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XVI. 



AN IMPROVED SIMPLE FORM OF POTOMETER. 



By G. H. PETHYBRIDGE, Ph.D., B.Sc. 



[Read, November 17 ; Received for Publication, December 18, 1903 ; 

 Published, February 13, 1904.] 



In view of the demand which exists for simple forms of apparatus 

 for use in the study of plant-physiology, the following account 

 of a form of potometer suitable for studying the action of various 

 external circumstances on the rate of transpiration may he of 

 service to those interested in this subject. 



Most of the forms of potometer described in the text- books are 

 complicated to arrange, and inconvenient to remove quickly from 

 place to place. This applies, for instance, to the form described 

 and figured by Darwin and Acton, 1 where it will be seen that, 

 in addition to the potometer proper (for which a special form of 

 U tube has to be made), a retort-stand with two clamps as well 

 as two blocks of wood are required to complete the apparatus. 

 Moreover, I have found this form of potometer especially dis- 

 advantageous, from the fact that, owing to there being a constant 

 pull of some twelve to eighteen inches of water on the cut surface 

 of the leafy shoot, unless the pull exerted by the transpiring shoot 

 is considerable, air is drawn into the limb of the potometer from 

 the intercellular spaces in the shoot, so that its cut surface soon 

 comes to be standing not in water, but in air. 



A very simple form of potometer has been described and 

 recommended, consisting merely of a wide-mouthed bottle, pro- 

 vided with a double-holed rubber cork. Into one of these holes 

 the leafy shoot is inserted, and into the other an L-shaped piece 

 of glass tubing of narrow bore, the long limb of the L being 

 horizontal. On filling the bottle with water to the brim, and 

 inserting the cork, the surplus water is forced out through the 

 glass tube. On the leafy shoot commencing to absorb water, and 



1 " Practical Physiology of Plants." Exp. 92, fig. 15, p. 80. 2nd Edit. 1895. 



SCIENT. PKOC. R.D.S., VOL. X., PART II. N 



