SUNFLOWER YOUNG STEM 45 



* * Young- Stem. 



IV. Cut transverse sections of a young branch of the Sun- 

 flower about one-eighth of an inch in diameter. The sections 

 may be cut from the hypocotyledonary stem, in which case 

 they will be found to correspond in all important points to the 

 following description, but they will differ in some of the 

 details ; thus hairs will be absent, the bundle-sheath will be 

 more obvious, &c. : it will therefore be best to cut the sections, 

 as directed, from a young branch. 



Mount in glycerine, and passing from the periphery inwards 

 observe successively under a low power 



1. The epidermis, as before a single layer, with hairs of 

 various complexity and shape. Beneath this 



2. Cortical tissue, which is more or less differentiated into 

 a. Collenchyma. 



#. Cortical parenchyma. 



y. Resin-passages. 



5. Bundle-sheath, or endodermis. 



These severally hold the same position, and have the same 

 characters, as were above observed in the older stem, but the 

 tissues are less bulky, and less clearly differentiated from one 

 another. 



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

 than in the older stem, though it will subsequently be still 

 better seen in roots. It is a continuous layer of cells, which have 

 the characteristic dark dot on each radial wall : this is 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. 



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 



