278 PRACTICAL BOTANY 



elliptical form as seen in the transverse section : several 

 of these are ranged side by side, the longer axes of the 

 ellipses being horizontal as the plant grows, and parallel 

 to one another : the masses of xylem may be irregularly 

 connected one with another towards the centre of the 

 stem. 



III. Before proceeding to the study of these several tissues in 

 detail, cut transverse sections from the young stem, at about one- 

 eighth of an inch below the apex : treat some of these with 

 dilute potash or " eau de javelle," others with chlor-zinc-iodine- 

 Observe that in these sections the tissues at the centre of the 

 vascular cylinder are still thin -walled, and have plentiful proto- 

 plasm, i.e. they are not fully developed ; towards the periphery, 

 however, will be seen a series of groups of tissue showing the 

 characters of developed xylem, and alternating with these is a 

 series of groups of phloem tissues. Other preparations may be 

 made successively from points further from the apex, and from 

 these the conclusion may be drawn that the vascular tissues at the 

 periphery of the vascular cylinder are matured first, and that the 

 development proceeds towards the centre. 



IV. Returning to the sections of the old and mature 

 stem, examine the vascular tissues under a high power. 



i. Immediately within the endodermis is a band 

 of tissue, which abuts directly upon the periphery of 

 the xylem and phloem, and having cell-walls which 

 stain blue with chlor-zinc-iodine : this may be regarded 

 as the phloem-sheath or pericycle. 



ii. On examining the masses of xylem, observe 

 that 



a. The constituent elements are much smaller at the 

 periphery of the vascular cylinder than towards the 

 centre : the former are the first developed or proto- 

 xylem elements. 



