, comprising 20 

 species, found widely distributed over the whole world. The genus can be 



