SELAOINELLA. 359 



and contain large numbers of the much smaller micro- 

 spores. 



(/) The dehiscence of the sporangia, by a slit running 

 transversely across from side to side. 



507. Structure of Stem. In transverse sections of 

 the stem of S. Martensii, note (1) the epidermis, a layer 

 of thick- walled cells, covered by cuticle ; (2) the green 

 cortex, the outermost part of which is usually compact, 

 thick-walled, and lignified ; (3) the vascular cylinder 

 (stele), surrounded by an air-space which is bridged by 

 trabeculae consisting of cell-rows (here and there a single 

 elongated cell may be seen forming a trabecula) . 



In the vascular cylinder, note (1) the pericycle, 

 a layer of rather large cells ; (2) the phloem, forming a 

 continuous band around (3) the solid central xylem, 

 which is oval or spindle-like in cross section, with a group 

 of small protoxylem tracheids at each end (the xylem is 

 therefore exarch and diarck). With the high power, note 

 that there is a ring-like transverse band on the innermost 

 cell of each of the trabeculae consisting of a row of cells, 

 or at the middle of the long unicellular trabeculae ; the 

 trabeculae represent the endodermis. 



In cross sections of the young parts of the stem, taken 

 an inch or so behind the stem, the development of the 

 air-space can be followed, and the trabecular tissue more 

 readily made out ; note also that the polar portions 

 (protoxylem) of the wood are fully formed and lignified, 

 while the large-celled middle portion is still thin-walled 

 and contains protoplasm. 



In longitudinal sections of the stem, note the tissues 

 as described above ; the xylem consists of spiral (pro- 

 toxylem) and scalariform tracheids, as in Lycopodium ; 

 note the leaf-traces, one running into each leaf the 

 leaf bundle carries with it into the leaf a continuation of 

 the air-cavity surrounding the central cylinder. 



508. For comparison, examine sections of the steins of other 

 species of Selaginella. In S. spinosa there is a single stele as in 

 S. Martensii, but in the creeping lower portion of the stem the 



