TYPES AND KINDS OF STEMS 49 



GROWTH OF STEMS. 



84. Definite and indefinite growth. In woody stems, shrubs 

 and trees, there are two types of growth in length of the new shoots 

 each year, definite growth, or determinate growth, and indefinite 

 growth, or indeterminate growth. In the larger number of trees 

 and shrubs of the North Temperate region the growth is definite. 

 It is usually completed by the middle of July. A terminal bud 

 is formed from which the following season the shoot continues 

 its growth. In some of these buds all the leaves of this shoot for 

 the coming season are already formed in miniature in the bud and 

 are covered and protected by the outer dull colored scales. Dur- 

 ing the growth of the shoot the next season these leaves mature 

 and unfold as the shoot elongates, and then a new terminal bud is 

 formed. In other cases not only do the young leaves already 

 formed in the bud expand, but new leaves are formed as the shoot 

 elongates. In indefinite growth, however, growth in length of the 

 shoot continues until late in the summer or autumn, and the ter- 

 minal bud as well as the terminal portion of the twig dies. One 

 of the lateral buds then acts as the terminal bud to continue the 

 growth the following season. In the spice bush a considerable 

 portion of the dead terminal shoot remains and the following year 

 the new growth comes from the living lateral buds some distance 

 back from the tip. In the elm the terminal portion of the shoot 

 which dies is small, falls away, and the latest lateral bud to be 

 formed appears to occupy the end of the shoot. This forms an 

 axillary terminal bud. 



85. Annual growth of stems. Annual growth of most stems 

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



86. 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 



