OF THE SAP-VESSELS. 37 



In a young branch of a tree or shrub, or in the 

 stem of an herbaceous plant, are found, ranged round 

 the centre or pith, a number of longitudinal tubes or 

 vessels, of a much more firm texture than the adjacent 

 parts, and when examined minutely, these vessels often 

 appear to be constructed with a spiral coat. This may 

 be seen in the young twigs and leaf stalks of Elder, 

 Syringa, and many other shrubs, as well as in nume- 

 rous herbaceous plants, as the Peony, and more espe- 

 cially many of the Lily tribe. If a branch or stalk of 

 any of these plants 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 spectacle even to the naked eye. In other 

 cases, though the spiral structure exists, its convolu- 

 tions are scarcely separable at all, or so indeterminate 

 as to be only marked by an interrupted line of perfora- 

 tions 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 ad- 

 vanced 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 

 then}. Both judged them to be air-vessels, or, as it 

 were, the lungs of plants, communicating, as these 



