26 STUDIES IN HISTOLOGY. 



with the substitution of a salt of lithium for the dj-e, and deter- 

 mine the rate of ascent. 



It will be well for the student at this point to review carefully 

 the principal facts regarding the amount of moisture which the 

 atmosphere can take up at different temperatures. In all trans- 

 piration experiments he should determine the percentage of 

 moisture in the atmosphere to which the leaves of the plants 

 are exposed, and for this purpose the well-known Hygrodeik, 

 or Hygrophant, may be employed. But if only the simple wet 

 and dry thermometer bulbs are at hand, the student can find 

 all necessary data for his calculations in the tables published by 

 the Smithsonian Institution. 



Place in a watch-glass under the microscope water containing 

 finely powdered indigo, and immerse in it the clean-cut surface 

 of a leafy shoot. Observe in which direction the indigo particles 

 move. 



X. TRANSPIRATION, OR EXHALATION. 



Repeat the following experiment devised by Henslow : " Take 

 six or eight of the largest, healthiest leaves you can find, two 

 tumblers filled to within an inch of the top with water, two 

 empty dry tumblers, and two pieces of card each large enough 

 to cover the mouth of the tumbler. In the middle of each card 

 bore three or four small holes just wide enough to allow the 

 petiole of a leaf to pass through. Let the petioles hang suffi- 

 ciently deep in the water when the cards are put upon the tum- 

 blers containing it. Having arranged matters thus, turn the 

 empty tumblers upside down, one over each card, so as to cover 

 the blade of the leaves. Place one pair of tumblers in the sun- 

 shine, the other pair in a shady place. In five or ten minutes 

 examine the inverted tumblers." 



Tie a piece of thin rubber-cloth around the flower-pot and 

 lower part of the stem of any young leafy plant, and weigh the 

 whole upon a common balance capable of turning with a deci- 

 gram, under a lead of two or three kilograms. If nothing better 

 can be procured, one of the best forms of small platform balance 

 will answer. A thistle-funnel should be tied up with the stem, 

 so that water can be supplied to the plant as required. Ascer- 

 tain the amount of transpiration from the foliage of the plant 

 during twenty-four hours under the following conditions: (1) at 

 a temperature not falling below 60 F. (about 16 C.) ; (2) at a 

 temperature not rising above 40 F. (about 4 C.). 



What is the loss of moisture in one hour under direct exposure 

 to the brightest sunlight? Note temperature and moisture in the 



