572 Comparative Physiology. 



ascent, are undoubtedly always along the fibro-vascular tissue, 

 though probably not confined to it. If the tubes and ducts 

 are air-vessels, which they appear to be, the stimulus of 

 oxygen is likely to assist in the ascent. The normal destination 

 of the woody fibre seems to be, like the bones of animals, 

 to strengthen and solidify the system ; their toughness and 

 straitened diameter do not fit them well for absorption and 

 conveyance rapidly : that of the ducts seems to be air-vessels, 

 their vascular structure being generally allowed. Independently 

 of the experiments of Mr. Knight, which would seem decisively 

 to point out that they do not generally carry sap, the lengthened 

 tubes and ducts do not seem so capable of producing the 

 phenomena of endosmose as cells and intercellular passages. 

 If we confine the power of ascent to endosmose, and dismiss 

 capillary attraction, stimulus of air-vessels, and excitability, 

 the ascent would be most likely principally through the cellular 

 tissue, which does not appear to be the case. The contractile 

 power of the tissue of vegetables, when excited by sap and 

 its motions, seems generally allowed, and is very probably a 

 main agent in the ascent, assisted by the endosmose power of 

 absorption, capillary attraction, and the stimulus of the air- 

 vessels. It is a vital power which probably exists in all 

 organised beings; it is concentrated in the higher animals, as 

 well as diffused; but diffused only in the lower animals and 

 plants, and will be greatly promoted by heat, as ascent of sap is 

 found to be. Larger air-vessels are probably needed in such 

 rapid circulation as that of the vine ; and the impenetrable dense 

 system of the oak, &c. ; and the larger glandular tubes of pines 

 seem to fit them as substitutes for ducts. The spaces found 

 between the cells, ducts, and fibres are likely to be the main 

 channel, if contractile force is the main agent ; if endosmose is 

 the main power, then the cells and spaces adjoining the tubes 

 and ducts are most likely to be the channels, probably the cells 

 principally. 



On the descent and circulation of the sap, Mr. Knight 

 thought the course of descent to be by the proper vessels through 

 the bark to the roots, giving off the elaborated sap in its descent 

 to form the new layers of alburnum and liber, and communi- 

 cating by the medullary processes and intercellulary passages 

 with the interior of the stem. Gravitation and vibration he 

 considered great helps to descent, as he found the free parts 

 of trees, partly tied for experiment, to thicken much more in 

 diameter than the parts kept from moving ; but, as the elabo- 

 rated sap must proceed upwards in inverted cuttings and 

 pendent branches, he considered the principal force to reside in 

 the vital action of the proper vessels themselves. Mr. Niven says 

 that, in a stem of elm tree, excavated in the centre, and left 



