JIVERSiTY 



OF 



101 



The formation of bark may be readily traced in the Elm, by 

 cutting transverse sections successively of stems of increasing age : 

 in those in which the external surface of the stem appears smooth 

 the primary formation of cork which originated from the hypo- 

 dermal layer will probably be the only one present, and in that 

 case a band of cortical tissue will intervene between the cork and 

 the secondary phloem ; but transverse sections should also be cut 

 through the superficial tissues of a stem of which the surface is 

 conspicuously fissured : in these one or more layers of corky tis- 

 sue will be seen traversing the secondary phloem, while tissues of 

 the nature of phloem will be seen included in the brown effete 

 mass of bark which is outside the innermost layer of cork. 



Transverse sections should also be made of the stems of Vitis of 

 various ages : here the formation of successive bands of cork takes 

 place at a relatively early age, and their number is greater than 

 in most woody stems. 



V. Cut radial sections from a four-year-old stem of 

 Elm ; soak them for ten minutes or more in alcohol to 

 remove the air-bubbles, and mount in glycerine. Other 

 sections may be mounted in Schulze's solution, and 

 these will perhaps be found the most useful ; examine 

 first under a low power. 



It will be found difficult to cut good sections so as to include 

 the whole radial surface : it is therefore better not to attempt 

 it, but to study the several structures in a number of succes- 

 sive sections, each extending over only a part of the radial 

 surface. 



Starting from the outside, observe the same suc- 

 cession of tissues as already seen in the transverse 

 sections, viz. : 



1. Epidermis, which is often dried up and dis- 

 organized. 



2. Cork (including the cork-cambium and phello. 

 derm), with the short cells arranged in radial rows. 



