AiSU THE STRUCTURE OF PLANTS. 



80 



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 

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

 elaborated sap from the leaves to th© 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 nunute 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 

 communicate nutriment in a perpendicular direction : and, when the stem 

 of a plant is cut horizontally, they instantly appear in circles. Interior to 

 these 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 situ- 

 ated 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.f They seem to be the true genuine lac- 

 teals issuing from the root, as, in animals, they issue from the villous coat- 

 ing of the intestinal canal. They are delicate membranous tubes, stretch- 

 ing 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 hgneous plants. These vessels also are very minute, 

 and, according to numerous observations of Hedwig made with the mi- 

 croscope, 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.| 

 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 common balsamine,§ or of various other plants, and cut it horizontally 

 at its lower end, and plunge it, so cut, into a decoction of Brazil wood, or 

 any other coloured fluid, we shall perceive that the arteries or adducent 

 vessels, as also the lacteals, will become filled or injected by an absorption 

 of the coloured hquor ; but that the veins, or reducent vessels, will not be- 

 come filled ; of course evincing an obstacle, in this direction, to the ascent 

 of the coloured fluid. But if we invert the stem, and in like manner cut 

 horizontally the extremity which till now was uppermost, and plunge it so 

 cut into the same fluid, we shall then perceive that the veins will become 

 injected, or suffer the fluid to ascend, but that the arteries will not : proving 



* Introd. to Botany, p. 56. See also Wildenow's Introd. p. 236. 

 t See Smith's Introd. p. 47. 



\ This seems to acquire additional probability from Mr. Knight's experiments. See Phil , 

 Trans. 1804 ; and Thomson's Chemistry, v. 385. See Wild. p. 236. 



§ Impatiens hahaminq, .-—This is the plant recommended by M. Wildenow for this pur 

 jpose, as affording the clearest results. 1 



