50 GROWTH AND WORK OF PLANTS 



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.* 



87. 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 being larger which are formed in spring 

 and early summer, while they are smaller and the wood more 

 compact which is formed later in the summer. The age of trees 

 or their branches can be determined by counting the number of 

 these annual rings. 



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

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

 sive nodes is called an internode. In some plants, especially the 

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

 coincide with the " joint." 



* 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. 



