454 ELEMENTARY BIOLOGY. [CHAP. 



thickened walls (protophloeni) and then a 

 single row of large thin-walled cells (sieve- 

 tubes or bast vessels] between which lie smaller 

 thin-walled cells containing starch granules 

 (past parenchyma). 



B. The whole of the space within the phloem 

 is occupied by the xylem or wood. Note the 

 greatly thickened walls of its large vessels, and 

 their central cavity containing no protoplasm ; 

 the groups (usually two in number) of much 

 smaller vessels (protoxylem) ; scattered here 

 and there, in the spaces between the angles 

 of the vessels, note the small parenchymatous 

 cells (wood parenchyma) containing starch 

 granules. 



e. Treat with iodine : observe the protoplasm 

 stained brown ; the starch granules deep blue, 

 rendering some of the cells quite opaque and 

 almost black-looking; the cell-walls of the 

 xylem light brown. 



5. Cut a thin longitudinal section of the stem and 

 examine with i inch and then with f obj. Make 

 out the various tissues described in 3 and 4. 



a. The epidermis, hypoderma and parenchyma, much 

 as in the transverse section, except that the 

 hypodermal cells are longer. 



b. The sclerenchyma is seen to be made up of greatly 

 elongated cells, tapering towards each end. 



c. The vascular bundles ; note in them 



a. The cells of the bundle sheath and of the 

 phloem sheath much as in the transverse 



