ANNUAL RINGS 



677 



assumes the form of an ellipse, with its 

 major axis vertical ; in this way the bending 

 moment is very considerably increased. The 

 investigations of Hartig, Metzger, Sonntag 

 and Ursprung have shown, that there is not 

 only a quantitative but also a qualitative 

 difference between the adaxial and the 

 abaxial wood in branches with a distinct 

 epinastic or hyponastic tendency. The 

 adaxial wood (the " white " wood of Picea 

 exeelsa and other Conifers) is mainly inex- 

 tensible in character, while the abaxial xylem 

 (the " red " wood in Conifers) is incompres- 

 sible. According to Metzger, Conifers have 

 an inherent tendency to produce a relatively 

 large proportion of incompressible wood, 

 whereas deciduous trees are on the whole 

 inclined to form inextensible woody tissue. 

 It is, therefore, not surprising that Conifers 

 concentrate their secondary xylem on the 

 lower or " compression " side of the hori- 

 zontal branches, whereas deciduous trees de- 

 velop the bulk of the wood on the upper 

 or " tension " side ; in other words, Conifers 

 tend to develop hyponastic, and deciduous 

 trees epinastic lateral branches. 363 



The extreme epinasty exhibited by the 

 flattened plank-roots which are characteristic 

 of a number of tropical trees (spp. of Fictcs, 

 Sterculia, Canarium commune, Parkia cifri- 

 cana, etc.) must be regarded as an adaptive 

 phenomenon, since the lateral compression of 

 the roots greatly increases their effectiveness 

 as buttresses of the trunk. A transverse 

 section of such a plank-root of Parkia afri- 

 cana, which the author brought back with 

 him from Buitenzorg (Fig. 280), has a total 

 vertical diameter of 104 cm. ; but the organic 

 centre is 92 cm. distant from the upper, and 

 hence only 12 cm. from the lower surface of 

 the root. Epinasty is not much in evidence 

 in the zone represented by the first twelve 

 annual rings (which amount in toto to 9 cm. SSSs^SSt^t^ 



Fig. 280. 



T.S. through one l the 

 buttress-roots of Parkia 



plank-like 

 n i icana. 



size. Fur 



