34 VASCULAR CR YPTOGAMS 



parent-cells of the first root, of the young stem, of the first leaf or 

 cotyledon, and of the foot by which the young embryo is attached to 

 the prothallium. In Marsilea a second cotyledon is formed from the 

 fourth octant of the lower half of the embryo. The layer of tissue 

 surrounding the central cell becomes double after impregnation ; a few 

 grains of chlorophyll are formed in it, and its outer cells develop into 

 long rhizoids, which are especially luxuriant if no fertilisation has taken 

 place. 



The sporophyte of the Marsileaceae differs very widely in external 

 form in the two genera ; but its internal structure agrees in its essential 

 features with that of the Salviniaceae. The stem, root, and leaves all 

 originate from a single apical cell, which always divides into three rows. 

 The stem is procumbent on damp soil or at the bottom of stagnant 

 water, and is traversed by a single central ' vascular ' cylinder filled with 

 fundamental tissue, each bundle consisting of a central xylem with spiral 

 or scalariform tracheides, surrounded by a phloem with large sieve-tubes 

 and sieve-plates, and the whole enclosed in a brown sclerenchymatous 

 bundle-sheath, composed of a single layer of cells with wavy lateral 

 walls. A single ' vascular ' bundle traverses each root and leaf-stalk ; 

 in the lamina of the leaf of Marsilea this branches into a dichotomous 

 venation. The fundamental tissue abounds, in' both genera, in large 

 schizogenous intercellular cavities, as is usually the case with water- 

 plants. Those of Marsilea form a complete intercommunicating system. 

 In those of Pilularia are remarkable spiral hairs. The leaves develop 

 basifugally, as in Salviniacese ; they are formed in two alternating rows 

 on the dorsal side of the stem ; but, as in Salviniacese, it is not every 

 segment of the stem that produces a leaf In this respect the Rhizo- 

 carpese agree with Filices, and differ from Equisetaceae and Muscineas. 

 The leaves are circinate in vernation, resembling in this respect true 

 ferns only. Tannin-sacs occur in the petiole. In Marsilea (see fig. 6) 

 all the leaves except the first, which is filiform and destitute of a lamina, 

 have a very long slender petiole and a four-lobed lamina ; they are 

 larger when growing in water than on dry land. M. quadrifolia (L.) has 

 stomates on both surfaces of the aerial, on the upper surface only of the 

 floating leaves. The stomates are depressed in the tissue of the leaf by 

 the growth of the adjoining epidermal cells over the guard-cells. The 

 mesophyll of the aerial leaves is characterised by the presence of rows of 

 sclerenchymatous cells. In Pilularia the petiole is elongated and quill- 

 like, and entirely destitute of a lamina (fig. 5). 



The sporocarp or conceptacle of the Marsileaceae is an even more 

 complicated structure than that of the Salviniacese. In Pilularia it is a 

 roundish, shortly-stalked capsule springing from the axil of a leaf-stalk 



