I9I2] BROWN— PINUS RIGIDA 387 



as a homogeneous mass. The formation of this cambial layer takes 

 place the first year, and is brought about by the linking together, 

 so to speak, of the fascicular cambium of the primary bundles by 

 the formation of interfascicular cambial zones, the result being a 

 cylinder of merismatic tissue capable of division. There are, in 

 addition to this, however, certain other growth phenomena. In 

 the cortex of many trees, either near or remotely distant from the 

 primary cambium, secondary cambial zones arise, whose function 

 it is to form cork, the so-called cork cambiums. They are not 

 united in a ring, as is the primary cambium, but extend for com- 

 paratively short distances in a peripheral direction.^ Again, as met 

 with in the Cycadales and Gnetales (Coulter and Chamberlain 

 4), successive bundle-forming cambiums sometimes arise toward 

 the periphery of the stem, and in such cases the life of the primary 

 cambium is generally very short. Further, among dicotyledons 

 there are a number of modifications of secondary thickening, par- 

 ticularly in underground parts. In the present studies, however, 

 it is the intention to confine investigation to growth as it normally 

 occurs in trees, that is, to the activities of a cambium which has 

 certain active and certain dormant periods. 



A number of specimens of Pinus rigida in the Cornell pinery as 

 well as others in the wild state were used. Those in the nursery 

 consisted of a number of individuals standing in a row which ran 

 approximately east and west. The land sloped gently to the south- 

 west and drainage conditions appeared to be good. The individual 

 trees were about 22 years of age and seemed to be in a thriving 

 condition. The height varied from 6 to 7 m., depending on the 

 vigor of the individual, and the average diameter at breast height 

 was 12 cm. In 1909, when investigation began, the branches 

 extended to within 1.2m. of the ground. However, during the 

 year above mentioned, the trees were pruned to a height of 1.9m. 

 above the ground. Experiments were carried on with six indi- 

 viduals of this series, which were numbered I-VI. 



The trees in the wild state had better be described separately, 

 since each was of different age and external factors varied with the 

 individual. For the sake of clearness they were designated as 



= Exceptions to this rule occur, resulting in the so-called "ring-barked" trees. 



