52 TRANSPIRATION AND ASCENT OF SAP ch. 



according to Janse, consists in an absorption of water 

 on one side of the cell and a giving off of it on the other, 

 a result which is effected, he suggests, by the harmonious 

 action of enzymes and protoplasmic streaming. An enzyme 

 fixes water in the cavities of the protoplasm on one side 

 of the cell, and streaming brings it to the other side, where 

 a reversing enzyme sets it free. 



In order to get a clearer idea of the demands of this 

 theory, it is interesting to apply to it the figures already 

 obtained for the velocity of the transpiration current, and 

 those for the rate of protoplasmic streaming, and at the 

 same time to take into account the structure of stems. 



Ewart has shown that the rate of upward movement 

 during transpiration in stems of Conifers is about 7 cm. 

 to 10 cm. per hour. Higher figures are sometimes 

 given. According to Janse, this movement must be 

 due to the difference between the actual amount raised 

 by the medullary-ray-cells and the leakage back by filtra- 

 tion. The pressure under which this filtration takes place 

 is the pressure of a head of water equal to the height of 

 the tree. As will appear later, this head causes a flow 

 approximately equal to the rate of the transpiration 

 current. Therefore the pumping action of the cells must 

 continually pass an amount of water which would, if 

 there was no leakage, be twice that of the transpiration 

 current. It is obvious, however, that the motion 

 through the cells must be much more rapid than this, 

 as they occupy but a fraction of the cross-section of the 

 wood. 



A cross section of the wood of Pinus silvestris shows 

 that 



The lumina of the medullary ray cells occupy 6 9% i r . ,1 



,, ,, tracheids ,, 61*2% V , 



The walls of all 31'9%J cross sectlon - 



I am indebted to Mr. W. R. G. Atkins for these 

 measurements. They were made by cutting out and 



