52 THE BOOK OF NATURE STUDY 



In order to determine the exact amount of water lost by plants 

 in transpiration, experiments may be tried with two sets of plants, 

 one set being allowed to grow under the ordinary conditions in 

 the greenhouse, whilst the plants of the second set, of the same 

 species as those in the greenhouse, and as far as possible the same 

 size and age, are kept in an experimental greenhouse under con- 

 ditions that can be controlled. In an actual experiment con- 

 ducted in this way, the amount of water transpired was determined 

 by weighing, and special precautions were taken to prevent loss 

 of water through evaporation. The result of the experiments 

 has been thus recorded : " There are two daily extremes, a 

 maximum loss around noon when the sunlight is most intense, heat 

 usually the greatest, moisture least in the atmosphere, but a 

 good supply of water in the soil around the roots ; the minimum 

 loss occurs some time during the night when the temperature is 

 low, the atmospheric pressure approaches saturation, the dark- 

 ness is complete, and in most plants the stomata are closed." 

 The figures given showed that the amount of water transpired is 

 about five times as much on the average per hour per square 

 metre by day as by night. Another important fact brought out 

 by such experiments is the extreme sensitiveness of transpiration 

 to even slight changes in external conditions. The plants which 

 are well adapted for class experiments are Chrysanthemum 

 frutescens, Tropceolum majus, and several species of Pelar- 

 goniums. 



It is interesting to inquire by what tissues the water travels 

 through a plant. If a young twig with the cortex stripped off 

 between two leaves is fixed in a cork, so that the cut end dips 

 into a jar containing water, it will be found that the twig does not 

 wither ; the leaves remain spread out and do not droop, as they 

 would do if they were not supplied with water. Clearly the water 

 is not travelling along the cortex of the stem. In order to find 

 out along which tissue it does travel, red ink may be added to 

 the water, and after some hours, sections of the stem should be 

 cut. The woody part of the stem is stained. The water travels 

 along the wood. This tissue can be recognised with the naked 

 eye, and looks like white lines. With care it can be traced from 

 one internode to another, in such plants as the Dead-nettle. 



