STEM AND BRANCHES. 9 



of other plants, their roots penetrating the bark and sucking up the juices 

 already elaborated for their needs. To this class belong the beech-drops 

 (Epiphegus Virginiana Barton), parasitic upon the roots of the beech, and 

 American mistletoe (Phoradendron flavescens Nuttall), a woody parasite 

 upon the branches of forest trees, common in the Southern States. 



Still other plants are mainly nourished by roots which hang in the air. 

 These, called air-plants, are almost exclusively inhabitants of warm, moist 

 regions. 



Another important office of some roots is to serve as storehouses of 

 nourishment for the future needs of the plant. The great mass of plants 

 are annuals, living but a single season, during which they germinate from 

 the seed, attain their full development, flower, produce fruit, and die. 

 These have no need for a reserve store of nourishment, hence their roots 

 are fibrous and not thickened. 



But many plants, termed biennials, germinate from the seed in 

 spring, produce a cluster of radical leaves, and develop a very large tap- 

 root during the first season. The next spring, drawing upon the store of 

 nourishment laid up in the root, they send up vigorous flower-stems, pro- 

 duce seed, and die. Many such roots, as the beet, carrot, and turnip, are 

 of great importance as articles of food. 



Plants which endure for several years, termed perennials, not un- 

 frequently have roots of the same character. These thickened roots in 

 many instances contain the active medicinal principles of the plants. 



THE STEM AND BRANCHES. 



We have seen that the stem is the ascending axis of a plant ; that it 

 grows upward toward the light at the same time that the root is develop- 

 ing in an opposite direction. We have now to consider more particularly 

 its mode of growth and some of its more common forms. 



In the case of the bean it was observed that after the seed-leaves came 

 a pair of green leaves (Fig. 5) ; after these another pair, and so on. The 

 points where these leaves appear are termed nodes or joints, and the 

 spaces between them inter nodes. Now, during the earlier stages of 

 growth the internodes increase both longitudinally and in diameter also 

 by cell-proliferation, so that though two nodes of a growing shoot may, 

 when their leaves first unfold, be quite close together, in the end we find 

 them separated by an interval of perhaps several inches. In this particular, 

 as stated above, the growth of the stem differs greatly from that of the 

 root. 



Again, it was noted that in the axils of the leaves were buds which nor- 

 mally developed into branches subject to the same laws of growth as the 

 main stem. We have now only to suppose that these axillary buds keep 

 pace with the development of the main stem, and every opposite-leaved 



