STUDIES OF SEED-PLANTS 167 



with the main stem, e.g., compare Lombardy poplars with 

 ordinary trees of our forests and parks. 



160. Elongation of Stems. Careful studies have shown 

 that elongation is most rapid just below the growing tip of 

 stems and their branches, but that elongation occurs else- 

 where to some extent so long as hard woody tissue has not 

 developed in the internodes of the stem. For example, 

 the distance between the first true leaves and the cotyledons 

 increases for some time while the tissues in this internode 

 are soft. If one measures the distance between marked 

 branches on a young fruit-tree in early spring and again in 

 autumn, it will be found that the older branches have not 

 grown farther apart or higher from the ground; but the* 

 branches which appear during the growing season grow 

 farther apart during some weeks while the tissues are soft. 

 This is a matter of practical importance for one who culti- 

 vates trees. In pruning ( 166) branches must be left at 

 the height where it is desired to have them when the tree is 

 fully grown. Also by cutting the ends (terminal buds) of 

 the main stem and of its branches it is possible to control 

 the height of the tree, and thus keep fruit-bearing branches 

 near the ground for convenience in collecting fruit. 



161. Stem of Monocotyledons. The bean plant has 

 the type of stem found in all dicotyledons and cone-bearing 

 plants. The stem of monocotyledons is different in that 

 its fibro-vascular bundles (bundles of fibers and wood-tubes) 

 are arranged irregularly in the pith (Fig. 44), and not in 

 rings as in stems of dicotyledons (Fig. 48). Moreover, the 

 monocotyledon stem does not increase in diameter each year 

 by addition of layers to the outside of the wood, but instead 

 grows to the full diameter during its primary growth. Thus 

 the trunks of palms and cocoanut trees are nearly the same 

 diameter almost to the leafy top ; and although the trees 

 continue to grow for many years, there is little increase in 

 diameter except near the growing top. Such increase as 



