684 SECONDARY GROWTH IN THICKNESS 



towards the distal end of the trunk, where it finally diminishes once 

 more. 



The gradual increase in the average width of the annual rings from 

 below upwards, is correlated with a tendency on the part of the tree 

 to develop its trunk as a " girder of uniform strength " in relation to 

 the bending action of the wind. 3 ' This interpretation was first definitely 

 put forward by Schwendener ; subsequently Metzger proved, by means 

 of numerous measurements of the radius at various levels, that the 

 trunk of the spruce does actually conform very closely to the 

 mathematical equation for girders of uniform strength of circular 

 cross-section. The elaborate researches carried out upon Pint's sylvestris 

 by F. Schwarz led to similar results. At the base of the trunk the 

 annual rings considerably exceed the width necessary in the case of an 

 imaginary girder of constant resistance, the greater thickness at the 

 base depending upon the fact that this region of the trunk has to cope 

 with the special demands which arise out of the necessity of anchoring the 

 stem in the ground. Metzger accordingly compares the base of the 

 trunk to a rigid socket, in which the rest of the stem is embedded. 

 The secondary diminution in the width of the annual rings, at the 

 distal end of the trunk, is evidently correlated with the fact that the 

 surface exposed to wind decreases (in the upper part of the crown) 

 as one passes from older to more recent whorls ; the reduction of 

 pressure permits of a corresponding weakening of the trunk. Taking 

 them all in all, the difference in the width of the annual rings, and 

 consequently of the cross-sectional area of the trunk at various heights, 

 is the result of very perfect adaptation to the mechanical demands that 

 are made upon the trunk by the bending action of wind. 



4. Normal changes in the physical condition of the wood. 3 ' 1 



There are comparatively few deciduous trees (such as Acer 

 Pseudoplatanns, A. platanoides, Buxus sempervirens, Betula <dli<<, 

 Populus tremula) the wood of which retains its original characteristics 

 all over the cross-section of the trunk ; as a rule, the most recently 

 added annual rings form a definite zone of sap-wood (alburnum), which 

 is clearly demarcated from the older heart-wood (duramen). The 

 sap-wood presents the yellowish-white appearance that one ordinarily 

 associates with lignified tissue, and is entirely composed of cells which 

 are still functional. Hartig, Wieler and Strasburger have, however, shown 

 that, even in the case of the alburnum, every part is not engaged in the 

 conduction of water to the same extent. Under normal conditions, in 

 fact, it is only the outermost annual rings that serve to convey the 

 transpiration-current, the more internal portions of the sap-wood 

 serving for the storage of water and synthetic products. The dark- 



