NO. I GROWTH LAYERS IN TREE BRANCHES — CLOCK ET AL. I23 



It should be emphasized that two to four cross sections only are 

 taken roughly equal distances apart out of a 50-cm. length of branch. 

 Perhaps this number could be considered too few to serve as random 

 samples, yet partial growth layers do have a longitudinal dimension 

 thus permitting each cross section to represent more of the branch 

 length than just its own thickness. This probability of two to four 

 sections revealing all cases of multiple growth layers apphes naturally 

 to those cases where multiplicity is caused by partial growth layers 

 only. 



In view of the mass of evidence accumulated in our cross sections 

 and in view of the nature of growth conditions in the lower forest- 

 border regions, it may well be that few, very few, annual increments 

 throughout their extent over roots, trunks, and branches possess only 

 one growth layer. Growth factors first, and species second, seem to be 

 of importance in determining the incidence of multiple growth layers 

 in annual increments. 



It is of interest to note that 1,036 out of a total of 1,245 sectioned 

 blocks taken from branches and trunks were dated accurately. The 

 remainder were obtained almost wholly from New Mexico on field 

 trips so brief that the growth layers could not be dated by our 

 methods. 



STATEMENT OF THE PROBLEM 



Here we are dealing with normal growth layers in normal trees; 

 that is to say, those not subjected to the sudden, or prolonged, impact 

 of such accidental factors as fire, wind, disease, defoliation, or insect 

 attacks. 



Discussion has cropped up intermittently through the years on the 

 unity or multiplicity of growth layers in annual increments. In many 

 cases, certainly, both aspects of the problem are correct, the response 

 of the trees being dependent upon the particular set of growth factors 

 present. That a tree forms one and only one sharply bounded growth 

 layer a year, or that a tree can form more than one a year — either of 

 these is an assumption unless supported by adequate evidence. 



Certain implications follow from the idea of rigid unity. First, it 

 implies that growth begins in the spring and goes to completion, a 

 completion signalized by a sharply defined outer surface everywhere 

 throughout the extent of the growth layer. Second, it implies that 

 growth, especially cambial activity, cannot be reinitiated during the 

 same general growing season after growth has once completely 

 ceased. This must be true no matter how brief or how prolonged the 

 original growth flush might be at the beginning of the season. Third, 



