314 CONDUCTING SYSTEM 



as water. But he maintains, that the outermost water-channels which 

 are known to convey the greater part of the transpiration-current 

 always contain the smallest proportion of air. If both vessels and 

 tracheides are present, the former, which have the wider cavities, con- 

 tain the larger amount of air ; where, however, true vessels alone are 

 present as in the wood of Ficus, Acacia and Salix they never contain 

 much air. The protoxylem vessels of roots likewise contain very few 

 air-bubbles. 



A number of investigators have devised experimental methods for 

 determining the water- and air-content of vessels and tracheides. Boehm 

 was the first who endeavoured to prove the presence of water in vessels, 

 in opposition to the opinion prevalent at the time. He attempted to 

 force air into branches (of Sycamore, Horse-chestnut, Birch, Lime, etc.) 

 from 1-2 cm. in diameter and about 50 cm. in length, through the cut 

 end, with the aid of a column of mercury. He found all the vessels, 

 or at any rate those near the periphery, to be impervious to air under 

 an additional pressure of one atmosphere, though this pressure often 

 caused water to exude from the tracheal elements for a short time. 

 Boehm inferred from his experiments that the wood vessels contain 

 water. 



Hartig was the first to suggest that the vessels of transpiring twigs 

 and branches contain rarefied air. This fact was, however, definitely 

 established for the first time by Von Hohnel, who showed that when a 

 transpiring branch is severed under mercury, the latter rises in the 

 vessels to a considerable height (up to 70 cm.) in spite of the great 

 resistance opposed by capillary forces. Yon Hohnel's experiment clearly 

 proves that the tracheal air is in a highly rarefied condition, a phe- 

 nomenon which can only be explained by assuming that during periods of 

 active transpiration water is removed from the vessels faster than it can 

 be replaced by the osmotic pumping action of the adjacent living cells. 

 Since the vessels and tracheides are separated by almost air-tight walls 

 from the ventilating spaces of the plant, air only gains access very 

 slowly to the partially depleted water-channels; hence the reduction of 

 pressure which Yon Hohnel found to result from active transpiration. 



II. Hartig endeavoured to estimate the proportions of water and 

 air in vessels and tracheides by yet another experimental method, 

 namely, by accurate determinations of the weights of samples of wood. 

 He came to the conclusion that the water-conducting elements of the 

 woody cylinder contain both sap and air at all seasons ; he further 

 estimates that the proportion is sufficient, assuming the tracheal vvalls 

 to be completely saturated, to occupy at least one-third, but often as 

 much as two-thirds, of the lumen of each element in the case of 

 deciduous trees provided with true vessels. In Coniferous wood, which 



