CHAPTER II. SPECIAL PHYSIOLOGY OF THE NUTRITIVE FUNCTIONS. 703 



water being absorbed and then returned under pressure into the 

 vessels by the adjacent parenchymatous cells. This view, based 

 principally on anatomical facts, is supported by but little direct 

 evidence, although it has been ascertained in certain cases that 

 the parenchymatous cells of the stem are capable of forcing 

 liquid into the xylem : for instance, when a piece of a grass- 

 haulm is placed with its lower end in wet sand, drops of water 

 may be observed to exude from the upper cut surface after a 

 time. But in view of the fact that the transpiration- current can 

 be maintained through a considerable length of stem killed by heat, 

 as also of the fact that fatally poisonous solutions may be thus 

 conveyed upwards through the wood for a length of time which 

 ensures the death of all the adjacent parenchymatous cells, it is 

 difficult to see how this theory can be maintained. 



It has also been suggested that the current is due to purely 

 physical causes, such as capillarity and the difference between 

 the pressure of the external atmosphere and the lower pressure of 

 the gases in the xylem of the plant, or the differences of pressure 

 of the gases in the lower and upper parts of the plant. With 

 regard to capillarity, it need only be pointed out that in 

 many cases {e.g. Coniferee) the conducting-tissue of the xylem 

 does not consist of continuous capillary tubes, but of closed 

 tracheids; here it is impossible for a column of water to be 

 raised by capillarity, and yet the current is maintained ; and 

 even in plants with continuous xylem-vessels, the force of capil- 

 larity would be altogether inadequate for the maintenance of the 

 current. With regard to the " gas-pressure-theory," it will suf- 

 fice to point out that, even if it were well-founded, it could only 

 account for the raising of water in the plant to a height of thirty- 

 two feet at the utmost ; but it is not well-founded, for inasmuch 

 as the xylem-system is air-tight, being shut off from all commu- 

 nication with the external air (see p. 678), the movement of fluids 

 within it is in no degree affected by the atmospheric pressure, 

 and the internal differences of gas-pressure are altogether in- 

 adequate. However, though neither capillarity nor differences of 

 pressure can be regarded as the active cause of the current, the 

 maintenance of the current is affected both by the capillarity of 

 the vascular tissue through which it travels, and by the varying 

 pressure of the gases which that tissue may contain. 



Two facts have been made clear by the foregoing considerations : 

 first, that the water contained in the wood is readily mobile, a 



v, s, B. z z 



