54 VEGETABLE ORGANOGRAPHY. 



to say, in the intercellular passages : though I incline 

 to the first opinion, yet I do not regard it as demon- 

 strated. 



2. In microscopic observations it is not rare to see 

 bubbles of air in certain vessels, especially in the annular 

 or dotted ; but a bubble of air is only visible when the 

 rest of the cavity is full of fluid. 



3. In the hypothesis that the vessels meet the 

 stomata, and that the stomata serve for the evapora- 

 tion of the water, it must be concluded that the vessels 

 contain it also. 



4. In the opinion of those who deny the existence of 

 intercellular canals, it is almost necessary to admit that 

 the lymph passes in the vessels ; and this is also the 

 opinion maintained by Mirbel and Dutrochet, whilst the 

 contrary opinion is defended by Kieser and Amici, who 

 admit the existence of intercellular canals. 



It seems then that the theoretical ideas upon the 

 general structure of plants have more influence than 

 direct observation upon the conjectures which are made 

 of the use of the vessels. Among these contradictory 

 arguments, it is without doubt difficult to arrive at a 

 final opinion; however, the great analogy which vessels 

 bear to one another, the probabihty of their transforma- 

 tion from one into the other, do not permit us to think 

 that their use can be very different ; but, as the trachea 

 in particular appears evidently an aerial canal, I am in- 

 clined to believe that all the other vessels perform the 

 same office. When a stem with a loose tissue is cut 

 transversely, the juice is never seen to exude from the 

 orifice of the vessels, but always from the cellular tissue ; 

 bubbles of air, on the contrary, are often seen to proceed 

 from them. I think then, definitely, with Kieser, that 

 the vessels are aerial canals ; but I do not mean to deny 

 that, in some particular cases of vegetation, they may 



