724 RESISTANCE OF FOLIAGE-STEMS TO STRAIN, PRESSURE, AND BENDING. 



EESISTANCE OF FOLIAGE-STEMS TO STRAIN, PEESSURE, AND BENDING. 



When the weight of the individual parts of these huge trees is considered, it 

 is diiEcult to understand how their comparatively slender main stems are able to 

 support a crown weighing many thousand kilogrammes, and how it is that the 

 boughs extending far out from the trunk horizontally do not crack and break 

 under the weight of the branches and leaves they carry. The culms of grasses 

 and the stems of bushes and herbs are also so loaded as to astonish us, and we 

 cannot help asking how it is they are able to keep erect, and how, when their 

 equilibrium is disturbed, they can resume their normal resting position almost 

 at once. If we wish to investigate the mechanisms which make it possible 

 for these plants to maintain their stems in this position without assistance, we 

 must in the first place consider the lowest portion of the erect main stem, since 

 it is that part which would naturally have the heaviest burden to carry. Given 

 that the pressure caused by the loading operates in the direction of the axis, the 

 main stem must exhibit contrivances enabling it to resist the vertical pressure; 

 in other words, it must possess what is known as columnar strength. With the 

 exception of some palms whose erect stems rise up like pillars from the ground, 

 and whose leaves project equally in all directions, such a pressure, acting exactly 

 in the direction of the axis of the stem, is but rarely found. As a rule some 

 inequality in the stem or crown, although perhaps but slight, causes the pressure 

 to be diverted from the central axis; the stem is bent by the one-sided burden, 

 and has need not only of columnar strength, but of resistance to flexion as well. 

 Winds also will effect a bending, not only by direct impact, but also inasmuch as 

 they displace the centre of gravity of the load sustained by the lower part of the 

 stem. Observation shows us that this bending is only rarely followed by the 

 fracture of the stem. Not only grasses and reed culms, but also the thinner 

 erect branches of trees, shrubs, and bushes, and even palm caudices may be bent 

 down a considerable distance, but, when the wind subsides, quickly return to their 

 erect position without having sufiered the least harm. 



Formerly but little attention was given to these phenomena, perhaps because 

 they were so common and frequent, or perhaps because it was thought to be 

 impossible to give a scientific explanation and reason for the swaying of branches 

 in the wind. It was reserved for modem times to explain the mechanisms 

 underlying the returning of bent stems to a definite position of rest, and the 

 contrivances which permit such stems to bend but not break, even when con- 

 siderably loaded and under strong pressure. Investigations into this subject 

 have demonstrated that the bearing capacity and power of resisting bending 

 moment in plant stems are obtained by structures exactly similar to those 

 used by man in spanning a river with bridges, in fixing the supports of a roof, 

 of wooden partitions, &c. Further that the principle so important to every 

 builder, the obtaining of the greatest strength possible with the smallest outlay 



