40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I40 



under any magnification as to the abrupt termination of the dense- 

 wood or to the sudden start of the liglitwood. The characteristics of 

 each are possessed to a superlative degree. It is this type of contact 

 which has been emphasized by some as especially characteristic of an 

 annual increment and, in contrast, as not existing among intra-an- 

 nuals. In the case of the sharp contact, no doubt exists that growth 

 ceased entirely at least for a time, and that the physiologic conditions 

 responsible for the densewood were in marked contrast to those re- 

 sponsible for the following lightwood. Also, the cambium presumably 

 came to rest. A sharp contact, it must be noted, depends as much, if 

 not more, upon the superlative development of the following light- 

 wood as it does upon the accentuation of the preceding densewood. 

 This is less true with stained sections and high powers of magnifica- 

 tion. 



The term definite is applied to a contact if the outer margin is not 

 quite so smooth, not quite so decided as that identified as sharp (pi. 4, 

 fig. 2 : 1938). Under high power, it may show slight irregularity. Ob- 

 viously, however, cambial division ceased for a time. Definite is dis- 

 tinguished from sharp, as a matter of fact, not only for purposes of 

 discussion but also and more particularly for purposes of understand- 

 ing the nature of the processes responsible for the various types of 

 growth-layer contacts. Such discussion will be held over until the 

 types have been defined. In assembling and synthesizing the results of 

 our microscopic analyses, we have classified all definite contacts with 

 the sharp. 



The term indefinite refers to undoubted irregularity at the contact or 

 a certain amount of transition from the densewood to the following 

 lightwood (pi. I, fig. 2 ; pi. 2, fig. 2 : 1939). Under lowest powers of 

 magnification, there is a strong inclination to assume that growth ac- 

 tivities merely had declined for a time but had not ceased. High power 

 is necessary to distinguish a contact indefinite because of irregularity 

 or because of transitional features. In the one case growth activities 

 may have ceased entirely ; in the other, they probably did not. 



The term dijjuse refers to a contact which lacks any abruptness or 

 contrast between densewood and following lightwood (pi. 2, fig. 2 : 

 1940). Under lowest powers of magnification the transition appears 

 to be gradual and complete. Growth processes responsible for dense- 

 wood were at first accelerated and later declined, whereas cambial ac- 

 tivity probably was continuous but at a decreasing and then increasing 

 rate as the densewood was formed. Under high power the true nature 

 of contacts becomes more clearly defined. Indefinite contacts have 

 been united with the diffuse for purposes of classification in this 

 report. 



