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SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



the densewood feathers out into, and the lightwood merges into, the 

 lightwood of the general growth layer. This appearance on cross sec- 

 tion applies equally well to that in area over the whole tree ; that is, 

 part of the densewood termination lifts away from contact with other 

 densewood and "floats" free in lightwood. 



Half -lenses are simple or compound depending upon the presence 

 of one, or more than one, in a single system at one locality. Text 

 figure 9 shows three simple and one compound half -lenses. On a dif- 



FiG. 9. — Various types of half-lenses. In order from top down: interior half- 

 lens, exterior half-lens, thick interior half-lens, compound exterior half-lens. 



ferent basis, half-lenses are either interior or exterior. In the interior 

 variety, the cusp terminates against the densewood of the preceding 

 growth layer (text fig. 9) ; in the exterior, the cusp terminates 

 against the densewood of the succeeding growth layer. Text figure 

 10 shows not only an exterior half-lens but also an exterior lens. 



Two phases of the subject merit somewhat more extended dis- 

 cussion. They are the identity of the half-lens and the nature of cam- 

 bial activity. In the structural picture the half -lens on cross section 



