THE TEANSPIEATION CURRENT 425 



has been shown to be the young wood of the axial stele, into 

 which it is forced by a kind of filtration from the over-turgid 

 cortical tissue of the root. The wood vessels and tracheids 

 form a closed system, quite separated from the atmosphere, con- 

 taining ah- and a certain quantity of water. Sometimes they 

 are filled with the latter, often they contain a much larger 

 quantity of air. In no case probably are they during life free 

 from either, though the proportions of the two vary very greatly. 

 Much controversy has arisen as to whether the transport of the 

 water takes place through the substance of the cell-walls or 

 through the cavity of the vessels and tracheids. The character 

 of the former suggests that very possibly water may be carried 

 through them ; that indeed the structiu-e may be regarded as a 

 column of water held together by the molecules of the lignified 

 walls. This view is supported by the behaviour of lignin, 

 which while refusing to absorb much liquid aiad swell, as 

 cellulose does, yet can contain a certain quantity which it will 

 part with very easilj'. On the other hand compression of the • 

 vessels by a vice, if carried so far as partially or entirely to 

 obliterate their cavities, materially interferes with the rate of 

 flow. 



Two main causes appear to co-operate in maintaining this 

 upward stream. We have the constant pumping action of the 

 cortex of the root, giving us the force known as root-pressure. 

 We have also the modified evaporation from the surface of the 

 green parts of the plant, which we have spoken of as transpira- 

 tion. Both these may now be discussed in greater detaU. 

 Besides these two, other factors have been held to co-operate, 

 though much less certainly than they. The walls of the vessels 

 having extremely narrow calibre, capillarity has been suggested 

 as playing a part. This cannot, however, have much effect 

 in a system of closed tracheids, like those of the secondary 

 wood of the Conifers, which nevertheless conduct the water. 

 It has been thought that the living cells of the parenchyma which 

 abut upon the woody tissue of the stele may play a pa.rt similar 

 to the pumping action of the root. Against this theory we have 

 the fact that if the transpiration current be made to contain sub- 

 stances that are poisonous to the living ceUs, and the latter 

 are consequently kOled, the current still goes on. Nor do 

 differences of gaseous pressure within and without the plant, 

 or at different portions of the axis, explain the matter more 

 satisfactorily. 



RooT-PHESsuRE, — We have shown how the absorption of water 



