compression-lines of the old. The biological interest connected with this principle lies chiefly in the mechanical construction of the rush or the straw, or any other typically cylindrical stem. The material of which the stalk is constructed is very weak to withstand compression, but parts of it have a very great tensile strength. Schwendener, who was both botanist and engineer, has elaborately investigated the factor of strength in the cylindrical stem, which Galileo was the first to call attention to. {679} Schwendener14 shewed that the strength was concentrated in the little bundles of ãbast-tissueã but that these bast-fibres had a tensile strength per square mm. of section, up to the limit of elasticity, not less than that of steel-wire of such quality as was in use in his day.
For instance, we see in the following table the load which various fibres, and various wires, were found capable of sustaining, not up to the breaking-point, but up to the ãelastic limit,ã or point beyond which complete recovery to the original length took place no longer after release of the load.
| Stress, or load in gms. per sq. mm., at Limit of Elasticity | Strain, or amount of stretching, per mille | |
|---|---|---|
| Secale cereale | 15ã20 | ã4ôñ4ã |
| Lilium auratum | 19ãã | ã7ôñ6ã |
| Phormium tenax | 20ãã | 13ôñ0ã |
| Papyrus antiquorum | 20ãã | 15ôñ2ã |
| Molinia coerulea | 22ãã | 11ôñ0ã |
| Pincenectia recurvata | 25ãã | 14ôñ5ã |
| Copper wire | 12ôñ1 | ã1ôñ0ã |
| Brass wire | 13ôñ3 | ã1ôñ35 |
| Iron wire | 21ôñ9 | ã1ôñ0ã |
| Steel wire | 24ôñ6 | ã1ôñ2ã |
In other respects, it is true, the plant-fibres were inferior to the wires; for the former broke asunder very soon after the limit of elasticity was passed, while the iron-wire could stand, before snapping, three times the load which was measured by its limit of elasticity: in the language of a modern engineer, the bast-fibres had a low ãyield-point,ã little above the elastic limit. But nevertheless, within certain limits, plant-fibre and wire were just as good and strong one as the other. And then Schwendener proceeds to shew, in many beautiful diagrams, the various ways in which these strands of strong tensile tissue are arranged in various cases: sometimes, in the simpler cases, forming numerous small bundles arranged in a peripheral ring, not quite at the periphery, for a certain amount of space has to be left for living and active tissue; sometimes in a sparser ring of larger and {680} stronger bundles; sometimes with these bundles further strengthened by radial balks or ridges; sometimes with all the fibres set