i 9 i 2 ] BROWN PINUS RIGIDA 399 



The living portion of the cortex, on the contrary, follows a law 

 exactly the reverse. In the upper portions of the crown the cortex 

 is necessarily thin, inasmuch as it contains a relatively small series 

 of bast parenchyma and sieve tube areas. Below, the thickness of 

 the cortex increases markedly, so much so in fact that it often 

 attains 3-5 cm. in width. The storing capacity of the cortex as a 

 result must be greatest in the basal portions of the trunk. Assum- 

 ing that food abundance alone was concerned in cambial awakening, 

 the latter would result first below. Inasmuch as it does not, there 

 are certainly other determining factors, chief among which is 

 probably insolation. 



Investigations on the older trees revealed a number of factors 

 of sufficient interest to demand mention in this paper. A curious 

 feature long known to former workers was especially prevalent. 

 I refer to the often noted lessened density of the wood on the south 

 side of trees. This is due to the fact that the proportion of summer 

 wood on the north side is greater as compared with the width of 

 the ring than on the south side. This disparity in wood formation, 

 however, is not so marked in young individuals. The ring forma- 

 tion is much more regular and it is only in the older trees that the 

 phenomenon above described is seen. As to the cause of this 

 lessened density on the south side, no reasonable conclusion was 

 attained in these investigations, nor has it ever been satisfactorily 

 accounted for. It is without doubt correlated with insolation in 

 some way, but further study is necessary to determine this defi- 

 nitely. 



The manner of cambial awakening likewise presents an interest- 

 ing study. It was observed that even on different sides of the same 

 section a noticeable disparity often occurred. In some cases 

 growth had proceeded to the extent of one or two partly formed 

 tracheids, while in closely neighboring spots the cambium appeared 

 as yet in the resting condition. Nor was one tracheid completely 

 formed as to size before another began. Often rows of three or 

 four small tracheids were visible, none of which had yet attained 

 half the size of those formed first the previous year. In such cases 

 it would appear that cell division was so rapid in the cambial region 

 during favorable seasons that new elements were laid down before 



