Minnesota Plant Life. 



169 



face of the water in tanks. It has a much branched stem and 

 tiny, rather ovate leaves. The whole body resembles that of a 

 scale-moss. As they grow the leaves form peculiar cavities, 

 opening by a narrow aperture through which a little alga in- 

 serts itself and is a constant companion of the AzoUa plant, for 

 in all AzoUa leaves are found growths of this little blue-green 

 alga. Like the leaves of the scale-mosses, those of Aaolla have 

 two lobes, one, the floating lobe, lying upon the surface of the 

 w^ater and the other, the submerged lobe, lying below. The 

 spore-cases are borne in groups upon the submerged lobe. 



Fig. 61. A fern-plant embr>-o imbedded in the enlarged egg-organ, where it arose V)y seg- 

 mentation of an egg. S, tip of rudimentary stem; I,, tip of first leaf; R, tip of primitive 

 rootlet; F, nursing foot. Much magnified. After Atkinson. 



There are two kinds of spores, large and small, and several of 

 the spore-cases which produce small-spores are developed in 

 clusters and inclosed by a general protective wall. The spore- 

 cases which develop the large-spores occur singly within such 

 a wall and each large-spore-case produces a single large-spore. 

 When the small-spore-cases open, simultaneously several small- 

 spores escape imbedded in a lump of frothy mucilage upon 

 which curious little anchor-shaped barbed hairs are disposed. 

 The large-spore, when its case opens, is found to have one end of 

 its wall provided with low flat-topped excrescences from each 



