118 



MONTHLY JOURNAL OF AGRICULTURE. 



considerable ; water holding half its weight 

 of sugar in solution raised pure water through 

 membrane with a power capable of sustain- 

 ing the pressure of a column of mercury of 

 1'27 inches in hight, a power nearly four and 

 a half times greater than the pressure of the 

 atmosphere which sustains the mercury in 

 the barometer and raises the water in a com- 

 mon pump. On the cause of this curious 

 jjhenomenoa j)hilosophers are not agreed : it 

 is probably only one of those numerous in- 

 stances that are from time to time brought to 

 light by incjuiry into the hidden processes of 

 Nature, of the general tendency of matter to 

 maintain an equilibiium among its particles ; 

 but the permeability of tlie vegetable mem- 

 brane being established, it is not necessary 

 to our present purpose to pursue the subject 

 farther : we have only to consider, as a nat- 

 ural consequence of the development of the 

 living plant, that the sap becomes thicker as 

 it ascends from the root, drawing after it the 

 thinner and moi-e recently imbibed fluid, to 

 account tor its successive rise through the mi- 

 nute vessels above desciibed — a rise so rapid I trio rings, or perhaps of the coils of spiral 



naked eye in a transverse section of wood, 

 are the divided cavities of the vessel so 

 called, which are sometimes distiibuted 

 through the layers of wood, but more com- 

 monly foiTn themselves distinct layers or 

 bundles. Many vessels of difterent structure 

 have been confounded under the general nam© 

 of duct ; and being wholly unacquainted with 

 their offices in the economy of the plant, 

 much diversity of oj^inion exists among 

 physiologists respecting their classification 

 and relative uses. Some are angidar, and 

 veiy evidently formed by the breaking of 

 cells longitudinally into each other, or by 

 the absorption of the dividing membrane ; 

 others are dotted with apparent pores ; a 

 third kind are more or less distinctly marked 

 with spiral lines, sometimes continuous, oc- 

 casionally broken at irregular intervals, as 

 though a spiral thread contained within a 

 membrane had had its coils separated and 

 ruptured in places by the longitudnial growth 

 or extension of the membrane ; a fourth se- 

 ries, called annular ducts, consist of concen- 



that several quarts or even gallons are obtain' 

 able daily for weeks together by tajiping the 

 trunks of some tropical trees. 

 '' The fibre of hemp, flax, and of many other 

 plants employed in diflerent parts of the 

 world hi the mauufactme of cloth, cordage, 

 &c., consists of these woody tubes ; and some 

 idea of their extreme tenuity may be fonned 

 from the examination of the finest flaxen 

 thread by a microscope, which shows it to 

 consist of a considerable number of tubes. 

 The membrane composing them, though 

 delicate and transparent, possesses much 

 Btrength and elasticity in most plants, 

 and in all cases is greatly superior in 

 that respect to cellular tissue, the mem- 

 brane of wliich is cfimparatively brittle ; 

 the ditference in the strengthof cotton 

 and linen thread is an instance of this, 

 the former being cellular, the latter 

 woody tissue. The wood of the fir tribe 

 appears to be very dissimilar to that of 

 ordinary trees and herbs, the tubes being 

 marked with dots or apparent {lores, sur- 

 rounded by a series of concentric circles ; 

 similar markings are observable in the 

 woody tissue of a few other tiibes, but 

 only such as, like the plants in question, 

 produce aromatic or resinous secretions, 

 whence they have been generally regard- 

 ed as glands ; their structure is, however, 

 at present very imperfectly understood, 

 ancl not anything is known respecting 

 their functions ; if really openings, they 

 constitute a remarkable exception to the 

 closeness of the corresponding tissue in 

 other plants. 



The name of duct has been given to 

 various comparatively large tubes or 

 vessels, generally associated with those 

 of the wood, but always distinguishable 

 from them by theirgreater diameter. The 

 large pores, frequently observable by the 



(262) 



threads so broken as to present the appeai- 

 ance of such, and held together by the mem- 

 braneous tube containing them. These larger 

 ubes are disposed in a similar manner to 

 the smaller ones of the wood, and resemble 

 them in being more or less narrowed or con- 

 ical at their extremities : several of their forms, 

 and there are some intermediate between those 

 described, approach so nearly to the following 

 fonn of tissue, that it is difficult to conceive 



Fie. 3. 



SPIRAL VESSELS. 



them other than more imperfect or disurbed 

 modifications of the same, viz., the spiral vessel. 



