294 PRACTICAL BOTANY 



parenchyma : the globular head contains when fresh a resin- 

 ous secretion, which is soluble, but not readily, in alcohol. 



c. The vascular bundles of elliptical outline : they 

 are embedded in the ground- tissue, and are sharply 

 circumscribed by a narrow, light brown layer of cells 

 without intercellular spaces : this is the bundle-sheath 

 or endodermis. Among the tissues inclosed by this 

 sheath, note that a large central mass may be distin- 

 guished as consisting for the most part of elements with 

 large cavity, no cell-contents, and rather thick walls 

 with a peculiar marking : this is the xylem. Between 

 this and the bundle-sheath is a broad band of tissue 

 with thin, bright looking walls, and with protoplasmic 

 contents : this is the phloem. Since the xylem is 

 surrounded by the phloem, this bundle is said to be of 

 the concentric type. 



In the sections treated with chlor-zinc-iodine note 

 that the 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 constituents of 

 the xylem stain yellow (lignified). 



VII. 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 



