234 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



to the Salviniaceae on one side, and the Marsiliaceae (Marsilia and Pilularia] 

 on the other. 



The embryo of Salvinia, like that of all the Filicineae, has at first, as may be 

 gathered from what has been already said, a three-sided apical cell, but its segmen- 

 tation soon passes into that of a two-sided cell. Leitgeb says that the segments lie 

 right and left from the very first as in the full-grown stem. The first leaf, the 

 cotyledon or scutiform leaf, is placed medio-dorsally, and is followed by a second 



and third aerial leaf each standing alone, and the 

 final position of the leaves in a whorl is not 

 established till the formation of the fourth node. 

 Each whorl of leaves consists from that time for- 

 wards of a submerged leaf which springs right or 

 left from the ventral face of the stem and at once 

 ramifies into a tuft of long filaments hanging down 

 in the water, and of two others leaves with flat 

 expansions which grow from the dorsal face and 

 touch the water only with their under surface 

 (Fig. 194). These whorls of three leaves alter- 

 nate and so form two rows of ventral submerged 

 leaves and four rows of dorsal aerial leaves ; the 

 succession of the leaves, according to age in the 

 whorl and the position of the whorls which are 

 antidromous as regards one another, is indicated 

 in Fig. 1 88. The node of the stem, which pro- 

 duces a whorl of leaves, is formed, as Pringsheim 

 showed, from a transverse disk of the long vegeta- 

 tive cone, the length or height of the disk cor- 

 responding to that of a half segment, while every 

 internode corresponds to the whole height of a 

 segment. A nodal disk, like every internode, is 

 made up of cells of the right and left row of 

 segments of different ages (Fig. 189). In each 

 whorl the submerged leaf is the oldest, the aerial 

 leaf which is further from it is the next in age, the 

 aerial leaf nearer to it is the youngest. Every leaf 

 is formed from a cell in a definite position; this 

 cell arches outwards (Fig. 189, B, L v Z 2 ) and 

 developes into the apical cell of the leaf forming 

 segments on two sides. In Azolla too, which has 

 been studied by Strasburger, the apical cell of the 

 stem which floats in a horizontal position but is curved upwards at the apex forms a 

 row of segments lying on the right and on the left, and each segment is divided 

 by a lateral longitudinal wall into a dorsal and a ventral half; each half is divided by 

 a transverse wall into an acroscopic and a basiscopic portion, and each of these four 

 cells is again divided into two cells by a longitudinal wall inclined obliquely upwards 

 or downwards. Thus the stem, apart from later divisions, is composed of eight 



FIG. 187. Marsilia salvatrix. Longitudinal 

 section through the spore, pro thallium and embryo; 

 am starch grains of the spore, t inner coat of the 

 spore torn above into lobes, ex the episporium 

 with prismatic construction, c the space beneath 

 the convex diaphragm, on which the basal layer 

 of the prothallium rests, pt the prothallium, -wh its 

 rhizoids, a the archegonium, f the foot of the 

 embryo, TV its root, s the apex of the stem, 6 the 

 first leaf (cotyledon) which stretches the pro- 

 thallium, si the mucilaginous envelope of the 

 spore, which at first forms the funnel above the 

 papilla and still surrounds the prothallium fifty 

 hours after the dissemination of the spores. Magn. 

 about 60 times. 



