RHIZOCARPE.^. 389 



In order to understand the subsequent processes of growth, Fig. 287 must be 

 compared with Fig. 289. The growing embryo bursts the prothallium ; from the 

 whole of the first segment (Fig. 289 B, r, r, r) arises the so-called Fooi of the 

 young plant (Fig. 287 a); from the whole of the second segment is formed a 

 peculiar foliar structure, differing from all the subsequent leaves, the Scutifonn leaj 

 (Fig. 287 B,h), by the growth of which the terminal bud of the stem becomes 

 directed downwards (Fig. 287 A,v). The anterior part of the embryo faces the 

 anterior side, its posterior part the posterior side of the prothallium ; its axis o: 

 growth lies in the same plane with the median line of the latter. 



The first divisions of the embryo of Marsilea salvairix agree in all essentia 

 points, according to Hanstein's observations and my own, with those of Salvinia ; anc 

 Hanstein states that this is also the case with Pilularia ; but in both these genera the 

 rudiment of the first root is visible at once in the first segment. The stem in these 

 genera also creeps or floats in a horizontal direction from the first, as in Salvinia, 

 forming a number of roots in acropctal succession. Fig. 290 shows the first divisions 

 of the embryo of Marsilea salvafrix. The oospore is divided by a nearly vertical 

 wall into an anterior larger and a posterior smaller cell ; the former splits up by an 

 almost horizontal wall into an upper segment which forms the first leaf ; the latter 

 {i. e. according to the type of Salvinia the first segment of the apical cell of the 

 stem) also splits up into two cells, lying one over the other, the upper of which 

 produces the first root. The union between embryo and prothallium is brought 

 about by the foot, which is formed out of the posterior inferior segment together 

 with the posterior half of the anterior inferior segment (Fig. 290 Eff). The apical 

 cell of the stem, the anterior half of the anterior inferior segment. Fig. 290 E s, thus 

 lies, after the formation of the first three segments, between the anterior margins of 

 the first leaf and of the foot. In the stage represented in Fig. 299 (p. 398) this 

 origin of the first leaf, first root, and foot, may still be recognised from the arrange- 

 ment of the cells. 



The further growth of the three genera, otherwise very different in their habit, 

 agrees in maintaining the bilateral structure already manifested in the embryo in 

 connection with the decidedly horizontal growth, although, as we shall see, the 

 position of the apical cell and of its segments varies. In contrast with Muscine^ 

 and Equisetacese, but in accordance with Ferns, a leaf is not produced in the Rhizo- 

 carpeae from every segment of the stem ; some of the segments remain sterile, and 

 these then go to the formation of internodes. The leaves grow, as in Ferns and 

 Ophioglossacece, basifugally by means of an apical cell which forms two rows of 

 alternating segments. Before the development has assumed a constant course, an 

 increase of vigour of the young plant takes place, which is shown in the enlarge- 

 ment of the leaves and the greater perfection of their forms, as well as in a change 

 of their relative positions. But in order to make this clear, it is necessary to 

 observe separately Salvinia on the one hand, and the Marsileacese (Marsilea and 

 Pilularia) on the other. 



The embryo of Salvinia, as long as it is enclosed in the prothallium, forms, as 

 we have seen, the segments of its apical cell alternately above and below ; but when 

 the apex of the stem is exposed in consequence of its elongation, a torsion takes 

 place to the extent of about 90°, so that the two rows of alternate segments of the 



