PTERIDOPHYTA 



283 



these the plants often increase very rapidly, and completely cover 

 the surface of the water over large areas. 



Apical Growth. The stem-apex is extended beyoud the youngest leaves, in 

 the form of a slender cone, which is bent upward in Azolla. It grows by an 

 apical cell, from which two rows of segments are produced. Each segment divides 

 into a dorsal and a ventral cell, so that a transverse section made just back of 



FIG. 249. A-C, Azolla filiculoides. A, sporophyte (X2). B, branch with two 

 microsporangial sporocarps (X6). C, macrosporangial, ma, and microsporangial, 

 mi, sporocarps (x 10). D, E, Salvinia natans. D, small plant (X 2). E, hori- 

 zontal section of stem-apex; x, apical cell; L, young leaf (X400). F, Azolla 

 filiculoides, section of young macrosporangium enclosed in the indusium, id; 

 n, filaments of Anabaena (X 200). 



the apex shows two dorsal and two ventral cells. From the former the dorsal 

 leaves arise, from the latter the roots (or, in Salvinia, the ventral leaves) and 

 the lateral branches. 



The stem is traversed by an axial vascular bundle, like that of the stem in 

 the Hymenophyllacese and Schizgeacese. The bundle is typically concentric in 

 structure. As in all aquatics, large air-spaces are developed, forming a series 

 of longitudinal canals separated by thin plates of cells. 



