2o6 



BOTANICAL GAZETTE 



[march 



doubt these two processes overlap. For the sake of clearness in 

 the subsequent discussion, however, growth in length will be con- 

 sidered in the general sense of primary growth, growth in thickness 

 as secondary growth. 



Awakening of secondary growth in aerial parts' 



The awakening of secondary growth in aerial parts is first 

 manifested in the cambium and the adjacent phloem tissue. It 

 is evident in all cases several weeks before actual cell division begins. 

 The cambial cells and last formed sieve tubes (6-10) open out 

 radially, so that they attain several times their former diameter. 

 Reference to the following table will show the changes which 

 occurred between February 22, 1913 and March 29, 1913, in the 

 width of cambium and last formed phloem. 



TABLE B 

 Growth without cell division, tree i 



In all cases, enlargement of the tissues in question occurred 

 between the first two dates, and amounted from over a quarter to 

 nearly three-quarters of thefi: original size. The place of greatest 

 enlargement was in cutting III, 17 feet from the ground. This 

 does not correspond with the place of greatest ring thickening (table 

 A) the previous year. Whether any significance can be attached 

 to this discrepancy cannot be decided with certainty. It seems 

 reasonable, however, to ascribe it to heightened temperature from 

 the rise of soil water in the xylem.* It would be natural to assume 



7 No observations have been made on secondary growth in the leaves. Meissner 

 (21) has observed a marked increase in the number of phloem elements and a very 

 slight increase in the xylem elements in a number of species of Pinus. 



*RoBT. Hartig (ii, p. 262) made note of the temperature of soil water as a 

 factor potent in forwarding growth. 



