NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 65 



An overlapping lens ^ occuj)ies in some respects an intermediate po- 

 sition between the two foregoing varieties. If the lens is compound 

 but the two units are not centered on the same radius, an overlapping 

 results as shown in text figures i, 4, and 8, and plate 22, figure i. 

 Such lenses are not too common in cross section. However, if the 

 whole plant body is considered, it may be doubtful that a true com- 

 pound lens ever exists ; somewhere over its area it will no doubt be 

 overlapping. 



Fig. 8. — XSC 1-2-a. Increment for 1938. Outside the frost circle there 

 appears: psce; long sL (nearly see) ; thick sL, compound with former sL, and 

 containing overlapping ^L's and compound ^L's. 



The last variety of lens to be discussed here is called, for want of a 

 better name, a low-power lens. Under low power, and especially under 

 a wide-field binocular or a hand lens, only a single lens or a concur- 

 rent lens is visible. Under high power, on the contrary, it is seen that 

 only the lightwood terminates at the cusps, the densewood continuing 

 around the circuit as a layer one to several cells thick flush against the 

 densewood of the growth layer next interior to, and indistinguishable 

 from, it (pi. 7, fig. 2). In TTP 21-6-a the increments for 1939 and 



5 At first this was called transgressive because a band of densewood appeared to 

 transgress the lightwood separating two major bands of densewood. 



