218 PRACTICAL BOTANY. 



of the Xylem, and are disposed, roughly speaking, in 

 the form of a V, while the apex of the V is occupied in 

 each bundle by a large air-cavity. There are origin- 

 ally four groups of xylem elements in each bundle, two 

 bordering on the cavity, and two nearer the bundle- 

 sheath ; the elements of the former are often only im- 

 perfectly seen in transverse sections, since they are apt 

 to become disorganised during development. Between 

 the air-cavity and the bundle-sheath lies a mass of 

 tissue of the Phloem, with relatively thin cellulose 

 walls ; Sieve-plates may sometimes be observed in 

 surface view in this tissue. 



Y. Compare transverse sections of the underground 

 axis, or root-stock, with those of the aerial axis : the 

 sections may be prepared in the same way as the above. 

 Note that 



1. The superficial cells have brown walls, and often 

 grow out as long, brown, root-hairs : there are no 

 stomata. 



2. The sub-jaccnt cortex is thin-walled, and colour- 

 less, and often contains much starch. 



3. The ridges, intercellular cavities of the cortex, 

 and vascular bundles have the same relative positions 

 as in the aerial stem. 



4. The structure of the vascular bundles is similar to 

 that in the aerial axis. 



5. There is no cavity at the centre of the axis. 



Cut transverse sections of one of the tubers ; treat with potash, 

 and mount in glycerine. Observe 



1. The brown- walled epidermis with many hairs, similar to 

 those on the root-stock. 



2. A sub-epidermal layer with thickened walls. 



