690 



ECOLOGY 



greatest in spring, because the ascent of sap is more active then than 

 later. The new cells take part in this movement, and from the ascend- 

 ing materials derive the substances used in their development. Here, 

 then, as in primary hadrome, maximum growth is correlated with a 

 large movement of materials. 



Variations in the width of annual rings. — The width of the annual 

 ring is subject to considerable variation, which is dependent in part 

 upon the age of the tree and in part upon seasonal conditions. In 



early life there is a 

 / _ period of acceleration, 



^i^^^, ^ during which the width 



of the rings usually in- 

 creases from year to year, 

 probably because of the 

 increased absorptionand 

 food supply which are 

 made possible by a more 

 extensive root system 

 and by a greater ex- 

 panse of foliage. Even 

 if the rings are of equal 

 width year by year, 

 there is an increasing 

 increment of tissue, 

 owing to the enlarging 

 circumference. After 

 some years, which may 

 be few or many accord- 

 ing to the species, there comes a period of maturity, characterized by 

 approximate constancy in the annual increment. Finally, there is 

 a period of retardation, which is marked by an actual decrease in the 

 amount of tissue laid down year by year, the annual increment 

 approaching zero in extreme old age. While new roots and branches 

 develop yearly throughout life, the loss of old branches by death 

 finally e.xceeds the gain, and it is possible also that in an aging tree a 

 given amount of leaf or root surface becomes less effective. In long- 

 lived trees (such as the oak, chestnut, or yew) the final period may not 

 begin for 150 or 200 years, and may continue some hundreds of years 

 thereafter. 



Fig. 1019. — A cross section through an annual ring 

 in the secondary wood of the Austrian pine {Pmus 

 Laricio), showing the large-calibered vessels of the spring 

 wood (5) and the small-calibered vessels of the preceding 

 autumn wood (ii); note the relatively sharp line (/) be- 

 tween the autumn wood and spring wood; h, bordered 

 pits in cross section ; highly magnified. 



