276 PRACTICAL BOTANY 



sporangia, are erect and elongated ; their lower part 

 is covered sparsely with leaves of small size : about 

 12 inches below the apex they usually divide into 

 two or three branches, covered with rather broader, 

 closely imbricated leaves : from the upper surface of 

 each of these rises one sporangium, which is yellow 

 when ripe, and opens by a split parallel to the plane of 

 the leaf which bears it. 



II. Cut transverse sections of a fully developed stem : 

 mount some of them in glycerine, others in chlor-zinc- 

 iodine, and examine with a low power : externally will 

 be found 



1. An epidermis, consisting of a single layer of 

 cells: their outer walls are thick, and covered by a 

 continuous layer of cuticle, which may be recognized 

 in thin sections by its high refractive power, and yellow 

 stain in chlor-zinc-iodine. 



2. Below the epidermis lies a broad band of cortical 

 tissue, which appears differentiated into successive 

 thinner bands according to the thickness of the cell- 

 walls : thus there may be distinguished 



a. An external sclerenchymatous band, with thick 

 lignified walls (brown with chlor-zinc-iodine) and scanty 

 protoplasm : small intercellular spaces may be seen at 

 the angles between the cells. 



b. Within this is a broad band of thin-walled 

 tissue, in which the cell-contents are not apparent : 

 the cell-walls are tinged with pink in chlor-zinc-iodine. 

 There is a gradual transition from this to 



c. The most central part of the cortex, which 

 is strongly sclerenchymatous : it has intercellular 

 spaces, and retains its cell-contents. It forms a dense 



