THE GROWTH OF ROOTS AND STEMS. 163 



(g) Make two cuts round the lower part of a Willow twig, about an 

 inch apart, and remove the soft outer tissue of the stem between 

 these cuts, so as to leave only the hard woody portion of the stem 

 for this distance. Then set the twig in water (which should be 

 changed every day) or in culture-solution, and notice that it begins 

 to sprout after a few days (Fig. 54). Below the injury the develop- 

 ment of buds and new roots takes place but slowly, whereas above 

 it new roots are rapidly formed and nourished by food conveyed from 

 the upper parts of the branch by the tissue lying on the outer side of 

 the wood. This experiment usually succeeds best in spring or early 

 summer. Later in the year it is advisable to remove the leaves in 

 order to diminish the loss of water, since there are no roots on the 

 cutting to keep up the supply of water. The rapid development of 

 buds and the formation of roots above the ringed part show that food 

 passes down chiefly through the soft outer region of the stem. 



194. Chief Functions of the Stem. The leaves must 

 be exposed to air and light in order to carry on their work of 

 food-making, and in this work they require supplies of water 

 containing dissolved salts. The food made by the leaves has 

 to be carried to other parts of the plant which are living and 

 growing or acting as storage- organs. The roots, having no 

 chlorophyll and not being exposed to light, cannot, of course, 

 carry on photosynthesis, and must therefore be supplied with 

 organic food made in the leaves. 



From these considerations, and from the results of our 

 simple experiments, it is clear that the ordinary functions of 

 a stem are (1) to bear the leaves and help in spreading them 

 out to light and air; (2) to convey water with dissolved 

 salts from the roots to leaves and other parts of the 

 shoot ; (3) to carry organic food from the leaves to other 

 parts. These are the primary functions of the ordinary 

 stem, but in addition to this stems often have to take on 

 special functions. Thus they may serve as organs of vegeta- 

 tive propagation, as store-places of nourishment, etc. 



195. Structure of the Stem. In Dicotyledons, both in 

 herbaceous stems and in woody stems when young, isolated 

 bundles run through the stem, forming a hollow cylinder (a 

 ring, as seen in cross section). In Monocotyledons the 

 bundles are scattered through the ground-tissue of the 

 stem. This is because in the former case the bundles (veins) 

 which run into the stem from the leaves remain near the 



