OF THE SAP-VESSELS. 47 



than an entirely new theory of vegetation, by 

 which the real use and functions of the prin- 

 cipal organs of plants are now for the first 

 time satisfactorily explained. 



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 numerous herbaceous plants, as the Peo- 

 ny, 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, exhi- 

 biting a curious spectacle even to the naked 

 eye. In other cases, though the spiral struc- 

 ture exists, its convolutions are scarcely se- 

 parable at all, or so indeterminate as to be 

 only marked by an interrupted line of perfo- 

 rations or slits, as shown by M. Mirbel. 

 Indeed the very same brandies which exhibit 



