412 



MOSSRS AND FERNS 



CHAP. 



Pilularia retain the same structure as the cotyledon. In 

 Marsilia they are ahvays four-lobed. The spor^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 considerable 

 numbers — twenty or more in M. polycarpa — and are globular 

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



Fig. 215. — Marsilia vestita (Hook and Grev.). A, Vertical longitudinal section of the stem apex, 

 x8o ; L, leaves ; st^ stem apex ; r, roots ; B, the stem apex, X450 ; C, horizontal section of very 

 young leaf, X450; D, similar section of an older one, X450; E, cross-section of mature stem, 

 x8o. 



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

 both genera. 



A longitudinal section of the stem (Fig. 215, A) shows the 

 decidedly pointed apex occupied by a large and deep apical 

 cell with very regular segmentations. 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 

 developing the cortex of the stem, and the leaves in the dorsal 

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



