92 



THE VEGETATIVE ORGANS 



contents of the vessel. Weigh again, and note the difference. 

 To what is this increase due and whence has the water 

 come ? This giving off of water by living shoots is called 

 transpiration. A vessel with the dry calcium chloride 

 weighing together 16 grammes has been found by experi- 

 ment to have gained I gramme in weight in twenty-four 



hours ; i. e. the leaves have given off 

 during twenty-four hours i gramme 

 or i cubic centimetre of water. 



If this water has come from the 

 leaves, has it come equally from 

 the two surfaces, and are leaves 

 similar in this respect ? To deter- 

 mine these points, prepare a few 

 sheets of cobalt paper by dipping 

 pieces of filter-paper in cobalt 

 chloride solution and allowing them 

 to dry. Test a small piece of this 

 paper and note the changes in 

 colour that occur (i) when the 

 paper is warm, and (2) when ex- 

 posed to ordinary air or when 

 breathed upon. Obtain a dry duster, 

 fold it several times so as to make 

 a pad, and lay on it a few leaves, 

 some with the lower, others with 

 the upper surface to the duster. 

 Place over these a sheet of dry (blue) cobalt paper, and 

 cover immediately with a sheet of thick glass to exclude 

 the air from the cobalt paper. Try this with several kinds 

 of leaves, including some evergreens, and note the changes. 

 Which surface gives off the more water ? Are there any 

 differences in this respect in the different leaves you have 

 examined ? From what we have seen in the structure of 

 a Box leaf, is it likely that the differences noted can be 



Fig. 55. Experiment 

 to determine the 

 Amount of Water 

 given off by a plant. 

 a, vessel containing 

 calcium chloride. 



