SUNFLOWER YOUNG STEM 77 



central part of the radial walls. The oblique part of each wave 

 acts as a reflector, so that the greater part of the light is diverted 

 before it reaches the eye : hence the origin of the dark dot. The 

 bundle-sheath lies immediately outside the vascular bundles, 

 curving slightly towards the centre of the stem in the spaces 

 between the bundles. It is more prominent in the hypocotyle- 

 donary stem, and especially when this is young. The cells are 

 then filled with starch, and the layer may be readily recognized 

 in sections treated with iodine. Under ordinary circumstances it 

 is brought into greater prominence by treatment of the sections 

 with potash, or " eau de javelle." 



Within the bundle-sheath, and arranged in a ring, 

 lie 



3. The vascular bundles, which are of variable 

 number, they are wedge-shaped and of unequal size, 

 and are composed of elements essentially similar to 

 those described above in the older stem ; but the bun- 

 dles are here much smaller, they consist of fewer ele- 

 ments, and the tissues are immature : thus the scleren- 

 chyma which lies directly within the bundle-sheath has 

 its walls as yet thin ; it may already be distinguished 

 r rom the soft bast by the more uniform character of its 

 elements : the cambium may be distinguished by the 

 regularity of arrangement of its cells in radial rows. 

 Of the wood only the vessels nearest to the pith will be 

 matured, and between them and the cambium various 

 young vessels may be seen, having large cavity, but the 

 walls not yet thickened or lignified. 



Note that, if the stem be young enough, the bundles 

 are not joined laterally as in the older stem, but are 

 separated from one another by broad bands of ground 

 tissue. In slightly older stems the cells of this tissue 

 may be found actively dividing, by tangential and occa- 



