392 



MOSSES AND FERNS 



CHAP. 



upper cells of the ventral segment, and alternate with the leaves 

 on the same side of the stem. 



The mother cell of a leaf is distinguished by its size and 

 position (Fig. 201, B, III, L), and the first division wall, as in 

 the cotyledon, divides it into two nearly equal lobes. No trace of 

 an apical cell can be found in the young leaf, and in this respect, 

 as well as the secondary divisions of the stem segments, Azolla 

 differs from Salvinia, where for a long time the young leaves 

 grow, as in most Ferns, by a two-sided apical cell (Fig. 203, A). 

 Each leaf lobe m. Azolla is divided into an inner small cell and 

 an outer larger one, and the latter is then divided by a radial 

 wall. This formation of alternating tangential and radial walls 



B. 



Fig. 202. — A, Young microsporangial sorus oi A._fiHcnloides, X 80 ; col, columella ; id, indusium ; 



B, nearly ripe microsporangium, X 225. 



is repeated with great regularity, and can be traced for a long 

 iime. It is not unlike the arrangement of cells figured by Prantl ^ 

 in some of the Hymenophyllaceae. 



The fully - developed leaves of Azolla are all alike. In 

 A. filiculoides the two lobes are of nearly equal size, the lower 

 or ventral one, which is submersed, somewhat larger, but simpler 

 in structure. The dorsal lobe shows a large cavity near its base 

 (Fig. 204, A), which opens on the inner side by a small pore. 

 On the outer side the epidermal cells are produced into short 

 papillate hairs, which in some species, e.g. A. Caroliniana, are 

 two-celled. Stomata of peculiar form (Fig. 204, B) occur on 



1 Prantl (i), PI. I. Figs. 2, 3. 



