166 THE GROWTH OF HOOTS AND STEMS. 



(d) Next cut across a piece of fresh stem with a dry knife or razor, 

 and notice the juice which oozes out of the soft greenish outer and 

 inner portions of each bundle. These portions are the bast, l and they 

 also consist chiefly of tubes ; but the bast-tubes, instead of carrying 

 water up the stem as the wood-tubes (vessels) do, carry organic food- 

 substances from the leaves to other parts of the plant. Notice that 

 the juice which oozes out of the bast- tubes is thicker than water ; 

 collect some of it, place it on a glass slide, and test it (1) for starch, 

 (2) for proteids, (3) for sugar. To test for (1) and (2) add a drop of 

 iodine solution ; for (3) place some Fehling's solution on the slide with 

 the juice and warm it, or collect enough juice to place in a test-tube 

 and heat with some Fehling's solution. 



(e) In sections through the youngest parts of the Marrow stem, 

 notice that the ground-tissue is complete right across the stem ; the 

 cavity found in the older parts is formed oy the central region of 

 ground-tissue (pith) becoming torn as the stem grows thicker. Also 

 notice that the wood contains only the narrow spiral and ringed vessels, 

 and that the collenchyma and sclerenchyma are not yet distinguished 

 sharply from the ordinary ground-tissue. 



197. Sunflower Stem. Cut transverse and longitudinal sections 

 of stem of Sunflower, and compare its structure with that of the 

 Marrow stem (see Fig. 57). In the Sunflower the stem is cylindrical, 

 not furrowed ; the central ground-tissue does not become torn to form 

 a cavity ; the hard tissue (sclerenchyma) is in separate strands, one 

 immediately outside the bast of each of the larger bundles. In an old 

 stem, notice that there is a continuous layer of wood, with a complete 

 layer of growing tissue (cambium) on its outer side. 



198. Maize Stem. (a) Cut transverse sections of stem of Maize. 

 Notice that the bundles, although "scattered" through the ground- 

 tissue, are most crowded towards the outside, and that in each bundle 

 the bast side points towards the epidermis and the wood side towards 

 the centre of the stem. In thin sections stained with aniline chloride, 

 notice (1) the epidermis of small thick -walled cells ; (2) the narowband 

 of sclerenchyma below the epidermis ; (3) the ordinary ground-tissue 

 (parenchyma) of large thin- walled cells, separated by air-spaces at the 

 corners ; (4) the vascular bundles, each with a sheath of scleren- 

 chyma. 



In a single bundle, notice the four conspicuous vessels arranged like 

 a V, thus : Q Q 



8 



with narrower vessels lying between ; the patch of clear-looking tissue 



1 The Marrow, Cucumber, and some other plants are exceptional 

 in having bast on the inner side of the wood as well as on the outer 

 side. In most stems the bast occurs only on the outer side of the 

 wood. The bast-tubes are called sieve-tubes, because they are inter- 

 rupted by cross-walls which are perforated like a sieve (sieve-plates). 



