Survey of Plant Kingdom 139 



A 



B 



C 



Fig. 51. — The polypody fern (Poly podium) of the phylum Tracheophyta, class 

 Filicineae. A, Showing the horizontal rhizome, the slender adventitious roots, and 

 the fronds with numerous sori on the lower surface; a young frond is shown 

 unrolling; B, lower surface of a frond showing three sori (enlarged), each con- 

 sisting of numerous sporangia which produce spores; C, a sporangium much en- 

 larged, showing the stalk, the internal spores, the capsule with thin walls, and the 

 water-sensitive annulus (band of cells, each of which has three thick walls and 

 an outer thin wall) ; the annulus responds to changes in moisture and bends, back^ 

 thus throwing the spores to the outside. 



