144 STEMS 



an endless struggle for light. In that struggle it is the 

 stems which play the principal part, and those forms of 

 stems which exist to-day are those which have been suc- 

 cessful in that struggle. Old competitors of theirs which 

 were not successful have disappeared. 



In Chapter I you learned of stems as helping organs. 

 You learned that they place flowers and fruit as well as 

 leaves in advantageous positions. You learned .that 

 through them run paths of movement from rooft to leaves 

 and from leaves to all other living parts of the plant. 

 You learned, in Chapter II, that the root-to-leaf move- 

 ment is through the xylem or wood and the leaf-to-root 

 movement largely through the phloem or bast. 



You have learned something of the structure of stems; 

 you know of the nodes and internodes. You know that 

 green stems make food just as leaves make it, and that 

 in some plants all the food-making is done by stems. You 

 have learned something of the tissues of stems. You 

 learned that their arrangement is quite different from the 

 arrangement of the tissues of the roots. While the tissues 

 are arranged in practically the same way in all roots, there 

 are two quite distinct types of tissue arrangement in stems, 

 and neither of these is like the tissue arrangement of roots. 

 In one of the stem arrangements the vascular bundles 

 form a cylinder which in cross section appears as a ring 

 (see Figure 48) ; in the other arrangement the vascular 

 bundles are scattered (see Figure 49). 



All stems may be divided into those which are above- 

 ground and those which are underground. Since the 

 stems which are above the ground are more familiar, 

 we shall consider them first. We may call them aerial 

 stems. 



