NO, I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 



6l 



having major intervals of activity, whether one or several annually, 

 then local activity immediately preceding the major interval produces 

 an interior lens, and local activity immediately succeeding the major 

 interval produces an exterior lens. Examples of interior lenses are 

 shown in text figures 3 and 4 and in plate 20; examples of exterior 

 lenses are shown in figure 4 and in plates 8 and 13. 



Fig. 2. — XSC 8-4-a. Multiplicity and partial gls * by symbol and cell struc- 

 ture. 1938: Effects of natural frost; 2 s arcs becoming diffuse and disappearing 

 into lightwood ; see ; sL ; 1939 : see ; 2 sL, outer dw of inner sL is heavy, 

 outer sL made up of 2 rows of cells, i of Iw and i of dw; 1940: i s arc 

 becoming diffuse and disappearing; d 2L disappearing; 2 see. 



* Abbreviations explained on p. loi. 



A compound lens consists of two or more lenses in contact with 

 each other and in sequence radially (text figs, i, 2, 4 ; pis. 8, 13) ; they 

 represent two or more successive intervals of cambial activity. In 

 some cases the inner lens possesses the greater area, in others the 

 outer, dependent upon a decrease or an increase in area of cambial 

 activity. The rate of cambial activity may vary at one locality on the 

 circumference, thus giving a series of lenses which represent an alter- 

 nation of rapid cell division and little or none, during the general 

 growing season. This is well illustrated by a highly eccentric growth 

 layer whose bulge consists of a radial series of lenses (pis. 8; 13; 

 22, fig. i; 29). 



