VEGETAHLE ORGANIZATION. 65 



presenting resistance to any external force tending 

 to compress and close it.* 



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 regu- 

 lar intervals, and constituting what are called 

 annular vessels (Fig. 15). They are generally 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 ti^ansition from one of these forms to 

 the other, in consequence of the various kinds of 

 convolution, of branchings, or of transverse junc- 

 tions 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 Moldenhawer and Kieser, even in the 

 different 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 after- 

 wards, by a closer junction, present a reticulated 

 appearance, or a series of transverse lines, which, 

 becoming smaller and smaller, are at length mere 



* Vessels are sometimes met with which appear to be formed 

 simply by the coils of a spiral fibre in close juxtaposition, and unat- 

 tached to any external envelope, or connecting membrane. 

 VOL. I. F 



