566 Comparative Physiology. 



of the absorbing system. Besides tlie conveyance of tlie nutri- 

 ent fluid to the remoter parts of the system, the crude aliment 

 must be exposed to the influence of the air before becoming fit 

 for its ultimate purpose, and the fluid once passed through the 

 tissues must undergo a similar process to restore it to its proper 

 condition. This process, respiration, requires the circulating 

 fluid to pass through certain organs during some part of its 

 transit, which bring it into relation with the atmosphere. The 

 uninterrupted performance of this function is therefore necessary 

 to the continuance of life, since the nutrition of the tissues de- 

 pends wholly on the materials thus supplied ; and the constant 

 stimulus of the vital fluid is necessary to excite them to the 

 performance of their appropriate actions." 



Dr. Carpenter has adopted the opinion that the ascent of the 

 sap is by the woody fibre and dotted ducts (vasiform tissue) 

 of the alburnum. DeCandoUe and others, however, are of 

 opinion that it passes through the intercellulary passages of 

 the cellular matter around the fibre and ducts, that the cells 

 separate to allow of the passage (the fluid not passing through 

 them as imagined by some) and again close when it has passed, 

 acting like the valves of the veins in animals, and assisting in- 

 the ascent. The sap, on examination, has sometimes, especially 

 during the greatest ascent in spring, been found in the ducts ; 

 but this may have arisen from the wound made, or from the 

 excess of sap at peculiar periods. They are generally found to 

 contain air ; and, like other modifications of spiral vessels, this 

 is probably their destined function. Changes are supposed to 

 take place and continue in the sap so soon as it enters the plant ; 

 and though these may be partly produced by the vital nature of 

 the membranes it passes, or of the descending sap it is mixed with, 

 they are likely to be more promoted by aeration than any thing 

 else, and the sap probably passes principally along the ducts for 

 that purpose. Like insects, plants are pierced with air-vessels in 

 all directions, in the leaves, flowers, and young shoots ; the spiral 

 vessels communicating with the stomata perform the function 

 of aeration in its most perfect form, which is by permeation in 

 plants, the spiral vessels being closed at the ends, not in a con- 

 tinuous tube like the trachea3 of insects. The stomata are 

 always connected with vessels ; some say their number is de- 

 termined by them. In exogenous trees the function is pro- 

 bably continued through the whole trunk by the dotted ducts, 

 which, it is likely, serve the same purpose as the spiral vessels 

 interspersed through the wood of endogenous trees. It must 

 be evident that the spiral vessels interspersed through the 

 trunks of the latter are intended for the purpose of aeration ; 

 the wood is also more sparingly furnished with vasiform tissue, 

 the whole bundle of vessels being by many considered as ducts. 



