66 THE MECHANICAL FUNCTIONS. 



In many instances the inner fibres of the tube, instead of 

 forming a continuous spiral, appear in the shape of rings, 

 succeeding one another at regular intervals, and constituting 

 what are called annular vessels (Fig. 15.) They are gene- 

 rally larger than the spiral vessels. In other cases, as was 

 first observed by Hedwig, the adjacent coils are found to be 

 closely coherent throughout the greatest part of their course; 

 leaving, however, occasional intervals, where the external 

 membrane, being unprotected, appears, from its transparency, 

 as if spotted or perforated in various places (Fig. 16.) Every 

 intermediate stage may occasionally be seen in the transi- 

 tion from one of these forms to the other, in consequence of 

 the various kinds of convolution, of branchings, or of trans- 

 verse junctions of fibres, as well as the greater or less extent 

 of their lateral adhesions. All these varieties are met with, 

 not only in different vessels, but, as was observed by Mol- 

 denhavver and Kieser, even in the dififerent portions of 

 the same vessel, when followed by the eye throughout a 

 great extent of its length. Thus, in the course of the same 

 tube, (as seen in Fig. 17,) we find parts exhibiting spiral 

 fibres, which in other parts, bifurcate and again unite; and 

 in others, again, form rings: these may afterwards, by a 

 closer junction, present a reticulated appearance, or a series 

 of transverse lines, which, becoming smaller and smaller, 

 are at length mere points, arranged in circular rows around 

 the cylindrical surface of the vessel.* 



What are called the woody fibres have their origin, like all 

 other parts of plants, in cells. These are generally fusiform, 

 that is, of the shape of a double cone, very greatly elongated, 

 and placed close and parallel to one another, with the nar- 

 row extremities of one set wedged in between those of ano- 

 ther set (Fig. 18.) Their coats are more firm and elastic 

 than those of ordinary vessels, but do not appear to contain 

 any internal fibres, although they receive, in the progress of 



* Many disting-uisbed botanists, such as Rudolpbi, Link, Treviranus, and 

 Dutrocbet, consider these spots as being produced not by the deficiency of 

 the internal coating, but by the addition of granular bodies. See De Can- 

 doUe's Organographie Vegetale, torn. 1, p. 56. 



