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 1 . 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. 



