ADAPTATIONS TO EXTERNAL MECHANICAL FORCES. 



horizontal branches have often eccentric rings of wood, as seen in a cross - 

 section. This is due to the fact that the weight of the bough continually 

 increases, consequently the leverage is gradually increased, so the weight 

 must be overcome by strengthening the bough, on the underside espe- 

 cially, and at its insertion into the trunk of the tree. Such a bough may 

 often be found to have the centre of the annual rings considerably above 

 the actual middle of the bough. 



When a horizontal bough is over- weighted, as by snow, or has decayed, 

 the equilibrium is destroyed and it breaks ofif, not at the actual base of the 

 bough, where it issues from the trunk, but at a short distance from it. 

 The accompanying diagrams will explain why it is so. Fig. 278 represents 

 the distribution of forces when a man lifts a ladder. He puts his foot 

 against the lowest rung as a fulcrum (F), and holds one higher up (P, the 

 " Power "). He thus overcomes the resultant of F and W (the Weight of 

 the ladder). As long as he stands with the ladder in his hands without 

 pulhng it up, the three forces are in equilibrium. So is it with a bough of 



Fig. 278. — Diagram showing the dis- Fig. 279. — Diagram showing distribu- 



tribution of forces in raising a tion of forces keeping a bough in 



ladder. equilibrium. 



a tree as growing horizontally out of a tree-trunk. Fig. 279 represents 

 the usual curvature of such a bough. The arrows represent the three 

 forces normally in equilibrium. W is the weight of the bough, F the 

 force acting through the fulcrum at the base of the bough, P is the power to 

 resist the resultant of the other two (R). Now, if W becomes too great, 

 F remaining the same, this resultant (R) is greater than P, and the bough 

 snaps off at the place where this "pulling force" is acting, and conse- 

 quently it leaves a " snag " projecting from the trunk. 



It will be observed that "decimate" stamens, which support the 

 insect visiting a flower, are precisely of the same curvature, this being the 

 best form for supporting a weight at a distance from the fulcrum. 



Stem-girders. — Mechanical appliances invented by engineers are often 

 close imitations— though they may not have intentionally copied them — 

 of similar ones in Nature. Thus it has been noticed that the distribution 

 of the woody bundles in herbaceous stems is on the same principle as 

 is followed in the construction of iron girders. Thus Dr. Kerner observes, 

 speaking of the woody bundles in a stem : — " If they were confined to 

 the centre it would be anything but a suitable arrangement for an erect 



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