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the fact that, by means of the numerous stomata and large 

 intercellular spaces in the leaf tissue, the atmospheric air is 

 able to actually come in contact with the thin walls of the 

 living turgid cells in the interior of the leaf which are saturat- 

 ed with water. Room is thus made in these cells for more 

 water which accordingly flows into them from the vessels 

 and tracheids of the leaf vascular strands and continually 

 brings with it a fresh supply of the necessary salts. This 

 evaporation of water from the leaves is called transpiration. 

 Within certain limits plants are able to regulate and control Regulation of 

 the amount of water which shall thus escape from them by Transpiratioiu 

 closing the stomata, by developing a thick coating of hairs, or 

 other protective covering, from the epidermis, or by other 

 devices, and consequently the quantity of water transpired 

 from a given surface of living leaves is always less than that 

 which would have evaporated from an equal surface of water 

 while a dead leaf, also, loses water by evaporation quicker than 

 a living leaf. With this proviso, however, the factors on which 

 transpiration depends are essentially the same as those which re- 

 gulate ordinary evaporation, and the amount of water transpired 

 therefore depends on the temperature of, and amount of 

 water contained in, the surrounding air, the presence or absence 

 of air currents, and the area of the surface exposed to eva- 

 poration. As a general rule, the rapid removal of water 

 from the actively assimilating cells of the leaf is very necessary 

 to enable them to obtain the necessary quantity of salts they 

 require from the very dilute solution absorbed by the roots. 

 There are, however, obvious cases in which rapid transpiration 

 may be highly injurious, such as when the roots can obtain 

 very little water from a dry soil, and it is essential that plants 

 should be able to regulate this loss of water. That this can 

 only be done within definite limits is indicated by the fact 

 that, during a hot dry day, plants may frequently be seen to 

 droop and become flaccid, the amount of water which has been 

 lost by transpiration, in such cases, being so great that the 

 living cells have been unable to obtain sufficient water from the 

 roots to enable them to remain turgid. They have accordingly 

 become inactive and unable to perform their vital functions. 

 Unless the loss of water has been too great such plants usually 

 recover in the night, owing to the diminished transpiration. 

 In some cases when little or no evaporation can take place,, 

 owing to the saturation of the atmosphere, or other cause, 

 plants may become filled with an excess of water and salts 

 in solution which is pumped into them by the active roots. 



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