224 BOTANICAL GAZETTE [MARCH 
the conclusion that cessation of growth occurred later below and 
last of all in underground parts. R. Hartic (11), following up 
these studies, made cuttings from species of Pinus, Picea, Larix, 
and Abies, in order to determine the condition of the cambium in 
different parts of the shaft. Cambial activity in each case was 
farther advanced above than below. It gradually diminished in 
intensity during the months of May, June, and July in the higher 
parts, while below the same applied to the months of June, July, 
and August. KNupson’s data (16) indicate the same condition 
of the tissue for Larix laricina, except that in the larch the phenom- 
enon occurred in July instead of August and September. 
The disparity in growth in different parts of a tree is without 
doubt dependent on conditions of temperature. The primary 
cortex persists in white pine for a long period, in some cases as long 
as 50 years. This condition is brought about through the division 
of the original cells of the cortex by anticlinal walls, and the sub- 
sequent enlargement of the two cells thus resulting. Meanwhile, 
cork formation remains superficial, so that the upper portions of 
the tree, even where the bole is 15 inches in diameter, are clothed 
by a layer of living, chlorophyll-bearing, primary cortex. Sooner 
or later, however, and varying markedly in different individuals, 
deep cork formation begins. This is evident first through the 
formation of isolated areas of brown tissue which stand out sharply 
from the surrounding living cortex. These increase in number, 
finally become confluent, and the characteristic old bark of white 
pine is formed. With this change in the type of cork formation 
there is correlated a modification of at least one factor potent in 
forwarding growth. The first phellogen is continuous around the 
whole circumference and functions until deep cork formation begins. 
New cork cells are added to the outside, and with the increase in 
circumference the older ones on the extreme outside slough off. 
So long as the primary cortex persists, the corky mantle remains 
thin and its protective value is in like proportion restricted. With 
deep cork formation, however, the conditions are altered to a large 
extent because the corky layers which are then formed through 
the activity of each phellogen accumulate. Protection of the 
cambium in the basal portions of the tree is thus greatly increased. 
