537 



presence of air in the ducts that traverse wood, can scarcely be 

 held anomalous if when the wood is formed their function ceases. 

 The canals which ramify through a Stag's horn, contain air after 

 the Stag's horn is fully developed ; but it is not thereby rendered 

 doubtful whether it is the function of arteries to convey blood. 

 A gain, that air should frequently be found even in the vessels of 

 petioles and leaves, will not appear remarkable when we call to 

 wind the conditions to which a leaf is subject. Evaporation is 

 going on from it. The thinner liquids pass by osmose out of the 

 vessels into the tissues containing the liquids thickened by evapora- 

 tion. And as the vessels are thus continually drained, a draught is 

 made upon the liquid contained in the stem and roots. Suppose 

 that this draught is unusually great, or suppose that around the 

 roots there exists no adequate supply of moisture. A state of 

 capillary tension must result a tendency of the liquid to pass into 

 the leaves resisted below by liquid cohesion. Now, had the vessels 

 impermeable coats, only their upper extremities would under these 

 conditions be slowly emptied. But their coats, hi common with all 

 the surrounding tissues, are permeable by air. Hence, under this 

 state of capillary tension, air will enter ; and as the upper ends of 

 the tubes, being both smaller in diameter and less porous than tho 

 lower, will retain the liquids with greater tenacity, the air will 

 enter the wider and more porous tubes below the ducts of the 

 stem and branches. Thus the entrance of air no more proves that 

 these ducts are not sap-carriers, than does the emptiness of tropical 

 river-beds in the dry season prove that they are not channels for 

 water. There is, however, a difficulty which seems more serious. 

 It is said that air, when present in these minute canals, must be a 

 great obstacle to the movement of sap through them. The investi- 

 gations of Jam in have shown that bubbles in a capillary tube resist 

 the passage of liquid, and that their resistance becomes very great 

 when the bubbles are numerous reaching, in some experiments, as 

 much as three atmospheres. Nevertheless the inference that any 

 such resistance is offered by the air-bubbles in the vessels of a 

 plant, is, I think, an erroneous one. What happens in a capillary 

 tube having impervious sides, with which these experiments were 

 made, will by no means happen in a capillary tube having pervious 

 fides. Any pressure brought to bear on the column of liquid con- 

 tained in the porous duct of a plant, must quickly cause the expul- 

 sion of a contained air-bubble through the minute openings in the 

 coats of the duct. The greater molecular mobility of gases than 

 liquids, implies that air will pass out far more readily than sap. 

 "Whilst, therefore, a slight tension on the column of sap will cause 

 it to part and the air to enter, a slight pressure upon it will force 

 out the air and reunite the divided parts of the column. 



To obtain data for an opinion on this vexed question, I have 



