GROWTH IN TRHES. 
By D. T. MacDovaat. 
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' 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 ina 
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 gymnosperms and 
their cytological significance. Proc. Nat. Acad. Sci., 5: 283-285. 1919. 
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