ROOT PRESSURE 



77 



therefore (p. 143) the swelling must be attributed "to some other 

 cause than the stoppage of the sap in its return downwards," 

 because the first gap in the bark should be sufficient to check 

 the whole of the flowing sap^ He must in fact have seen that 

 there is a redistribution of plastic material in each section of 

 bark. ) 



We^now for the moment leave the subject of transpiration 

 and pass on to that of root-pressure on which Hales is equally- 

 illuminating. 



Figure from Vegetable Staticks showing a vine with mercury gauges in place to 



demonstrate root-pressure. 



His first experiment, Vegetable Staticks, p. 100, was with 

 a vine to which he attached a vertical pipe made of three 

 lengths of glass-tubing jointed together. His method is worth 

 notice. He attached the stump to the manometer with a " stiff 

 cement made of melted Beeswax and Turpentine, and bound it 

 over with several folds of wet bladder and pack-thread." We 

 cannot wonder that the making of water-tight connexions was 

 a great difficulty, and we can sympathise with his belief that he 

 could have got a column more than 21 feet high but for the 

 leaking of the joints on several occasions. He notes the 



1 He notices that the swelling of the bark is connected with the presence of buds. 

 The only ring of bark which had no bud showed no swelling. 



