52 OF THE SAP-VESSELS. 



plant$ be partly cut through or gently broken, and its 

 divided portions slowly drawn asunder, the spiral coats 

 of their vessels will unroll, exhibiting a curious specta- 

 cle even to the naked eye. In other cases, though the 

 spiral structure exists, its convolutions are scarcely se- 

 parable at all, or so indeterminate as to be only marked 

 by an interrupted line of perforations or slits, as shown 

 by M. Mirbel. Indeed the very same branches which 

 exhibit 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 phi- 

 losophers 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 di- 

 rect communication with the former. Thus the tubes 

 in question have always been called air-vessels, till Dar- 

 win suggested their real nature and use.* He is per- 

 haps too decisive when he asserts that none of them are 

 air-vessels because they exist in the root, which is not 

 exposed to the atmosphere. We know that air acts 

 opon the plant under ground, because seeds will not 



* Du Hamel, indeed, once suspected that they contained 

 « highly rarefied sap," but did not pursue the idea. 



