284 



PLANT MORPHOLOGY 



The bundles are amphicribral with mesarch xylem (Fig. 239). Each is 

 surrounded by an endodermis that encloses a continuous pericycle usu- 

 ally comprising only a single layer of cells. The ground tissue of the rhi- 

 zome consists of parenchyma surrounded by an outer zone of thick-walled 

 cells. In the central region are two transverse bands of sclerenchyma, 

 one occurring above and one below the medullary strands. The lower 

 band is larger than the upper one and slightly curved. 



Sorus. In most of the Filicales the sporangia are borne on the abaxial 

 surface of ordinary foliage leaves. In the Schizaeaceae the sporangia are 



Fig. 239. Cross section of a vascular bundle from the rhizome of Pteridium aqiiilinum, 

 X200; end, endodermis; per, pericycle; ph, phloem; px, protoxylein; mx, metaxylem. 



solitary, but in the other families they occur in groups called sori. Gen- 

 erally the sori are arranged on either side of the midrib, but in many 

 genera they are marginal or nearly so (Fig. 240). Ordinarily the leaf seg- 

 ments that bear the sporangia are unmodified. Often, however, a 

 marked differentiation exists between sterile and fertile leaflets, the latter 

 being conspicuously contracted. This condition prevails in Osmunda, the 

 Schizaeaceae, Onoclea, Blechnwn, etc. The same leaf may produce both 

 sterile and fertile leaflets, or the entire leaf may be made up of either one 

 kind or the other. 



Some ferns have naked sori, but usually each sorus is covered by a flap- 

 like membrane called the indusium (Fig. 240). Although absent in the 

 Osmundaceae and Gleicheniaceae, an indusium is present in the Schizaea- 

 ceae, Hymenophyllaceae, Dicksoniaceae, Cyatheaceae (except Alsophila), 

 and in most of the Polypodiaceae {Polypodium being a notable exception). 

 Generally the indusium represents a special outgrowth of the leaf, but it 



