AND THE STRUCTURE OF PLANTS. 



89 



of these are found alive when the tree is cut down, deriving both air and 

 nutriment enough from the surrounding vessels of the tree daring their im- 

 prisonment. In the Memoirs of the Paris Academy there is an example of a 

 toad found in a tree that was proved to be a century old.* 



As the series of concentric circles, produced in the trunk of a tree by the 

 growth of every year, are still visible after the conversion of every other part 

 into lignum, or hard wood, we can trace its age with a considerable degree 

 of certainty, by allowing a year for every outer circle, and about two or three 

 years for the complete lignification of the innermost.! 



Independently of these more solid parts of the trunk or stem, we generally 

 meet with some portion of parenchyma and cellular substance, and always 

 with the different systems of vegetable vessels disposed in one common and 

 uniform arrangement. The lower orders of plants, indeed, such as the an- 

 nuals and biennials, consist almost exclusively of parenchyma or cellular 

 substance, with an inner and outer bark, and the respective vessels of the 

 vegetable system. 



These vessels are adducent and reducent, or arteries and veins, lacteal 

 or sap-vessels, and lymphatics. Many of these may be seen by the naked 

 eye, and especially the sap-vessels: and the vascular structure of the whole 

 has been sufficiently proved by Gessner, by means of the air-pump. The 

 reducent or returning vessels are stated, by Sir E. Smith, to bring back the 

 elaborated sap from the leaves to the liber for the new layer of the existing 

 year.| 



The lymphatics lie immediately under the cuticle and in the cuticle. They 

 anastomose in different ways through their minute intermediate branches, 

 and, by surrounding the apertures of the cuticle, perform the alternating 

 economy of inhalation and exhalation. Their direction varies in different 

 species of plants, but is always uniform in the same species. 



Immediately below these lie the adducent vessels or arteries; they are the 

 largest of all the vegetable vessels, rise immediately from the root, and com- 

 municate nutriment in a perpendicular direction : and, when the stem of a 

 plant is cut horizontally, they instantly appear in circles. Interior to th-ese 

 lie the reducent vessels or veins : which are softer, more numerous, and more 

 minute than the arteries ; and in young shoots run down through the cellular 

 texture and the pith. Between the arteries and veins are situated the air- 

 vessels, as they were formerly called ; but which Dr. Darwin and Mr. Knight 

 have sufficiently succeeded in proving to contain, not air in their natural state, 

 but sap.^ They seem to be the true genuine lacteals issuing from the root, 

 as, in animals, they issue from the villous coating of the intestinal canal. 

 They are delicate membranous tubes, stretching in a spiral direction, the 

 folds being sometimes close to each other, and sometimes more distant, but 

 generally growing thicker towards the root, and especially in ligneous plants. 

 These vessels also are very minute, and, according to numerous observations 

 of Hedwig made with the microscope, seldom exceed a 290th part of a line, 

 or a 3000th part of an inch in diameter. 



The lymphatics of a plant may be often seen with great ease by merely 

 stripping off the cuticle with a delicate hand, and then subjecting it to a 

 microscope ; and in the course of the examination we are also frequently able 

 to trace the existence of a great multitude of valves, by the action of which 

 the apertures of the lymphatics are commonly found closed. |i Whether the 

 other systems of vegetable vessels possess the same mechanism, we have not 

 been able to determine decisively; the following experiment, however, 

 should induce us to conclude that they do. If we take the stem of a com- 



* M6m. de I'Acad. Par. 1731, p. 24. 



tThc palms form an exception t o this general rule, possessing neither proper bark, nor fascicles of ves- 

 sels displayed in any circular form : the bark being produced by a remnant of tlie leaves, and the vessels 

 running in a straight line \v itliout regular order, and surrounded by cellular substance 



X Introd. to Uolaiiy, p. 56. See also Willderiow's Introd. p. 2.36. § See Smith's Introd. p. 47. 



II Tins seems to acquire additional probability from Mr. Knight's experiments. See Phil. Trans. 1804 , 

 and Thomson's Chemistry, v. 385. See Willd. p. 236. 



