MACDOUGAL— GROWTH IN TREES. 13 



automatic registration of changes in circumference taken up by a steel 

 band in 1905/ but I have been unable to find any description of re- 

 sults obtained. Such results would include the errors due to the 

 high temperature coefficient of steel. Some measurements of expan- 

 sions of trunks as solid columns were made by Trowbridge and 

 Weil in 1918,^ but the most serious efifort with accurate methods 

 was that of Mallock in 19 18, who used an arrangement including 

 a tape of invar passed around the trunk and two superposed plates 

 of glass by which changes in circumference caused displacements 

 in interference bands of light. Direct and continuous observation 

 yielded accurate results of value with regard to daily equalizing 

 variations as well as of actual growth. ° 



The trunk of a tree is largely composed of dead cells, but enclos- 

 ing it is a thin sheet of spindle form cambium cells'^ in 2 to 10 or 

 more layers which in the growing season enlarge in thickness and 

 divide lengthwise, those on the outside becoming transformed into 

 phloem cells and those on the inner into wood cells or tracheids. 

 Extending from the center of the trunk are thin sheets or rays of 

 the medulla or pith of the young stem. The most recently formed 

 cells of these elements are still living and in Come trees the medullary 

 cells remain alive for several years, so that the woody cylinder of 

 the tree may comprise wood-cells or tracheids, vessels and thin-wall 

 ray cells, some of which are alive. Externally to the cambium are 

 sieve cells, bast fibers, etc., and cork cells, enclosed in a bark which 

 varies widely as to structure in different species. 



The greatest amount of increase or change in volume is that 

 which results from the multiplication by fission of the cambium ele- 

 ments, and the following enlargement of the derivatives. The facts 



Jorge, Uruguay, from Jan. 12, 1885, to Jan. 12, 1890," Trans. Bot. Soc. 

 Edinhurg, 18: 456, 1891. 



4 Friedrich, J., " Zuwachsautograpli," Centralh. fiir das gasamnite Forst- 

 wasen, 31. Nov., 1905, pp. 456-461. 



5 Trowbridge and Weil, " The Coefficient of Expansion of Living Tree 

 Trunks," Science^ 48: 348, 350, 1918. 



^ Malloch, A., " Growth of Tree*- with a Note on Interference Bands 

 Formed by Rays at Small Angles," Proc. Roy. Soc, 90, B, 186-191, 1918. 

 Submitted Dec. i, 1917. 



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

 escent Gymnosperms and their Cytological Significance," Proc. Nat. Acad. 

 Sc, 5: 283-285, 1919. 



