THE TEANSPIKATION CUEEEXT 425 



has been shown to be the 3'oung wood of the axial stele, mto 

 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 sej)arated from the atmosphere, con- 

 taining air 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 structure 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 and swell, as 

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

 part with very easily. 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 \\ith 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 a.s root-jyressure. 

 AVe have also the inodihed 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 detail. 

 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 camiot, however, have nmch etfect 

 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 part similar 

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

 the fact that if the transj^iration current be made to contain sub- 

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

 are consequently killed, 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. 



EooT-PRESsuRE, — Weliave shown how the absorption of water 



