THE VESSELS. 45 



we have described in the preceding chapters, preserve the 

 same form throughout its entire length? — The uncer- 

 tainty on this subject has been caused by Mirbel, who 

 admits the existence of mixed tubes (tubes mixtes) — that 

 is to say, tubes which in different parts of their length 

 may be dotted, striped, or spiral like a gun-screw. " The 

 same tube," says he, '■' exhibits successively these dif- 

 ferent forms : a trachea of the stem may terminate at 

 the root in a strangulated vessel — may become a false 

 trachea at the knot at the base of a branch — may pass 

 over this in the form of a dotted tube, and resume in 

 the leaves or petals the form of a true trachea." Almost 

 all the anatomists who have WTitten since this opinion 

 has been published, have written against it, or at least 

 against its general application : several acknowledge 

 that they find some tubes which bear at the same time 

 short and long stripes, so that it may be perhaps ad- 

 mitted that annular and dotted vessels pass one into 

 another, and in this very restricted sense the existence 

 of mixed vessels may be admitted ; — but the majority 

 deny the other combinations. Rudolphi says, that he 

 believes that it is impossible for an annular tube ever to 

 change into a spiral vessel ; and he explains the assertion 

 of Mirbel, in thinking that he has, unconsciously, passed 

 from one vessel to another under the field of the micro- 

 scope. Dutrochet, in particular, affirms that there are 

 no mixed vessels, in the sense which Mirbel assigns to 

 this word ; and that the tracheae preserve their organiza- 

 tion throughout their whole extent. Amici says, that 

 he has never met with any vessels compounded of tracheae 

 and of tubes. The assertion of Mirbel, he adds, may 

 perhaps be only a simple hypothesis : every one who 

 has applied himself to the anatomy of plants, easily 

 comprehends the impossibility of following the course 

 of a vessel for so long a distance. 



