CHAPTER VI. 



TRANSPIRATION. 



262. Transpiration. Various simple observations 

 show that water- vapour is given off by leaves and other 

 green organs, and that a current of water (transpiration 

 current) passes from the roots to the transpiring organs. 

 The root absorbs a very dilute solution of salts, in order 

 to obtain the essential elements for growth, and the excess 

 of water is got rid of by evaporation from the leaves ; but 

 transpiration is a process of evaporation controlled by the 

 living protoplasm of the plant. 



(a) Pull up a Broad Bean seedling and place it on a 

 dry piece of glass ; cover it with a tumbler, and note that 

 (a) the leaves and stem of the seedling become limp or 

 " wilted," (6) drops of water collect on the inside of the 

 tumbler owing to condensation of the vapour given off by 

 the leaves. 



(6) Gret three similar narrow-necked bottles filled with 

 water, and two cut leafy shoots of about the same size. 

 Put a leafy shoot into one bottle ; into the second a shoot 

 deprived of its leaves ; and leave the third as a control. 

 After some hours' exposure to light, compare the amounts 

 of water left in each bottle : which bottle loses most water, 

 and which least? Repeat the experiment, but put the 

 bottles in darkness, and note how the amount of loss of 

 water is affected. 



(c) Fix a long- stalked leaf, or a cut leafy shoot, in a 

 card, passing the leaf -stalk (or the stem) through a hole 

 in the middle of the card and sealing it up with putty or 

 plasticine. Place] several cards, each with a leaf or shoot 



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