SPIRAL VESSELS. AIR-TUBES OF INSECTS. 67 



are found in the leaf-stalks, from which their spiral fibres 

 can be uncoiled in the manner already described. They are 

 found also in a delicate membrane, surrounding the pith of 

 stem which possess one (. 135) ; and in the midst of the 

 woody bundles which form the strings of such stems as the As- 

 paragus. From this plant, indeed, they may be obtained more 

 readily, perhaps, than from any other. If a stem be boiled, or 

 softened by soaking in water for some time, and these bundles 

 be separated from the soft tissue which surrounds them, the parts 

 of each may be further separated from each other, by rubbing 

 them, with a little water, between two plates of glass. On look- 

 ing at them with a magnifying-glass, some portions of these 

 bundles will be seen to present a dark appearance, if still under 

 water. This is caused by the air they contain ; since bubbles of 

 air in fluids viewed with the microscope, will appear dark to the 

 observer, for reasons which will be mentioned in the Treatise on 

 Light. If one of these threads be then carefully torn, with a 

 pair of small needles fixed in handles, into finer ones, whilst 

 under a powerful single magnifier, it may be separated into the 

 individual spiral vessels which compose it, just as the thread of 

 flax may be resolved into its woody tubes. 



81. It is an interesting circumstance, that the air-tubes of 

 Insects are formed upon nearly the same plan with these spiral 

 vessels of Plants. The former consists, like the 

 latter, of an external membrane, which is main- 

 tained in its tubular form, in spite of pressure 

 from without, by the elasticity of a fibre, spirally 

 coiled in its interior. The principal difference 

 between the two structures is, that the air-tubes 

 of Plants are closed vessels, and that their con- 

 tents find their way gradually from one to another, BRANCH^ 1 AIR- 

 permeating the delicate membrane of their walls; VESSEL OF INSECT. 

 and that they give off, therefore, no branches : whilst the air- 

 vessels of Insects, whose office it is to convey air with great 

 rapidity into all parts of the structure, form a set of continuous 

 tubes, which branch and ramify with the most wonderful minute - 



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