48 OP THE SAP-VESSELS. 



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 al- 

 ways 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 ap- 

 pears from experiment that they have no di- 

 rect communication with the former. Thus 

 the tubes in question have always been called 

 air-vessels, till Darwin suggested their real 

 nature and use*. 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 con- 

 tained ** highly rarefied sap/' but did not pursue the 

 idea. 



