STEMS AND THEIR FUNCTIONS 



153 



. terminal bud 



JS. axillary bud 



leaf scar 



^ inte 



mode 



begin to swell with the rapid growth of the enclosed structures. The bud 



scales then open, and the young shoot pushes 



out. It elongates rapidly, chiefly by lengthening of 



the internodes, or intervals between the regions 



where leaves and axillary buds develop, which are 



called the nodes. Soon the shoot appears as a 



replica of last year's twig; the latter no longer 



bears leaves and has become a part of the branch. 



The bud scales at the base of the new twig drop 



off after a time, leaving a group of bud-scale 



scars. The space between two such groups of scars 



marks the amount of growth accomplished in one 



season, and each section of the branch between 



two groups of bud-scale scars was originally an 



embryonic shoot inside a bud. 



The structure of a bud. From the facts de- 

 scribed above, it is evident that each bud contains 

 a growing point. In fact, the bud, or stem tip, 

 resembles the root tip in fundamental respects. 

 Its growing point is apical, instead of being covered 

 by a cap of cells, as in the root ; but in both stem 

 and root the meristematic zone is followed first by 

 a region of elongating cells and then by a region of 

 cell differentiation in which the tissue groups are 

 formed. On the other hand, roots possess nothing 

 corresponding to the nodes and internodes of 

 stems, nor does the internal structure of roots and 

 stems correspond in all details. 



The structure of the young woody stem. There 

 are three principal types of stem structure among 

 the spermatophy tes : (1) the woody stem of the 

 dicotyledonous trees and shrubs (to which the stem 

 of pines and their relatives is essentially similar), 

 (2) the succulent stem of the herbaceous dicotyledons, 

 and (3) the very differently organized stem of the 

 monocotyledons (the palms, grasses, lilies, etc.). The 

 following account applies especially to the first of 

 these, the woody dicotyledonous-gymnosperm 

 type; the features of the other two can be men- 

 tioned only briefly. 



A cross section of a one-year-old branch (stem) of such a tree as an 

 oak shows the following arrangement of tissues. The surface is covered 

 with a layer of epidermis, one cell thick. The outer surfaces of the epi- 



leaf 



■ lenticels 



bundle scar 



scars of former 

 ~? terminal bud 



Fig. 11.1. A woody twig 

 (horse chestnut) in winter 

 condition, to show the 

 principal external fea- 

 tures of the stem. (Re- 

 drawn from Sinnott, Bot- 

 any : Principles and 

 Problems.) 



