360 SELAGHNELLA. 



protoxylem is at the centre of the xylem (the latter being therefore 

 endarch), while in the higher erect portions the stele is exarch but 

 is radially symmetrical and has numerous protoxylems all round 

 the outside, instead of two only ; this structure is obviously con- 

 nected with the spiral arrangement of the leaves, since the leaf- 

 bundles are given off at the protoxylems). In S. Kraussiana, the 

 stem has two steles, which are joined up at the points of branching 

 of the stem and then separate again ; each stele has its own air- 

 space, and they are evidently formed by division of a single stele. 

 In other species there may be three or even more steles, each 

 usually flattened and having a protoxylem at each end of the plate- 

 like xylem. 



509. Structure of Leaf. Carefully remove a few 

 leaves, and note (1) the single bundle; (2) the toothed 

 margin; (3) the small outgrowth (ligule) often fan- 

 like at the base of the leaf on the upper side ; (4) the 

 presence of single large chloroplasts in the cells of the 

 epidermis ; (5) the stomata on the lower epidermis, near 

 the middle line. 



In transverse sections of the leaf, note especially (1) 

 the spongy mesophyll, which is absent at the thin 

 margins of the leaf (here the upper and lower epidermis 

 layers come together) ; and (2) the small central vein. 



510. Structure of Rhizophore and Root. In trans- 

 verse sections of (1) the " rhizophore " and of (2) the 

 roots which arise from it, note that the general structure 

 is similar to that of the stem, except that the air-cavity is 

 either absent or represented only by small intercellular 

 spaces between the cells of the endodermis, while the 

 vascular cylinder has a central mass of xylem, with a 

 single protoxylem (monarch structure) at one side where 

 the phloem is interrupted and therefore forms in cross 

 section an incomplete ring. 



511. Structure of Sporangia and Spores. Examine 

 with the microscope (a) an entire cone, treated with 

 potash to make it more transparent ; (6) microspores 

 and megaspores, isolated by teasing out or pressing 

 open the sporangia ; (c) longitudinal sections of a 

 whole cone. 



