TRANSPIRATION. 207 



by ample watering under a bell- jar. Cut a shoot of each 

 under (1) eosin solution or red ink, (2) mercury, (3) 

 melted cocoa-butter coloured with lampblack. If a plant 

 with a very transparent stem (e.g. Balsam) is used, note 

 the height to which the liquid runs by simply holding the 

 shoot up to the light; if the stem is not transparent 

 enough for this, at once slit it longitudinally, or cut trans- 

 verse sections at different levels, and examine with the 

 microscope, to see how far the coloured liquid has reached 

 in the vessels. 



(c) Let a number of different plants become partially 

 dry and wilted, and cut from each plant two shoots, one 

 under water and the other in air. Place both with their 

 cut ends in water, and note which one remains wilted and 

 which one becomes fresh and flaccid. 



27O. Influence of External Conditions on Trans- 

 piration (Weighing Experiments). Various forms of 

 apparatus may be used for this inquiry experiments 

 with Potometers are given later. As might be expected, 

 transpiration is influenced largely by the same factors 

 as those controlling ordinary evaporation temperature, 

 humidity of the air, movement of the air. 



In weighing experiments the simplest plan is to set 

 a cut shoot in a bottle of water, then pour in olive oil to 

 form a layer on the water and prevent direct evaporation, 

 and weigh the whole apparatus. Or pot plants may be 

 enclosed with rubber sheeting, to prevent evaporation from 

 pot and soil, leaving only the shoot uncovered. Another 

 plan is to cover the soil with a divided disc of cork pre- 

 viously soaked in and smeared with wax, and to wrap 

 rubber sheeting over the pot ; to water the plant during 

 the experiment, fit a corked tube or thistle-tube into a hole 

 in the cork disc. 



A still better method is to use the " aluminium shells " 

 supplied by the Bausch and Lomb Optical Company 

 (Fig. 46) ; the rubber roof (made by cutting a hole with 

 a cork-borer in the middle of a piece of rubber sheet, cut- 

 ting a slit from the hole to the margin of the piece, placing 



