i 9 i2] BROWN PINUS RIGID A 391 



above the other. There is also a change in the pith rays. The 

 ray tracheids have given rise to ray cells, so that the pith rays con- 

 sist exclusively of the latter. These as well as the bast parenchyma 

 cells contain abundant starch. 



TANGENTIAL SECTION 



In order to study cambium and cortex in tangential section, a 

 series of mounts is necessary. The same general characters are 

 observable, but in addition it is evident that there is an entire 

 absence of sieve plates on the tangential walls of the sieve tubes. 

 The callus thickenings of those on the radial walls, however, are 

 particularly noticeable with proper staining (methyl blue). 



Cambial awakening 



In taking up the study of xylem formation as it normally occurs 

 in trees, one naturally begins the study before cambial activity 

 begins. Cuttings taken at different heights from tree III on 

 February 21, 1910, all showed in cross-section the general outline 

 of the completed ring. Growth was not manifest in any of the 

 sections. Each ring presented well marked areas of early and late 

 wood. The latter in Pinus rigida is sharply differentiated, owing 

 to its greatly thickened walls. The above statement does not hold 

 true, however, for the wood of the first two or three years at any 

 point in the trunk. Here there is no sharp demarcation between 

 early and late wood. This condition is probably brought about 

 by the fact that the main axis was elongating rapidly at this point 

 when the ring was formed, or else, as these investigations tend to 

 show, growth is slow in beginning in the apical shoot but progresses 

 very fast when once started, so rapidly in fact that there is not 

 sufficient time for the walls to thicken appreciably. In either 

 alternative, there is a gradual thickening in the walls of the late 

 wood of successive rings as the apical shoot progresses aloft. 



The next set of cuttings were taken on April 4, 1910, from tree 

 III. The* cambium was still in the resting condition. Figs. 1-3 

 and 7-9 show the changes which occurred (figs. 1-3) between April 

 4 and April 15. In fig. 3 growth is more advanced than in either 

 figs, i or 2. The latter are both in the resting condition. So far 



