1. A biologist named Stephen Hales described how he carried out an investigation in 1727. 
  • - I cut a branch (b) off an apple tree about 1 metre long, then sealed the cut end (p) and tied a piece of wet bladder over it.
  • - Then I cut off the other end of the branch at (i) and attached a glass tube (z) to it. 
  • - After filling the glass tube with water, I placed the lower end in a bowl of mercury (x).

(Reproduced from Vegetab[e staticks, Bales, S., by permission of the Royal Botanic Gardens, Kew)
  • - I left the apparatus outside on a warm afternoon. 
  • - By 3.00 p.m. the mercury .had risen over 30 cm. 
  • - When the mercury reached the cut end of the stem (i) air bubbles appeared and the mercury slowly ran back into the bowl (x).
(a) Suggest a hypothesis that was being tested in this investigation.


(b)  (i) Through which tissue in the stem is most water transported?


(ii) Give one structural feature of this tissue which enables it to transport water rapidly (as shown in Hales's demonstration).


(c) Explain, in terms of the cohesion-tension theory, 
(i) why the level of mercury rose during the investigation;



(ii) why the level of mercury fell towards the end of the investigation.




NEAB 1998