162 



THE GROWTH OF ROOTS AND STEMS. 



inner portion of each bundle when the bundles are separate 

 and through the hard woody part of a stem when the bundles 

 are joined to form a continuous cylinder. 



Since only the green parts of a plant (in most cases, the 

 foliage-leaves) can manufacture organic food for themselves, 

 they must supply with some of this food all the other parts, 

 which cannot carry on photosynthesis. Hence there must be 

 some channels for conveying the leaf -made food to the roots, 

 buds, and other parts which are either growing or storing 

 up food as a reserve for future growth. Through what 

 channels do the organic substances produced in the leaves 

 travel to other parts of the plant ? If these also travel along 

 the bundles, they must pass through the soft outer tissue in 

 the case of a woody stem ; and in that of a stem, with 

 separate bundles, these substances must 

 pass either through the part of each 

 bundle which was not stained by the red 

 ink or else through the other tissue of the 

 stem, or by both routes. 



Before considering this question, we 

 must know more about the structure of 

 the stem ; but the following experiments 

 will give a clue as to the paths by which 

 the leaf -manufactured food travels. 



(/) Place a Bean seedling or a Garden Nas- 

 turtium (try various other plants) in darkness for 

 a few days, then remove some leaves and test 

 them for sugar by boiling them in Fehling's 

 solution. There will be little or no red colora- 

 tion, showing that sugar is absent, or nearly so. 

 Expose the plant to sunlight for several hours, 

 then place it in darkness (after having tested 

 some leaves, or parts of leaves, for starch), and 

 after a time test some leaves for sugar, which 

 will be indicated by the red colour produced 

 A iviugcu. around the veins. If the presence of sugar is 

 Branch of a Willow detected in this way in the leaf-blade, test 

 sprouting in Water. sections of the leaf -stalk at different levels, 

 to find out by what paths the sugar travels 

 towards the stem. Also test sections of 



the stem itself. By using Fehling's test, we find that the sugar travels 

 in the leaf itself along the tissue surrounding the veins (bundles), and 

 in the stem it is to be found in the ground tissue in which the bundles 

 are embedded. 



54. A 



