50 CARNEGIE INSTITUTION OF WASHINGTON. 



Among other things estabhshed by these records is the fact that a 

 period of several weeks may intervene between the awakened activity 

 of the tops of the branches and increase in diameter of the trunk, 

 especially in the pines. These rough-barked trees show great diurnal 

 variation which, as shown by records of the variations of the woody 

 cylinder, may be due largely to the action of non-living tissues. Not 

 all rough-barked trees show such marked variations, while smooth- 

 barked trees or those with a green skin display a minimmn variation. 



Experiments in irrigation show that trees which have come to rest 

 may be awakened by a renewed supply of ground- water. 



Dendrographs have been used to obtain comparisons of the upper 

 and lower parts of the trunk, concerning which but little reliable infor- 

 mation was hitherto available. 



The measurements of the variations of woody cylinders offer some 

 opportunities for estimation of the changes in surface tension in wood 

 cells containing water and gas-bubbles and the probable part which 

 this may play in the ascent of sap and other purely physical phenomena. 



Measurement of a Seaso7i^s Growth of Trees by the Neuiy Designed Dendrometer, 



by D. T. MacDougal. 



In addition to the dendrographic record as to the course and daily 

 variations in the rate of growth of tree trunks, it is desirable to register 

 the total amount of growth of trees under observation and of a large 

 number of others. A separate instrument v/hich has been designated 

 as a dendrometer has been designed for this purpose and a number of 

 working models constructed in om^ own shop have been in operation on 

 pine and oak trees at the Coastal Laboratory during the summer of 

 1920. 



The dendrometer utilizes the principle of a number of ''plungers" 

 or short rods arranged radially to the tree, an encircling wire anchored 

 at one end, the other end being attached to a pivoted pointer passing 

 through holes in the outer ends of the radial rods in such manner that 

 the lengthening of the radii is registered by the pointer as growth takes 

 place. In practice the plungers are the short arms of L-shaped copper 

 bars, the long arms being thin and spring-like. The ends of the long 

 arms are attached to a belt of galvanized-iron strip. The principal 

 features of this design were first tested as a means of securing dendro- 

 graphic measurements, but as suitable wire of low- temperature coef- 

 ficient could not be procured, it was discarded. 



The readings of the dendrometer are taken weeks or even months 

 apart and may be obtained at identical temperatures. The instrument 

 is of simple design and inexpensive construction and promises to be of 

 some practical use in ordinary forestry operation. The readings of 

 dendrometers showing changes in circumference form a valuable check 

 on the results of the dendrograph, which measures changes in diameter. 



