266 PRACTICAL BOTANY 



prismatic tracheides, with peculiarly marked, Kgnified 

 walls. 



4. At the poles of the spindle-shaped xylem note 

 tracheides of smaller size : these compose the first- 

 formed protoxylem : the development of the xylem 

 thus starts from the periphery and proceeds towards 

 the centre of the oval xylem. 



To confirm this, cut transverse sections of the stem about one 

 inch from the apex, and treat as above. It will be seen that the 

 elements near the poles of the xylem are already fully formed, and 

 their walls lignified, while those at the centre are still thin- walled, 

 and have protoplasmic contents. The lacunar tissue will be found 

 to be better denned in these sections. 



IV. Cut longitudinal sections of a stem of S. Martensii. 

 Owing to its being fixed in its place only by the weak 

 trabeculae, the vascular bundle is apt to be detached in 

 cutting accurately longitudinal sections : it will there- 

 fore be found better to cut the sections slightly 

 oblique ; it must then be remembered in examining 

 them under the microscope that the sections are not 

 truly longitudinal. 



Mount some in glycerine, others in chlor-zinc-iodine, 

 and examine them under a high power, noting the 

 same succession of tissues on starting from the outside 

 as were seen in the transverse sections : thus 



1. Epidermis } these are hardly to be distin- 



2. Outer cortex ) guished one from another : the 

 cells of both are prosenchymatous, and thick- walled, 

 and show a gradual transition to 



3. The inner cortex, in which the walls are thinner, 

 and the form of the cells parenchymatous. 



