HEREDITY AND EVOLUTION IN PLANTS 



5. Types of Foliage-leaf.— In some ferns the foliage- 

 leaf presents a simple, unbranched blade, and petiole; 

 but in other species the blade is variously branched. In 

 such cases the larger, primary divisions are called pinna, 

 and the secondary subdivisions pinnules. Illustrations 

 of these various types are shown in Fig. ii. 



6. Sporangia. — As noted above, each sporangium con- 

 sists of a spore-case borne on a stalk (Fig. 1 2) . The struc- 

 ture of the case varies considerably in various groups of 

 ferns, but it usually possesses walls only one cell thick, with 

 a clearly differentiated region, the annulus, composed of 

 cells whose radial and inner cell-walls are greatly thick- 

 ened. Various types of spore-cases are illustrated in 



12.- — Sporangia of an undetermined species of fern; li, lip-cells; 

 an, annulus; st, stalk; sp, mature spores. Each of the four nuclei in the 

 upper cells of the stalk is in the terminal cell of one of the tour vertical 

 rows of cells that compose the stalk. 



Fig. 13. Among the group of ferns which are now most 

 common, and to which the bracken fern (or "brake"), 

 the maiden-hair fern, the common pol3q5ody, and others 

 belong, the sporangium always originates from a single 

 epidermal cell. Ferns whose sporangia thus originate are 

 called leptosporangiate ferns (C/. p. 29). The walls of 

 their spore-cases are always only one cell thick, and 



