THE MOLECULAR FORCES IN PLANTS. 209 



face at the higher temperature was sucked back again owing to 

 the cooling and consequent contraction of the contained air. 3 



1 See Moll, Botan. Zeitung, 1880. 



2 See Wilson, in Untersuchunfjen aus d. botan. Institnt zu Tubingen, Bd. 1, 

 Heft 1. 



8 See Sachs, Botan. Zeitung, 1860. 



81. The Organisation of Plant Structures and Transpiration. 



The tissues of plant structures are by no means all easily per- 

 meable to fluid water or to water vapour. On the contrary, there 

 are tissues which allow the passage of water only with extreme 

 difficulty, and to this class belongs especially cork tissue. This 

 fact is of considerable biological interest. Thus we find that 

 many watery structures, e.g. potatoes, which have to survive a 

 long period of dormancy, are clothed with a more or less thick 

 layer of cork, and in order to determine its significance, in the 

 maintenance of the watery condition of the parenchyma of the 

 tubers, it is sufficient to make the following experiment. 1 We 

 select two potatoes as nearly as possible of the same size. One of 

 them is peeled in order to remove the cork tissue, the other is left 

 unpeeled. We now weigh the two potatoes, and then place them 

 side by side. Further weighings made at the end of three, six, 

 and twenty- four hours respectively, indicate that the peeled 

 potato loses much more water than its neighbour. The small 

 amount of water which the unpeeled potato loses escapes chiefly 

 from the lenticels and from fine rents in the skin. 



If we select two apples as nearly as possible of the same size, 

 peel one of them, leaving the other unpeeled, and determine from 

 time to time, e.g. every twenty-four hours, the weight of each, it 

 appears that the apple deprived of its skin gives up far more 

 water to the air than the unpeeled one. Cuticularised epidermis 

 is therefore, like cork tissue, at any rate only very slightly per- 

 meable to water. 2 



The small quantity of water given off by unpeeled potatoes and 

 apples is chiefly to be accounted for by the presence of lenticels, 

 and in order to prove directly that these organs are not without 

 importance in determining the amount of transpiration, we make 

 the following experiments. Two pieces of ^Esculus or Ampelopsis 

 twigs as similar as may be, without leaves, covered with periderm, 



P.P. P 



