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the part where these cells are continuous with the vessels, extremely 

 minute dark points are visible in the walls of the former. These points 

 coalescing, form a number of lines, indicating the spaces between the 

 coils of the spiral fibre in the perfect vessel. After several of 

 these cells have been thus transformed into vessels, a new joint is 

 formed, which at first lies merely in contact with the extremity of a 

 previously finished joint, but afterwards communicates with it in the 

 manner before described, and thus a vessel is increased in length by 

 the successive addition of new joints. At first, the marking of many 

 of the recently formed vessels is so faint as not to be distinguishable, 

 excepting by the best lenses of a high magnifying power, but it gradu- 

 ally increases in distinctness as the spiral fibre becomes developed." 



The author then explains, by means of a reference to his plates, 

 that the larger vessels acquire their diameter by the union of cells, the 

 diameter of the vessel depending on the number of cells united : he 

 remarks that " the spiral fibre can be observed to become developed 

 in the walls of the cells ; " this does not appear very obvious even 

 w'ith the assistance of plates, and without that assistance we doubt 

 our ability to make it manifest to our readers. The fibre commences 

 in a minute oval spot in the centre of which a "thinning" makes its 

 appearance and increases continually in a transverse direction to the 

 axis of the vessel, until it joins other similar spots, forming a transverse 

 bar; a series of such bars frequently occupies the breadth of a single 

 cell without extending at all to the collateral cells : when these lines 

 or transversely extended spots become continuous with those of the 

 collateral cells and form several circles round the vessels, a spiral fibre 

 is the result, and the vessel is called a " spiral" vessel ; but when the 

 appearance is bar-like the vessel is called " reticulated." The author 

 concludes that as vessels are made up of cells ; as the same structure 

 is common to vessels and cells; and as vessels contain a fluid and in 

 many cases a solid material ; it is therefore probable that the functions 

 of cells and vessels are analogous, and that vessels receive their fluids 

 by endosmose from adjacent intercellular tissue and perhaps from 

 surrounding cells. 



"45. Now, each cell being isolated, and without any communica- 

 tion with those in its vicinity, the pressure exerted upon the inner 

 surface of its walls, in consequence of the passage of a larger quantity 

 of fluid into it by endosmose, than can escape from it in the same 

 time by exosmose (to which pressure the enlargement of the cells is 

 due), is altogether local and limited ; and thus the development of one 

 part of the cellular system of a plant is independent of that of another 

 Vol. II. 6 o 



