GROWTH IN TREES. 



BY D. T. MACDOUGAL. 



A tree may be considered as a tall cone of wood terminating in leafy 

 expanses- The base of the cone is subdivided into myriads of rootlets, 

 through the surfaces of which the soil-solutions enter, and the water, 

 passing upward, is transpired from the leaves. The trunk of a tree 

 is largely composed of dead cells, but inclosing it is a thin sheet of 

 spindle-form cambium cells 1 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 cork and phloem and those on the 

 inner into wood cells and vessels. 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 

 some 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. External to 

 the cambium are sieve cells, bast fibers, etc., and cork cells, inclosed 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 cells, and 

 their enlargement accompanied by the differentiations mentioned, all 

 based upon hydration of cell-colloids. 



When changes in the entire diameter are measured the variations 

 may be due to the action of any one of these tracts of tissue. The 

 colloids of the cell, including those of non-living walls or wood, are 

 practically never in a stable condition, but follow an ever-varying 

 environment in resultant adjustments, especially with respect to the 

 water which they contain. All plants except submerged forms are 

 always losing water at a varying rate from part of their surfaces, while 

 water may be entering absorbing surfaces at a varying rate, and as a 

 consequence of the inequality the volume may increase or decrease 

 according to the balance between gain and loss. 



Growth in organisms is essentially a hydration of colloids of the 

 protoplasm accompanied by metabolic changes which result in the 

 conversion of materials in the cell-sap to the emulsoid condition charac- 

 teristic of living matter. Both processes, one of adsorption and the 

 other a result of shifting equilibria in chemical systems, cause an 



1 Bailey, I. W. Phenomena of cell-division in the cambium of arborescent gymnoaperms and 

 their cytological significance. Proc. Nat. Acad. Sci., 5:283-285. 1919. 



