164 PKACTICAL BOTANY 



Surrounding the above tissues of the xylem and 

 phloem is a sheath of sclerenchyma. On its in- 

 ternal side may be found tissue-forms which are 

 transitional between sclerenchyma and certain of 

 the constituents of the bundle. 



II. Cut longitudinal sections of the same, treat as 

 before, and observe 



a. The epidermis, composed of oblong cells. 



I. The prosenchymatous cells of the sclerenchyma. 



c. The ground-parenchyma,, with roundish cells. 



d. The vascular bundles, pursuing a longitudinal 

 course parallel to one another, without lateral fusion. 



In the xylem observe 



i. The annular, spiral, and pitted vessels, and 

 note, especially in the latter, the clearly-marked joints, 

 pointing to their origin from a succession of cells. 



ii. The pitted tracheides. 



iii. The thin-walled parenchyma. 



And in the phloem, which is easily recognized 

 by its cellulose walls, blue with chlor-zinc-iodine, 

 distinguish 



iv. The sieve-tubes, which have a wide cavity, 

 intercepted here and there by transverse sieves. 



If it be found difficult to distinguish the sieve -plates, 

 a fresh section may be treated with potash ; the 

 character of the sieve-plate is then more easily seen. 



v. The cambiform cells, which are narrow and 

 parenchymatous. 



Note the prosenchymatous constituents of the sheath 

 of sclerenchyma, and observe transitional forms 

 between these and the pitted tracheides with square 

 ends, which belong to the xylem. 



