40 THE STORY OF PLANT LIFE 



leaf, gives off other veins, and these lead to the stomata 

 through the mesophyll, and so the water is given off, 

 as a rule, as vapour. 



If some leaves have the stalks closed with plasticine 

 and are placed under a dry bell-jar in a fairly warm 

 room in the light, it will be found that the inside of 

 the jar will, in a short time, be covered with drops 

 of water. This water is formed by the condensa- 

 tion of the water-vapour given off by the leaves. 

 The bell-jar prevents the removal of this vapour, 

 and the air inside the jar soon becomes saturated 

 so that water is deposited on the side of the jar 



(Fig. 4)- 

 The water passes from the tracheal vessels into 



the spongy parenchyma and palisade tissue, and 

 thence after exposure to heat, on evaporation, into 

 the intercellular spaces, and so out by the stomata, 

 but this can only happen at a proper temperature, as 

 a rule during the day, for transpiration and assimi- 

 lation go on simultaneously and the former assists 

 the latter. 



The palisade-cells appear to exercise the same sort 

 of osmotic pressure above that the root-hairs do 

 below upon the rising fluid. This water is required 

 in the leaves, enabling the chloroplasts to decompose 

 the carbon dioxide taken in by the leaves. 



In some cases water itself is given off in drops 

 by hydathodes or special cavities or exits for this 

 purpose. 



