8 HEREDITY AND EVOLUTION IN PLANTS 



cular plants. These leaves perform all the functions 

 performed by the foliage-leaves of other plants, the most 

 important of which are the manufacture of organic, car- 

 bohydrate food from inorganic raw materials (photosyn- 

 thesis), and the giving off of water vapor from within 

 (transpiration). 



4. Spore-bearing Leaves. — The second type of fern- 

 leaf bears, on its underside, numerous "fruit-dots" or sort 

 (singular sorus) (Figs. 7 and 8). These structures have 

 to do with reproduction. A single sorus of such a fern 



Fig. 9. — Cross-section through the marginal sorus of a sporophyll of 

 the bracken fern {Pteris aquilina). I, palisade layer; fb, vascular bundle; 

 sp, sporangium; in, indusium. (Greatly magnified.) 



as, for example, Polypodium, is composed of a cluster of 

 tiny stalked cases. The cases contain minute unicellular 

 reproductive bodies called spores, and the entire structure 

 is a sporangium. The place where the sporangia are 

 attached to the leaf is the sporangiophore^ (Fig- 9), and 

 over all is often found a thin membranous covering, the 

 indusium (Figs. 9 and 10). In some ferns the indusium 

 is lacking, and the sorus is naked. Spore-bearing leaves 

 are called sporophylls, and plants that bear sporophylls 

 are called sporophytes. 



^AlsQ called receptacle, 



