THE VESSELS. 53 



certainly found it once, but he considered it as an acci- 

 dental extravasation. 



It is also pretty certain that the vessels do not contain 

 the sap elaborated by the leaves, since they are for the 

 most part absent in the bark, v^^hich is the part of 

 plants v^rhere these elaborated juices pass in the greatest 

 quantity. 



The question then is reduced to this — Whether these 

 vessels usually, or by turns, conduct either the air, or 

 the lymph or unelaboSrated sap. That the vessels con- 

 tain air, appears to me to be shown by reasoning and 

 observation. 1. Since vessels only exist in plants 

 which have stomata, and as the stomata are orifices 

 open to the air, it is probable that the use of vessels 

 is in connexion with the air. 2. Most observers 

 affinn that the vessels have appeared to them to be 

 void of all liquid, or, in other terms, to be full of 

 air. 3. All those vessels which are perforated or 

 slit (and we have seen that in certain theories all 

 are so more or less) are very imperfect conductors of 

 fluid. 



On the other hand, those who pretend that the vessels 

 serve to carry the fluids, depend upon the following 

 facts : — 



1. In experiments where a plant has been compelled 

 to draw up a coloured fluid, we see clearly enough that 

 the walls of the vessels become coloured. I have seen it 

 very distinctly, especially in the annular vessels of plants 

 with a loose tissue. But it must be remarked that the 

 fact has only been well observed in the case where a 

 stem has been cut and the end plunged into coloured 

 water, consequently in circumstances slightly different 

 from the natural course of things ; and that in observa- 

 tions so delicate, it is impossible to distinguish if the 

 colouring takes place in the interior or exterior, that is 



