GROWTH IN TREES. 21 



measurements the old, flaking outer cork layers of the bark were cut 

 away and bearings were taken on the thin layer formed in the previous 

 year, which had not yet become heavily browned. Internal to this is 

 the bast, which forms a layer about 2 mm. in thickness, and internal 

 to this is the cambium layer. Growth or permanent enlargement would 

 depend upon the division, enlargement, and differentiation of the 

 elongated, spindle-formed elements of the cambium. 



While new cells are being formed by division, and these are under- 

 going the enlargement constituting one of the main features of growth, 

 these cells form a very thin layer external to the woody cylinder. 

 This woody cylinder, with its closed systems of tracheids or wood- 

 cells, is the conducting system by which solutions pars from the roots 

 to the transpiring surfaces of the leaves. 



The rate of water-loss from the leafy surfaces may vary widely 

 during the course of the day as a result of changing temperatures of 

 the air, while the temperature at which water passes into the trunk at 

 its base changes but slowly, and chiefly in response to eoil-moisture 

 proportions. It is obvious that such relations between absorption and 

 loss of water may result in more water being given off during any 

 period than is taken in. Such excessive transpiration can, of course, 

 occur only from a balance or surplus, and this accumulated reserve, 

 occupying the entire trunk of the tree, may be very great. Any 

 lessening of its volume, however, would cause variations in the dimen- 

 sions of the trunk to an extent determined by a set of conditions which 

 need not be elaborated here. 



The most valuable dendrographic data, so far available, are those 

 obtained from the Monterey pine. The separate trees, five in number, 

 to which dendrographs have been attached are briefly described as 

 below: 



No. 1 is about 25 meters in height, bearing only short, thin branches, 

 with the trunk 40 to 44 cm. in diameter a meter above the ground. 

 Cores taken out by increment borers show 28 well-defined layers or 

 "annual rings." The radial increment the first year was 14 mm., and 

 varied from 2 to 5 mm. in the succeeding ten layers, which may not 

 safely be taken to represent annual formations. The second series of 

 ten layers has a total thickness of 62 mm., the separate layers varying 

 from 2 to 10 mm., the thinner ones being in all probability formed as 

 the result of autumnal rains, such as occurred in 1918. The total of 

 the 11 outermost layers amounts to 60 mm. of radial thickness, varying 

 from 2 to 9 mm. in the separate layers. Thus, on the basis of experience 

 described below, by which the tree after having formed a layer 5 mm. 

 in thickness in the early part of 1920 was awakened to form a second 

 layer of equal thickness by irrigation, it is assumed that the layer 2 mm. 

 thick interior to the layer of 1919, which is 8 mm. in thickness, resulted 

 from a rainfall of 5 inches early in September 1918. Formation of a 



