TRANSPIRATION 23 



the weight at five or six successive periods, and then, 

 as directed by the instructor, plot on section-paper 

 a curve of your readings. Lay off the observed 

 weights as ordinates, the time-intervals as abscis- 

 sae. Be sure that in this and all subsequent ex- 

 periments your inferences are only those warranted 

 by your observations. 



Experiment 2. To ascertain one cause of loss of weight 

 of plants. 



1. Take four clean, dry glass beakers or tumblers, two 

 pieces of cardboard large enough amply to cover the 

 opening of the beaker, and a vigorous green leaf 

 having a leaf -stalk and a perfectly dry surface. 



2. Fill two of the glass beakers or tumblers three- 

 fourths full of water, insert the leaf-stalk through 

 a small hole in the center of one piece of cardboard, 

 make the opening as tight as possible about the 

 leaf -stem, using cotton if necessary, and place the 

 cardboard over one of the water-containing beak- 

 ers, so that the leaf-stalk extends down into the 

 water. Invert one of the dry beakers over the leaf. 

 Arrange the other two beakers and cardboard in 

 the same way, only omitting the leaf and the hole 

 through the cardboard. This second set of beak- 

 ers is the control (cf. p. 3, 1f6). 



3. Place both sets of beakers in a well-lighted window, 

 preferably in direct sunlight, and from time to time 

 observe and compare the appearance of the inner 

 surfaces of the inverted beakers. 



4. Do you notice any difference in the result on oppo- 

 site sides of the leaf? If so, describe. 



5. Can you see any water passing from the leaves? 

 In what state, therefore, does it pass off? From 

 what part of the leaf does it come? Why do you 



