14 MACDOUGAL— GROWTH IN TREES. 



are not yet available for a definite determination of the part t\'hich 

 growth in the phloem may play in the variations as recorded by the 

 dendrograph. Much however is known as to the cytological pro- 

 gram of the growing elements in cambium and phloem. A correla- 

 tion of facts of this kind and of the seasonal changes in food supply 

 will be necessary to interpret the " growth-impulses " lasting for a 

 few days late in the season, displayed by many trees. ^ 



Dendrographic data, especially records of experimental settings, 

 may be expected to afford further necessary corrections as to the 

 time element in the interpretation of seasonal layers or " annual 

 rings " upon which much reliance is placed as offering corroborative 

 evidence as to climatic periods and solar cycles. 



Illustrations. 

 Plate I. 



Fig. I. Earlier form of dendrograph which takes a bearing from a pre- 

 pared area on the bark of the tree by one end of a small lever, the other end 

 of which is connected with the short arm of a recording lever. An encirc- 

 ling" belt of wooden blocks serves as a base and support. Flexible wire stand- 

 ards with a base of thin sheet metal are clamped in position on the wooden 

 blocks and screw clamps which slide up and down on the wire standards 

 serve to hold the floating frame in a horizontal position. The entire appara- 

 tus is so adjusted that a contact rod of the floating frame on the opposite 

 side of the tree is held with gentle pressure against the tree and any varia- 

 tion in diameter is then expressed by movements in the lever set. 



Fig. 2. Improved dendrograph lever set. A, inner end of quartz rod 

 in contact with prepared surface on the bark of the tree. The outer end of 

 the quartz rod is fitted with a metal guide which engages the short arm of 

 the recording lever at B. The long arm of the recording lever D carries a 

 pen which makes a tracing on ruled paper on a revolving drum. The hori- 

 zontal member of the frame C which carries the recording lever may be 

 toward or away from the tree to adjust the pen at any point on the paper 

 record sheet. 



s Knudson, L., " Observations on the Inception, Season and Duration of 

 Cambium Development in the American Larch (Larix laricina Du Roi 

 Koch)," Bull. Torr. Bot. Cluh, 40: 271-293. June, 1913. 



Brown, H. P., " Growth Studies in Forest Trees. I. Pinits rigida IMill.," 

 Bot. Gaz., 54: 386-402, 1912. "II. Pinus sirobus L.," ibid., 59: 198-240, 1915. 



Bailey, I. W., " Phenomena of Cell-division in the Cambium of Arbor- 

 escent Gymnosperms and their Cj'tological Significance," Proc. Nat. Acad. 

 Sci., 5 : 283-285, 1919. " The Cambium and Its Derivative Tissues. II. Size 

 variations of Cambium Initials in Gymnosperms and Angiosperms," Ainer. 

 Jour. Bot., 7: 335-367, 1920. "II. A Reconnaissance of Cytological Phe- 

 nomena," Amcr. Jour. Bot., 7: 417-434, 1920. 



