4IO MOSSES AND FERNS chap. 



later ones. In Pilularia the later leaves are essentially like the 

 cotyledon, but in Marsilia all the later leaves show a distinct 

 lamina. This is at first narrow and undivided, and spatulate 

 in form. In M. vestita this is succeeded by five or six similar 

 ones, with constantly-broadening laminae, which finally divide 

 into two narrow wedge-shaped lobes, and these are then 

 succeeded by others with broader lobes, which finally are 

 replaced by four lobes, the central ones being narrower than 

 the outer ones. All of these early lobed leaves are folded flat, 

 and it is not until about ten or twelve leaves have been formed 

 that finally the leaf attains the form and vernation of the fully- 

 developed ones. 



The divisions in the stem apex take place slowly, but 

 apparently a complete series of segments is produced in rapid 

 succession, and there is an interval before any more divisions 

 occur, as there is always considerable difference in the ages of 

 any two succeeding sets of segments. The apical cell of 

 Pilularia in cross-section has the form of an isosceles triangle 

 with the shorter face below. Probably each dorsal segment at 

 first gives rise to a leaf, and each ventral one to a root. How- 

 ever, the number of roots exceeds that of the leaves, but the 

 origin of these secondary roots was not further investigated. 



TJie Mature Sporophyte 



In both Marsilia and Pilularia the fully-developed sporo- 

 phyte is a creeping slender rhizome, showing distinct nodes and 

 internodes. At the nodes are borne the various appendages of 

 the stem, and the elongated internodes are, except for occasional 

 roots, quite destitute of appendages. Leaves and branches 

 arise from the nodes, and in Marsilia are much crowded. The 

 plants are aquatic or amphibious, and the habit of the plant is 

 very different, especially in Marsilia, as it grows completely 

 submerged, or partially or entirely out of water. Some species, 

 like M. vestita, which grow where there is a marked dry season, 

 grow in shallow ponds or pools, which dry up as the end of the 

 growing period approaches, and the ripening of the sporocarps 

 takes place after the water has evaporated. In the first case 

 the petioles are extremely long and weak, and the leaf-segments 

 float upon the surface. In the other case the petioles are much 

 shorter and stouter, and the leaves are borne upright. The 



