224 BOTANICAL GAZETTE [march 



the conclusion that cessation of growth occurred later below and 

 last of all in underground parts. R. Hartig (ii), 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. Knudson's data (i6) 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. 



