NEPHRODIUM SPOROPHYTE 161 



walls of the inner ground-tissue stain blue, and that starch is 

 found in the cells ; that the bundle-sheath appears browner 

 than before ; that the walls of the phloem stain blue (cellulose), 

 and the contents yellowish ; that the walls of the chief con- 

 stituents of the xylem stain yellow (lignified). 



VI. As the vascular bundles of the leaf-stalk are better fitted 

 for minute observation, and are better types of the concentric 

 bundle of the Fern than those of the stem, cut thin transverse 

 sections of the lower part of the petiole. Having previously 

 noted with a low power that in their main features the tissues 

 resemble those above observed in the stem, examine the 

 structure of one vascular bundle under a high 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. 



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. 



b. Cells of the conjunctive parenchyma, with thin walls 



and protoplasmic contents. 



5. Centrally lies the xylem, in which also two constituents 

 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 peculiar structure 



M 



