XVI.] THE BEAN PLANT. 473 



cells are much thickened at the angles (collen- 

 chymatous). 



e. The medullary rays: radiating rows of paren- 

 chymatous cells passing between the bundles and 

 uniting b and d: not quite continuous, being in- 

 terrupted by the cambium zone (c. 8.). 



f. The epidermis: composed of a single layer of 

 somewhat squarish looking cells, containing no 

 chlorophyll. Note the stomata, their two small 

 guard-cells being seen in section. 



3. Cut a transverse section through a node, and com- 

 pare it with that through the internode. Observe 

 the bundles passing out from the stem into the leaf. 



4. Cut a thin longitudinal section through part of an 

 internode (if necessary the bit of stem may be im- 

 bedded in paraffin first), and mount it in water ; 

 working from the medullary cavity outwards, note 

 the following layers, using at first a low power : — 



a. The pith-cells : much as in the transverse section. 



b. The fibro-vascular bundles presenting — 



a. The spiral vessels : elongated tubes with a spiral 

 thickening on their walls. 



/?. The wood-cells: elongated and with much 

 thickened walls. 



y. The pitted vessels : much like a, but with their 

 walls pitted instead of spirally thickened. 



S. The cambium zone: made up of small, prismatic, 

 thin-walled cells, containing abundant proto- 

 plasm. 



t . The bast parenchyma: thin-walled elongated 

 cells. 



