[Chap. XXVI FORMS AND EXTERNAL FEATURES OF STEMS 247 



branches usually occur only at certain places on the stem called nodes. 

 The part of the stem segment between two nodes is the inter node. The 

 length of a twig or branch and the height of the plant depend upon the 

 amount of elongation of the internodes. 



We noted in an earlier chapter that deciduous trees lose their leaves 

 annually and that the scar on the stem left by the abscised leaf is called 

 the leaf scar. Within the leaf scar are smaller scars. These small scars 

 are the severed ends of vascular bundles that extended from the leaf 

 into the stem, and are known as bundle scars or vein scars. The age of 

 the twig may often be determined by counting the ring-like markings, 

 the terminal bud scale scars, found at intervals along the stem. These 

 rings may be used as a means of locating the position of a dormant 

 terminal bud of some previous year. The distance between two succes- 

 sive bud scale scars is a measure of the growth in length of the stem 

 during one year. This length may be used to compare the annual growth 

 of stems of different plants, as well as the variations in growth from 

 year to year on the same plant. 



Scattered over the surface of the internodes are numerous small, 

 roundish or elongated structures, the lenticels. When young, these are 

 made up of loosely arranged cells through which gases pass into or out 

 of the bark of the twig. Stomates are often present in very young stems, 

 and the formation of a lenticel may be initiated by the division of cells 

 immediately beneath a stomate. Later cork develops beneath the 

 lenticel. Owing to the peculiar growth of the bark of such trees as 

 birch and cherry, the lenticels become greatly elongated and partly 

 encircle the stem. 



Conspicuous external features of stems during dormant periods are 

 the buds. There may be only one bud at a node, as on most stems with 

 alternate leaves. Where the leaves are opposite there are two buds at 

 each node. On the stems of other species of plants there may be several 

 buds at each node either arranged in a whorl about the node or grouped 

 about the primary axillary bud. In species of soft maple and forsythia 

 these accessory buds are usually flower buds. The bud at the apical end 

 of the stem is the terminal bud; all others are lateral buds. The terminal 

 buds of some plants, such as willow, elm, hackberry, linden, and tree of 

 heaven, die and drop off in the spring of the year before the elongation 

 of the internodes of the new stem segment is completed. The last lateral 

 bud formed on twigs of these plants appears to be terminal in position, 

 if only a casual observation is made. 



