SUNFLOWER. STEM. 63 



hairs of various complexity and shape (cf. apical bud). 

 Beneath this 



2. Cortical tissue, which is more or less clearly 

 differentiated into 



a. Collenchyma. 



/3. Cortical Parenchyma. 



7. Resin-passages. 



8. Bundle-sheath. 



These severally hold the same position, and have the 

 same characters, though less strongly developed, as were 

 above observed in the older stem. 



The bundle-sheath in the young stem is more easily recog- 

 nised than in the older stem. It is a continuous layer of cells, 

 whose radial walls have a characteristic dark dot on each radial 

 wall, due to reflection of light from the peculiar sinuous waves of 

 the 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 hypo- 

 cotyledonary stem, and especially when this is young. The cells 

 are then filled with starch, and the layer may be readily recognised 

 in sections treated with iodine. Under ordinary circumstances 

 it is brought into greater prominence by treatment of the sections 

 with potash. 



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

 lie 



3. The Vascular bundles, which are wedge-shaped, 

 of variable size, composed of similar elements to those 

 described above in the older stem. 



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 



