these spiral vessels when young, show no signs of them at a more advanced period of growth, when their parts are become more woody, firm and rigid. No such spiral-coated vessels have been detected in the bark at any period of its growth.
Malpighi asserts that these vessels are always found to contain air only, no other fluid; while Grew reports that he sometimes met with a quantity of moisture in them. Both judged them to be air-vessels, or, as it were, the lungs of plants, communicating, as these philosophers presumed, with certain vessels of the leaves and flowers, of an oval or globular form, but destitute of a spiral coat. These latter do really contain air, but it rather appears from experiment that they have no direct communication with the former. Thus the tubes in question have always been called air-vessels, till Darwin suggested their real nature and use[1]. He is perhaps too decisive when he asserts that none of them are air-vessels because they exist in the root, which is
- ↑ Du Hamel, indeed, once suspected that they contained "highly rarefied sap," but did not pursue the idea.
