GENERAL BIOLOGY 



about y$ inch or less in diameter, and containing nitrogen-fixing 

 bacteria, may occur on some of the rootlets. 



2. The disappearance of the cotyledons and the scars on the 

 stem where they were attached. 



3. The primary leaves as in the younger plant. 



4. The stem growing up between the primary leaves, and con- 

 sisting of nodes, from which leaves and branches arise, and inter- 

 nodes. 



5. The secondary leaves are compound, each consisting of three 

 or more ovate leaflets, with rudimentary leaflets (stipels) at the 

 base of each. Are stipules present at the base of the petioles? Do 

 the secondary leaves occur in opposite pairs, as in the primary 

 ones? 



6. In the axils of the leaves are buds which may give rise to 

 branches and flowers. 



(B) Histology. 

 (I.) The Stem 



i. Study a thin transverse section through an internode, first 

 under the low power and then under the high power. Note: 



a. The central pith often with a cavity and with pith-cells 

 around cavity. 



b. The fibro-vascular bundles arranged in a ring outside the 

 pith. Commencing at the side nearest the pith in each bundle: (i) 

 Small openings which are cross sections of the spiral vessels; (2) 

 Larger openings, the pitted vessels; (3) Small thick-walled wood 

 cells between the vessels. These three constitute the wood or 

 xylem of the bundle ; (4) The cambium zone, composed of thin- 

 walled cells in radial rows 1(5) The bast or phloem, composed in- 

 ternally of bast-cells and sieve tubes and externally of rounded 

 bast-fibers with thickened walls. 



c. The cortex, consisting of several layers of large, rounded 

 cells containing chlorophyll. 



d. The medullary rays, radiating rows of cells passing between 

 the bundles. 



e. The epidermis, composed of a single layer of squarish- 

 looking cells containing no chlorophyll and some bearing hairs. 

 Between some of the epidermal cells are openings, the stomata, 

 each bounded by two small guard cells. 



Draw and label the section. 



[22] 



