400 BOTANICAL GAZETTE [November 



their predecessors had yet attained their maximum size and 

 strength. 



Double rings were often in evidence in the old trees. These 

 might easily cause miscalculation as to age. The phenomenon of 

 double ring formation has often been observed, especially in broad- 

 leaved trees. Here it was ascribed sometimes to partial or com- 

 plete defoHation, at others to favorable or unfavorable external 

 factors. The first assumption would not hold in Pinus rigida in 

 this case or in general, since defoliation rarely occurs. The cause 

 must be ascribed to external growth conditions, but what these are 

 would be difficult to determine. That they are most prevalent in 

 old trees is well known, and this would lead one to infer that their 

 formation is in some manner correlated with inhibition of growth, 

 since the effects of this are most marked on older less vigorous 

 individuals. 



Secondary thickening in the roots 



Little stress was put on the study of secondary root thickening 

 in the present investigation. Only one cutting was taken, on 

 April 27, 1910, for purposes of comparison, so that no reliable 

 deductions can be made. At this time cambial activity was not 

 manifest, although it must have been in process throughout the 

 aerial portion with the possible exception of the apical shoot. 

 T. Hartig (8) claimed that cambial awakening in the roots is 

 much later than in the aerial portions. He gave midsummer as 

 the time of first inception and said it continued far into October. 

 Whether the same appUes fo Pinus rigida further investigation only 

 can decide. Suffice it to say, however, that the growth in thickness 

 of roots must not be confused with growth in length. The latter 

 is manifest often as early as March and continues throughout the 

 season. 



Summary 



1. The histological characters of Pinus rigida present no wide 

 variation from the normal coniferous type. 



2. The secondary thickening in the root is similar to that in 

 the stem, but differs (a) in less sharp demarcation between the 

 annual rings, (.&) in the biseriate character of tracheids, and (c) in 

 less density. 



