ASPIDIUM SPOROPHYTE 295 



power, and starting from the periphery of it, note 

 successively 



1. The bundle-sheath or endodermis, a single 

 layer of cells, with yellowish walls, and yellow granular 

 contents : there are no intercellular spaces in this layer, 

 nor in any of the tissue surrounded by it. 



Treat a thin section with sulphuric acid, and note that the 

 walls of the endodermis retain a sharp contour, while those of 

 the rest of the tissue swell, and become more or less disorganized. 



2. The phloem-sheath or pericycle, which is a band 

 of tissue of varying thickness at different parts of the 

 bundle, being thin at the poles of the elliptical bundle, 

 and thicker at the sides : it consists of cells of roundish 

 form, with cellulose walls, and protoplasmic contents, 

 and starch. 



3. At the inner limit of the pericycle are found 

 elements with thick cellulose walls and narrow cavity, 

 these constitute the protophloem. 



4. Internally lies the broad band of true phloem, 

 which is composed of two tissue-forms 



a. Sieve-tubes, which appear polygonal in the 

 transverse section, with thin cellulose walls 

 which are lined by a delicate protoplasmic 

 membrane dotted with numerous highly 

 refractive granules. 



&. Cells of the conjunctive parenchyma, with 

 thin walls, and protoplasmic contents. 



5. Centrally lies the xylem, in which also two con- 

 stituents may be recognized 



a. Tracheides, which appear polygonal in section, 

 and have large cavities, with no cell-contents : 

 the walls are thick and lignified, and show a 



