38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



having to do with cell division for a period of time? Extensive obser- 

 vation answers this last question in the affirmative, but at the same 

 time indicates that the cambium is restive, in a manner of speaking, to 

 continue or reinitiate cell division. Witness the many cases of post- 

 seasonal growth. 



REACTION OR COMPRESSION WOOD 



Compression wood occurs as a common feature in branches of coni- 

 fers, and especially characterizes Arizona cypress. Although it no 

 doubt exists as a rule along the underside of branches and on the long 

 radius, it is by no means restricted to those regions. It exists as 

 patches, as successive waves, and as crescents anywhere on a section 

 or within a growth layer: on the long, the short, or the quarter 

 radius ; in the lightwood or densewood ; at the inner or outer margin 

 of a growth layer ; in successive growth layers or isolated growth 

 layers ; or concentrated on one radius throughout a section or on 

 different radii for each growth layer of a section. Compression wood 

 may extend from the start of a growth layer outward to the outer two 

 or three rows of cells which are densewood normal in all respects. 

 Where it lies along the inner margin of a growth layer and subsides 

 outward into normal lightwood, it may give the appearance of re- 

 versed sequence. 



The greatest difficulty with compression wood occurs where it lies 

 immediately adjacent to, or bridges, the contact between two growth 

 layers. In most instances, the effect of such wood was eliminated by 

 extra thin sections, by "discounting" its presence, and by the use of 

 high power. In a few cases, compression wood masked critical fea- 

 tures to such an extent that the sections could not be used. 



The terms "compression wood" or "reaction wood" can perhaps 

 create confusion because they imply an interpretation. A descriptive 

 term would of course avoid such implications. "Brown wood" is de- 

 scriptive but could be confused with the ordinary densewood of many 

 conifers. In order to avoid confusion, the German word "Rotholz" 

 might be adopted in English in its original spelling, thus making it a 

 technical term. 



CONTACTS BETWEEN GROWTH LAYERS 

 TYPES OF CONTACTS 



In 1937, Clock wrote : 



The annual ring as it has been used in tree-ring work among the conifers 

 includes the secondary wood formed by the cambium from the time growth starts 

 in the spring until it ceases because of the approach of winter or because of the 



