4i6 MOSS-ES AND FERNS chap. 



question to be considered leaflets, which remain confluent 

 except at the top. A Httle later a slight depression or pit 

 forms at the base of each lobe and the central area at the top. 

 These pits are separated laterally by the coherent edges of the 

 leaflets, which extend to the axis of the sporocarp and are 

 continuous with it. As the young fruit enlarges, the depres- 

 sions deepen owing to the elongation of both leaflets and the 

 axial tissue, which forms a sort of central columella (Fig. 216, 

 D). Thus are formed four deep cavities, separated laterally by 

 the united margins of the leaflets, and corresponding to the 

 much more numerous " canals " described by Russow in the 

 fruit of Marsilia ; like these they at first open at the summit by 

 a pore, and a study of longitudinal sections shows clearly 

 their strictly external origin. 



Up to the time the cavities begin to form, the young fruit 

 is composed of uniform tissue, but shortly after, the tissue 

 systems become differentiated, and the peduncle of the sporo- 

 carp is formed. At this time the vascular bundle of the 

 peduncle can be recognised, and joins that of the sterile 

 segment near its base. The peduncle is much longer in P. 

 Americana than in the very similar P. globidifera. The 

 circinate coiling of the sterile segment is repeated, though less 

 conspicuously, here, and the body of the sporocarp is bent at 

 right angles to the peduncle. 



The cavities rapidly become larger with the expansion of 

 the growing sporocarp, but the space between the inner surface 

 of the lobes and the columella remains narrow, owing to the 

 growth of the sorus, which almost completely fills it from the 

 first. The sorus forms an elongated cushion, extending nearly 

 the whole distance from the apex to the base of the lobe, along 

 the median line of its inner face. In origin and position it 

 corresponds exactly to that of most homosporous Ferns, except 

 that it arises from the upper instead of the lower side of the 

 leaf. The lamina separating the cavities are composed of 

 about four layers of cells. 



The vascular bundle of the peduncle divides into four 

 branches, where it enters the sporocarp, and one branch goes 

 to each lobe, of which it forms the midrib lying below the 

 sorus. From each of these, two smaller branches are given 

 off near the base, following the margin of the lobe (Fig. 

 217, A). By this time the outer epidermal cells begin to 



