WATER-ABSORPTION AND TRANSPIRATION. 119 



Weigh all the pieces of paper together, and also a piece a foot square. 

 The former weight divided by the latter will give the approximate sur- 

 face area of the plant in square feet. From these data the average 

 amount of water evaporated in a given time from a square inch of sur- 

 face can be calculated. 



(Jc) Another method is based on the fact that dry calcium chloride 

 greedily absorbs water vapour, and if exposed to the air for a few days 

 even in dry weather it will absorb so much as to dissolve the salt, show- 

 ing that water vapour is always present in the air. Fix a seedling in 

 a bottle full of water, plugging up the hole in the cork, through which 

 the stem passes, with plasticine ; or get a plant growing in a pot of soil 

 and cover the soil with a rubber sheet, or with a Ia3 r er of melted wax. 

 Place beside the plant a watch-glass containing dry calcium chloride, 

 weighing watch-glass and salt ; cover the whole with a bell-glass, and 

 expose to light ; after a few hours, weigh the watch-glass and chloride 

 again, and note the increase in weight. To make the experiment more 

 accurate, set up a "control," by placing under a similar bell-glass a 

 watch-glass containing an equal weight of dry calcium chloride, and 

 expose along with the other apparatus, containing the plant, for an 

 equal time, to find out how much of the water- vapour absorbed was 

 already present in the air before the plant began to give off water- 

 vapour by transpiration. 



Instead of calcium chloride, copper sulphate may be used, though 

 it is not quite so effective. This salt when dry is white, but when 

 moist it becomes blue ; thoroughly dry the salt and place a weighed 

 quantity of it in a dry weighed watch-glass, and arrange the experi- 

 ment in the same way as before. 



* (I) Another method is to use a small plant, e.g. a garden Geranium, 

 growing in a tin fitted with a split cork through which passes a tube 

 for watering. Make water-tight with plasticine, and note the loss of 

 weight. Set beside the plant a vessel, e.g. a saucer containing water, 

 and in order to compare the loss of water from the plant and from an 

 equal area of water determine the area of the transpiring surface and 

 that of the water in the saucer. 



* (m) Cut three healthy leaves of Indiarubber plant or of Rhododendron, 

 plug the cut ends with plasticine, cover the lower surface of one (A) 

 with vaseline, cover the upper surface of the second leaf (B) with vase- 

 line, and leave the third (c) untouched except for the plug over the cut 

 end. Tie a piece of wire or string to each leaf, weigh each leaf care- 

 fully, then hang them up near each other and weigh them each day. 

 After several days the leaf whose stomates are blocked (A) will be still 

 green and fresh, while the others will be more or less withered and 

 brown. 



* (nl Get some cobalt chloride (nitrate or sulphate) ; make a solution 

 (about 5 per cent.) in water, and soak some filter-papers or sheets 

 of thin blotting-paper in the solution. Dry the papers, and observe 

 that they turn blue. Put a drop of water on one of the dried papers, 

 or simply breathe on it, and notice the change in colour. These cobalt- 



