44 VEGETABLE ORGANOGRAPHY. 



wliicli is confirmed by this circumstance, — that all, or 

 almost all the orders of vessels exist simultaneously in 

 certain classes of plants, and are as regularly absent from 

 others. 



It is also confirmed by the extreme diificulty which 

 most anatomists liave encountered in distinguishing these 

 orders of vessels with any certainty. Thus, for example, 

 there are some observers, such as DutrochetandRudol- 

 phi, who admit the existence of tracheas incapable of being 

 unrolled ; a state which, if it were well demonstrated, would 

 seem to establish a kind of identity between tracheae 

 and annular vessels. Dutrochet particularly afiirms, that 

 when these vessels are subjected to boiling in nitric 

 acid, we can destroy the junctions of their spires, and 

 render them capable of being unrolled. Kieser remarks, 

 that the rings of the vessels are often oblique; and 

 that they are seen to pass by degrees, in the same vessel, 

 into the form of true spires. The transition of annular 

 vessels into dotted vessels has been pointed out and 

 figured, by several of those who have thought that these 

 dots, whatever may be their nature, being continued, 

 form the transverse lines. 



The analogy of dotted vessels with strangulated vessels 

 is so great, that several observers have made little or no 

 distinction between them. 



It seems then admitted by observers, whether col- 

 lectively or in detail, that all these different organs are 

 only modifications of a single kind ; but nevertheless, in 

 conunencing on this theoretical foundation, many doubt- 

 ful points still remain to be examined. We proceed 

 now to pass them in review, not for the purpose of 

 solving them v^ith certainty, but to show the contradic- 

 tory tendencies, and the probabilities of the different 

 opinions : — 



1st. Does each of these vessels^ the structure of ivhich 



