Water-Conductivity of the Wood in Trees and Shrubs. 



249 



grounds for believing that one of tbe principal sources of the differences 

 between the evergreen and deciduous trees, with their very different 

 transpiration values, depends largely upon the smaller bore, and especially 

 on the shorter length, of the vessels in the wood of the former group. The 

 short length of vessels obviously involves more resistance to the passage of 

 water. The Holly may be cited as an example. It was not possible to 

 force clean mercury* through more than 2"o cm. of one well-grown specimen, 

 even at a pressure of 90 cm. of mercury continuously applied for 18 hours. 

 On passing water in which Indian ink was suspended through another stem, 

 the Indian ink only emerged through three vessels in a length of 6"5 cm., 

 while at 3 cm. it had filled up 20 vessels in 18 hours at 90 cm. of mercury 

 pressure. 



Shoots of coppice Ash are also resistant, though not so effectively, the 

 passage of mercury and Indian ink through 15 cm. length being almost 

 entirely limited to the vessels situated close to the pith. But, as the 

 ash sapling grows, of course the new wood is more vascular, and that of 

 adult trees is very porous. Strasburger found he could force mercury 

 at a pressure of 40 cm. through the wood of Oak branches to a length of 

 3-4 metres. Tlie vessels of this tree are of rather exceptional length and 

 width for a timber tree. It is possible that Indian ink or sepia suspended 

 in water might give even higher values, as the resistance offered to the 

 passage of mercury must obviously be considerable, unless it turns out that 

 the mercury actually burst the thin cross walls in tlie vessels. 



The principal results incorporated in this and the preceding communica- 

 tion may be summarised as follows : — 



1. A quantitative method has been described for estimating the water 

 conductivity of trees, shrubs, and herbaceous plants. The method depends 

 on measuring the volume transmitted through 15 cm. (in length) of the stem 

 (or root) delivered at a pressure of 30 cm. of mercury for a period of 

 15 minutes. The amount so transmitted is called the absolute volume. 

 By ascertaining with precision the area in cross-section of the wood, it is 

 practicable to reduce the absolute volume to a specific volume, which is the 

 ratio of water volume transmitted through the stem under the foregoing 

 pressure for 15 minutes, and an area of wood, the cross-sectional area of 

 which is 1 sq. cm. This ratio, or specific volume, can be used as a basis for 



* Mercury, which has been used by Strasburger and others for injecting the vessels, 

 is open to objections from an experimental point of view. I obtained far more leliable 

 results 'with a specially fine sample of Indian ink, which my colleague and friend, 

 Prof. H. G. Plimmer, was good enough to place at my disposal. 



YOL. XC. — B. X 



