56 GROWTH AND WORK OF PLANTS 



occupy the end of the shoot. This forms an axillary terminal 

 bud. 



91. Annual growth of stems. Annual growth of most stems 

 takes place in two directions, in length and in thickness. 



92. Growth in length. In the case of stems with definite 

 growth there are external marks on the shoots which indicate 

 each year's growth for several years until the bark becomes so old 

 as to obscure the marks. These marks are formed by the scars 

 of the bud scales when they fall off at the time of the opening of 

 the -buds in the spring. These are known as scale scars or ring 

 scars, because they form closely crowded rings on the shoot. 

 Stems with indefinite growth usually do not show these annual 

 scale scars, though they are faintly shown in some stems with 

 indefinite growth which have axillary terminal buds, as in the 

 elm. In pine trees the annual growth in length is easily shown 

 for many years since one whorl of branches is formed each year 

 from a whorl of buds just below the terminal bud.* 



93. Growth in thickness. Growth in thickness of most 

 shrubs and stems is marked by "annual rings" seen in a cross 

 section of the stem, a new ring of tissue being added each year. 

 These rings are made distinct by the variation in the compactness 

 or porosity of the wood formed each season, those vessels (or 

 "pores") in the wood which are formed in spring being larger 

 than those formed in the summer. Thus the wood formed in the 

 spring is less compact than that formed in the summer. The 

 age of trees or their branches can be determined by counting the 

 number of these annual rings (see paragraph 106). 



94. Nodes and internodes of the stem. The point where 

 each leaf is borne is called a node. The space between two suc- 

 cessive nodes is called an internode. In some plants, especially 

 the grasses, corn, wheat, etc., the nodes are very distinct since 

 they coincide with the "joints." 



* These buds are not in a true whorl since they arise in the axils of scale 

 leaves arranged in a spiral on the stem, but the scale leaves are very numer- 

 ous and crowded and so the buds appear to be in a whorl. 



