28 SYLVA AMERICANA. 



called the sap, by which the whole body of the plant is nourished, 

 and from which the peculiar secretions are made. 



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, called by Mr. Knight 

 central vessels, of a much more firm texture than the ad- 

 jacent 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 the elder, 

 lilac and many other shrubs, as well as in numerous herbaceous 

 plants, as the peony, and more especially 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 convolutions are scarcely separable at 

 all, or so intermediate as to be only marked by an interrupted 

 line of perforations or slits, as shown by IVL 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. 



Besides the central vessels, Mr. Knight has described another 

 set that traverse the alburnum, whence they are distinguished by 

 the name of alburnous tubes. Through them the sap also 

 ascends ; for the destruction of a circle of bark does not prevent 

 the formation of buds and leaves; "but," says Mr. Knight, 

 " the alburnous vessels appear to be also capable of an inverted 

 action, when it becomes necessary to preserve the existence of 

 the plant." The cortical vessels of Mr. Knight, which can 

 hardly be considered the same with the vasa reducentia of 

 Willdenow, (although they are said to perform the same function) 

 exist in the bark, and serve to reconvey the circulating sap to 

 the root. It is suspected, that there may be two sets of these 

 vessels, one which nourishes the bark, and another that secretes 



