The Rev. P. Keith on the Internal Structure of Plants. 287 



appearance the tracheae of insects, and designated them by that 

 term, an appellation by which they are still very generally 

 known. 



They do not occur often in the root, or at least they are not 

 easily detected in it ; yet Kieser is said to have found them in 

 it in great abundance*. Dutrochet could find none, and de- 

 nies their existence in the root expressly f. Neither are they 

 to be found in the branch or stem of woody plants, except in 

 the annual shoot ; nor in the pith of any plant, except in the 

 several species of Nepenthes%. But in the stem and branches 

 of herbaceous plants they are generally to be found without 

 much difficulty accompanying the longitudinal fibres, and 

 forming part of the bundles. They are very easily detected 

 in the foot-stalk, whether of the leaf or flower. The leaf-stalk 

 of the Artichoke affords a good example, in which they are 

 not only remarkably large and distinct, but also remarkably 

 beautiful. They are discoverable also in the leaf itself; more 

 rarely in the calyx and corolla ; and more rarely still in the 

 other parts of the flower. Grew and Malpighi found them 

 both in fruits and seeds. I have myself met with them in the 

 external umbilicus of the Cherry at a very early period of its 

 growth, that is, about the time of the falling of its petals, but 

 not in any other fruits. 



Is it certain that the organs now under consideration are 

 tubes ? In the closely coiled up state in which they exist in 

 the growing plant they evidently constitute tubes by the union 

 of their spires. Divide a leaf-stalk of the Artichoke or of the 

 Elder longitudinally, cutting it partly, and tearing it partly 

 asunder, and place a portion of it under the microscope. In- 

 spect the exposed surface carefully and in a strong light, and 

 it will present to your view bundles of spirals in their coiled 

 up and united state. Divide a portion of the same leaf-stalk 

 transversely, cutting it partly and breaking it partly asunder. 

 Place it under the microscope in a clear and strong light, and 

 multitudes of spirals, not yet divided, but merely drawn out 

 and uncoiled, will be found to connect the fractured surfaces ; 

 and if you stretch them even till they give way, the fragments 

 will, as if by an elastic and inherent spring, coil themselves up 

 again nearly as before. 



Much has been said with regard to their functions. Some 

 have believed them to be air-vessels; others, as Mirbel, re- 

 gard them as being sap-vessels. Dutrochet believes them to 

 be conductors, not of sap, but of a diaphanous and peculiar 

 fluid, the product of insolation. This we state, however, 

 * Suppl. Encyc. Brit., Veg.Anat. f L y Agent, p. 20. 



J Phil. Mag., Oct. 1832, p. 317. 



