160 PRACTICAL BOTANY 



stem as this could be cut so uniformly thin that the structure of 

 all the tissues could be well seen ; it is better therefore to cut a 

 number of sections, each extending over a comparatively small 

 area, and to study the various tissues separately. Mount some 

 in glycerine or glycerine jelly, others in chlor-zinc-iodine. 

 Examine under a low power, and observe successively the 

 following tissues, starting from the periphery of the stem : 



a. An epidermis, consisting of a single, somewhat irregular 

 and ill-defined layer of cells, with dark brown outer walls : 

 their arrangement is disturbed at the point of insertion of the 

 scaly hairs, which appear as plates of cells one layer of cells in 

 thickness, rising obliquely from the epidermis. Beneath this 

 is 



b. The ground-tissue, which is differentiated as 



i. An outer narrow band of tissue, with rather thick, colour- 

 less, pitted walls, and cell-contents with much starch : there are 

 no intercellular spaces. 



ii. A band of sclerenchyma with thick, yellow, lignified, 

 obviously stratified, and pitted walls, cell-contents as in (i.;,and 

 no intercellular spaces. This merges gradually into 



iii. The bulky central mass of ground-tissue, in which the 

 vascular bundles are embedded. It consists of cells with 

 comparatively thin, pitted, cellulose walls, protoplasmic contents 

 with much starch, and with intercellular spaces. Internal 

 glandular hairs are also found in the intercellular spaces. 



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 distinguished 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 



