NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. 253 



INTERRELATIONSHIPS 



CAUSES 



The causes, whether physiologic, ecologic, or specific, underlying the 

 formation of the different types of growth layers and the intensity of 

 the "factor of multiplicity" are of consuming interest because a knowl- 

 edge of them will permit interpretations. Here will be mentioned only 

 such facts and suggestions as are immediately pertinent to the work. 



Climatic. — Those topics which bear upon the climatic causes may be 

 discussed under growth slowdown, frost-induced growth layers, post- 

 seasonal growth, and general rainfall. 



Growth slowdown here refers to a transitional stage between con- 

 tinuous cambial activity and its complete cessation within a single 

 growing season. The evidence for such slowdown and notes on 

 weather conditions have been somewhat fully discussed on pages i86- 



199- 



Among the Washington trees there was distinct evidence that low 



temperature during the rapid growth of spring distinctly retarded 

 elongation. A further bit of evidence may be added. The tree WCh 

 2 has been described heretofore. Tree WCh 3 grew within 50 feet of 

 WCh 2. Branch WCh 3-2 was measured during the first half of 1945. 

 From April 25 to May 13, scarcely any tip growth occurred. The 

 weather was cool to cold with some heavy frost. During the cold spell 

 the plum trees grew least, the cherries the most, and the peach an 

 intermediate amount. 



Interrupted lightwood and interrupted densewood give evidence of 

 growth slowdown which is in contrast with the acceleration in growth 

 processes indicated by divided lightwood and divided densewood. 



During the course of fieldwork, it became apparent that low temper- 

 ature, drought intervals as reflected in low soil moisture, and insect at- 

 tacks might separately or in combination be responsible for decreased 

 cambial and other growth activities. The cherry tree, WCh 3, gave 

 evidence, as above described, of the repressive effect of abnormally 

 low temperature after growth had started in the spring. In many in- 

 stances, as in TTP 20, the reason for decreased growth is not di- 

 rectly known. 



Growth slowdown, in summary, ranges all the way from an almost 

 undetectable decrease in activity to complete cessation, and the evi- 

 dence therefore is both external and internal on a branch. 



Further influence of low temperature, as well as the effects of low 

 rainfall and of insect attacks, will be discussed later. 



Frost-induced growth layers, by the intimate connection of dense- 



