THE STRUCTURE AND WORK OF THE STEM 101 



Excurrent Tree. — If, on the other hand, the terminal buds of the tree 

 get a better supply of light, food, or if other factors aid its growth, the 

 tree will be tall and ha\c but one main trunk, such as the Lombardy 

 poplar, and pines and cedars. Such a tree is named crcurrcnl. The 

 picture shows trees of these two shapes. 



Problem XVIL The sti'iu'tinr 

 and ivork of stems. {Lahorafdry 

 Manual, Prob. .YJ7I.) 



(a) External structure of a dh 

 cotyledonous stem, {optional). 



(b) Internal structure of a di- 

 cotyledonous stem. 



(c) Circulation in stems, 

 id) Condition of food passing 



through tlie stem. 



The External Structure of a Dicotyle- 

 donous Stem. — A horse-chestnut twig in 

 its winter condition shows the structure 

 and position of the buds very plainly. 

 As the twig grew last year the scales 

 which covered the outside of the terminal 

 bud dropped off, and the young shoot 

 developed from the opened bud. The 

 scales which dropped off left marks 

 forming a little ring upon the surface of 

 the twig. These rings, collectively named 

 the hud scars, enable one to tell the age 

 of the branch. 



Just above the lateral buds are marks, 

 known as leaf traces, that show the points 

 at which leaves were attached. A care- 

 ful inspection of the leaf traces reveals 

 certain tiny dotlike scars arranged more 

 or less in the form of a horseshoe. These 

 sears mark the former position of bundles 

 of tubes which we have already studied 



in connection with roots. They are, in fact, continuations of the same 

 fibrovascular bundles which pass from the root up through the stem and 

 out into the leaves, where we see them as the veins which act as the 

 support of the soft green tissues of the leaf. The most ivip<irtiiril use to 

 the plant of the fibrovascular bundles is the conduction of fluids from the 



Three-year-old apple branch, 

 showing terminal and lateral 

 buds and bud scars. 



