MALE FERN. SPOROPHORE. 193 



1. The Bundle-sheath, a single layer of cells 

 w ith yellowish walls, and yellow granular contents. 

 N.B. There are no intercellular spaces in this 

 layer, nor in any of the tissues surrounded by it. 



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

 walls of the bundle-sheath retain a sharp contour, while those o 

 the rest of the tissues swell, and become more or less disorganised. 



2. The Phloem-sheath, 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, 

 with starch. Note that each of the outermost cells of 

 the phloem-sheath is as a rule opposite one cell of the 

 bundle -sheath : this points to a common origin of the 

 two layers. 



In the bundles of many Ferns, e.g., in the "Rhizome of Pteris, 

 the phloem-sheath appears as a single layer of cells. 



3. At the inner limit of the phloem-sheath are found 

 elements with thick cellulose walls and narrow cavity : 

 these constitute the Protophloem of Eussow. 



4. Internally lies the broad band of true Phloem, 

 which is composed of two tissue-forms 



a. Sieve-tubes, which appear in the transverse 

 section as polygonal, with their thin, cellulose walls, 

 which are lined by a delicate protoplasmic membrane 

 including numerous highly refractive granules. 



&. Parenchymatous cells with thin walls and pro- 

 toplasmic contents. 



o 



