326 THE PRIMARY STEM 



PRACTICAL WORK 



(i) Sketch the piece of herbaceous shoot provided, marking 

 leaf blade, leaf stalk, axillary buds, nodes, internodes. Note and 

 indicate the arrangement of the vascular bundles as seen on 

 the cut surfaces of the stem and leaf stalk : also the epidermis, 

 cortex, pericycle, rays and pith. [A piece of the mature shoot 

 of almost any large herbaceous plant is suitable, e.g. sunflower, 

 vegetable marrow, bean.] Examine also the preparation of a 

 piece of shoot made transparent in Canada balsam so that the 

 course of the vascular bundles can be followed. 



(2) Pull off the outer leaves of the Brussels sprout provided. 

 This is a bud whose out6r leaves become mature with very little 

 elongation of internodes. With a sharp knife or a razor cut a 

 longitudinal section as accurately as possible through the tip, and 

 make a diagrammatic sketch showing (a) growing point (primary 

 meristem), (6) the youngest leaves, (c) older leaves with buds in 

 their axUs, {d) young vascular bundles. Pull ofi the remaining 

 leaves and note their insertions and their axillary buds. 



(3) Examine the prepared slide of the same for microscopic 

 details. Note especially the origin of the vascular bundles 

 (desmogen strsinds), the first spiral tracheids of the protoxylem 

 and the general appearance of the pith cells. 



(4) Examine in dilute glycerine a cross-section of the stem of 

 the sunflower, at first with the naked eye and with a hand lens ; 

 and then draw a diagram with the help of the low power. Mark 

 the outlines of the following tissues and tissue-systems : epidermis, 

 outer cortex, inner cortex, endodermis (starch sheath) ; vascular 

 cylinder, consisting of pericycle (composed of fibres opposite the 

 bundles, parenchjrmatous between), rays, pith, and vascular 

 bundles, showing xylem, phloem and beginning of cambium. 



Put a section in a drop of Schulze's solution for a few minutes, 

 and note the colour reactions of the various tissues, showing the 

 distinction between cellulose and lignified tissues. 



(5) Now make careful high-power drawings of a few cells of 

 each of the following tissues : {a) epidermis with cuticle ; (b) outer 

 cortex (collenchymatous) ; (c) inner cortex (parenchymatous) ; 

 (d) endodermis ; (e) pericycle fibfes ; (/) phloem, including sieve 

 tubes, companion cells and parenchyma ; (g) cambium ; (h) xylem, 

 including large (pitted) vessels, parenchyma and xylem fibres ; 

 and (j) protoxylem ; [j] cells of the medullary rays which are 

 beginning to form interfascicular cambium. 



Examine a prepared stained section of the same to verify 

 details and compare the staining. 



