On the transpiration current in plants

In a paper published in 1905, professor Ewart investigates the question as to what force is required to move water through the waterways of plants at the same velocity as the transpiration current. His general conclusion is that the resistance is so great that neither are sufficient forces generated in the leaves o raise the water at the required velocity, nor is the tensile strength of water adequate to tansmit these forces downwards, if such existed. In a subsequent paper he seems to have modified this latter view and quotes Berthelot as stating that air-free water support a tension of over 200 temperatures. It is needless to criticise Ewart’s calculations of the resisitance based on Poiseuille’s formula, which he himself admits is quite inapplicable to the case, owing to the presence of cross-partitions and irregularities in the cross-sectionsof the tracheal tubes. It may be noticed, however, that when care was taken that discontinuities were not present in the water columns of the wood experimented upon, the flow observed approximated to the flow calculated by the formula. In an experiment to the flow calculated by the formula. In a experiment on a piece of yew wood the approximation was very remarkable. The actual amount transmitted through a length of 15cm. Was 4·2 c. c. Per hour, while the calculated amount was 9·8 c. c. The average distance from one another of the cross-partitions in the fine tubes composing the wood is 0·25 cm. Therefore about 60 partitions must be traversed in passing through a length of 15 cm. This indicated that the resistance offered by the walls, or rather by the pits in the walls, to the passage of water is very slight.