6o 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



At 100 percent, of course, a lens becomes a complete, entire sheath. 

 On cross sections, lenses vary in length from o° (pi. 8) up to 360° 

 (pi. 14, fig. 2) around the circuit. Many lenses occur on the long radii 

 of branches and constitute the reason for the longer radius, but they 

 are not necessarily confined to that radius. If the long radius is due 

 to local thickening of the entire growth layers, lenses may be the 

 means of reestablishing concentricity of the woody cylinder. 



Fig. I. — Various types of lenses. In order from top to bottom: Simple lens, 

 compound lens with outer one the longer, compound lens with inner one 

 the longer, concurrent lens, overlapping lens. 



The term "lens" as here used refers to the same feature previously 

 designated as locally absent (Douglass, 1935, p. 68) or locally present 

 (Clock, 1937, pp. 8-9). These last two have been abandoned because 

 they are not sufficiently descriptive, not sufficiently flexible, and can- 

 not be applied to transitional forms. 



True lenses occur in several varieties. The simple lens (text figs, i, 

 2; pi. 8) is single and corresponds in cross section to the ideal 

 concavo-convex form previously described. It may be either interior 

 or exterior in its relationships with the complete, entire growth layer 

 to which it seems most closely related. If we think of the cambium as 



