THE WALLFLOWER 47 



In the Wallflower the tissue of the young stem, with 

 the exception of the vascular bundles, is soft and succu- 

 lent. A part of the soft tissue lies to the inside of the 

 vascular bundles, and between them ; the remainder is 

 situated to the outside of the bundles. The bundles 

 collectively, together with all the tissue that lies between 

 and within them, form the central cylinder ; the tissue 

 lying to the outside of the vascular bundles is called 

 the cortex. The surface of the stem is covered by a 

 definite skin, which we can easily strip off with the aid 

 of a pair of forceps. We find that it is a very thin, 

 colourless membrane, which, in spite of its delicacy, is 

 fairly tough and elastic. This skin, the epidermis, is 

 only found intact on the younger parts of the stem. 

 The older portions are covered by a much thicker 

 and harder skin, the lark ; all the external " pro- 

 tective" tissues are called by the general name of 

 dermal tissue, which thus includes both epidermis 

 and bark. 



We will now examine, with the help of the micro- 

 scope, a transverse section cut through the stem of 

 the Wallflower (see Fig. 20). This shows us the 

 cut ends of the vascular bundles, and we see that they 

 are arranged in a ring. The space inside the ring 

 of bundles is occupied by a large-celled tissue, the 

 iritli or medulla. A similar tissue extends between 

 the bundles as far as the cortex which surrounds 

 them. These bands of interfascicular tissue (if in 

 Fig. 20) are called medullary rays. Vascular bundles, 

 medullary rays, and pith collectively form the 

 central cylinder. This is surrounded on the out" 



