ECONOMIC WOODS OF THE UNITED STATES 41 



early wood to the face of the more compact late wood that serves 

 to define the zones of growth. 



The origin of growth rings is physiological. Plants, like 

 animals, seem incapable of indefinitely sustained activity, but 

 require periods of recuperation. In latitudes of decided seasonal 

 changes such periods of rest are provided by the alternation of the 

 seasons, in which case the zones of growth correspond very closely 

 with annual periods. This constancy of relation diminishes 

 towards the equator and, although in the tropics growth rings are 

 not uncommon, they provide no reliable index to the age of the 

 tree. In temperate climates trees occasionally produce secondarij or 

 false rings, usually attributable to some disturbance of the normal 

 course of growth of the season, such as the action of frost, drought, 

 hail, and insect damages. Such rings, however, can usually be 

 distinguished from annual rings by their less pronounced line of 

 demarcation. 



Variation in width of different growth rings is common to all 

 trees, and is determined by external conditions of light, heat, 

 moisture, and available food-supply. The cross section of a stem 

 presents in the variable form and size of its rings a history of its 

 growth and nutrition. 



The breadth of an individual growth ring may not be uniform 

 all round in consequence of unequal acceleration of the growth 

 on different sides, the ring thus becoming undulating or eccentric. 

 The growth centre is accordingly not coincident with the geometric 

 centre. The more nearly erect the stem and the more nearly per- 

 fect the crown, the more closely will the two centres coincide. In 

 some species (e.g., Carpinus caroliniana and Juniperus virginiana) , 

 irregularity of growth causes the trunks to become fluted or even 

 deeply scalloped. 



The growth rings near the centre of a stem usually exhibit 

 considerable difference in structure from those later formed. The 

 elements are usually thinner-walled, of shorter length, and less 

 densely aggregated, so that the inner core of wood is comparatively 

 soft and weak. In the wood of Dicotyledons, although the elements 

 characteristic of the species are all present, their characteristic ar- 

 rangement does not appear clearly until later. This is particularly 

 evident in the distribution of the vessels and wood parenchyma in 

 many woods. Consequently, in the employment of these features 

 for systematic purposes, it is important to use stems of consid- 

 erable thickness rather than small branches or young shoots. 



