432 MOSSES AND FERNS CHAP. 



ledon. In Marsilia they are always four-lobed. The sporo- 

 carps are modified outgrowths of the petiole, which are often 

 formed so near the base as to appear to grow directly from the 

 stem. They often are borne singly, but may occur in consider- 

 able numbers twenty or more in M. polycarpa and are glob- 

 ular in Pilularia, bean-shaped in Marsilia. The growth of the 

 stem and the origin of the various appendages are the same in 

 both genera. 



A longitudinal section of the stem (Fig. 253, A) shows the 

 decidedly pointed apex occupied by a large and deep apical 

 cell with very regular segmentation. Each segment divides 

 into an inner and an outer cell, the former in all the segments 

 forming the central plerome cylinder, and the outer cells devel- 

 oping the cortex of the stem, and the leaves in the dorsal seg- 

 ments, the roots in the ventral ones. The young leaves are 

 separated by distinct intervals or internodes, and apparently 

 all of the dorsal segments do not give rise to leaves, but just 

 what the relation is between the nodes and internodes was not 

 determined. The roots arise in strictly acropetal order from 

 the ventral segments, but their number does not seem to be 

 constant. In Pilularia Americana the number of roots con- 

 siderably exceeds that of the leaves, as it does in the young 

 sporophyte of P. globulifera. 



The single axial vascular bundle is truly cauline, and ex- 

 tends considerably beyond the base of the youngest leaf. The 

 later leaves in Pilularia, both in their growth and complete 

 structure, correspond to the primary ones. They grow for a 

 time from a three-sided apical cell, in which respect they differ 

 from Marsilia, 1 The development of the leaf of the latter has 

 been carefully studied by Hanstein in M. Drummondii, and M. 

 vestita corresponds exactly with that species. A section of the 

 very young leaf (Fig. 253, C) parallel with the surface shows 

 a large two-sided apical cell. The leaf-rudiment assumes a 

 somewhat spatulate form, and on either side a projecting lobe 

 is formed, the rudiment of one of the lateral segments of the 

 leaf. The apical cell is now divided by a median wall, after 

 which periclinal walls are formed, and from this time the 

 growth of the leaf can no longer be traced to a single initial cell. 

 The first longitudinal wall in the apical cell establishes the two 



1 Pilularia globulifera, according to Johnson (2) and Meunier (i) has 

 the typical two-sided cell found in Marsilia. 



