46 VEGETABLE ORGANOGRAPHY. 



Kieser does not admit mixed vessels as a particular 

 class ; but he agrees much with the opinions of Mirbel 

 in this, — that he admits the passage of one form into 

 another. Thus, according to him, a tube may be partly 

 a trachea, and partly an annular vessel, as he has repre- 

 sented in the Balsam ; or partly a trachea, and partly a 

 reticulated vessel, examples of which are fomid in the 

 same plant. He thinks that, when they grow old, 

 most tracheae become dotted vessels, by the separation 

 of the spires or the rings, and by the development of an 

 intermediate dotted membrane : consequently, interme- 

 diate forms ought to be foimd from time to time between 

 these two periods. Lastly, he affirms that all kinds of 

 vessels become strangulated vessels at the articulations. 



For my own part, I have not any theoretical objection 

 to admitting transitions of form in the vessels; but I 

 confess that I have never seen them but in an obscure 

 manner, and one which has appeared to me doubtful, 

 either for fear of passing from one tube to another luider 

 the field of the microscope without suspecting it, or 

 because of the same difficulty which is also found in 

 classing these different forms. I am inclined to think 

 that these changes of form of one and the same tube 

 have rather originated from theoretical ideas than been 

 clearly seen by direct observation ; and I venture still 

 to direct observers to the verification of the facts. 



2d. Are vessels always simple, or do they ramify ? 



This question is as difficult as the preceding, and 

 equally merits the attention of observers. We find 

 several figures of the old anatomists where the vessels 

 appear ramified ; but it is difficult to recognise, either 

 in these figures or in the descriptions, whether they be 

 in reality ramified vessels, or bundles of vessels which 

 are distinct. Mirbel has most positively affirmed that 

 tubes really branched do exist, and he has given a figure 



