Mechanism of Transpiration 47 



used by them, it will be very helpful to know in what manner 

 this water is moved through and from the plant, and just what 

 part it plays in plant life. 



We may understand the essentials of this complex process 

 best if we compare it with our own breathing ; for transpiration 

 and respiration of land plants have much in common with the 

 breathing of animals. Both the plant and animal breathe air, and 

 while breathing it, both give off large quantities of water from the 

 organs of respiration. If you hold a cold, clean mirror in front 

 of a person breathing, its surface becomes at once clouded with 

 the moisture from the breath. So, too, if you hold the same 

 cold mirror close to the foliage of a growing plant, the moisture 

 escaping from that will also cloud the mirror. 



Now, the primary object of the lungs in our case is not to 

 remove water from the system, but to provide a means for oxy- 

 gen to enter the blood from the air, and for the carbonic acid 

 gas to escape from the blood into the air. This can take place 

 rapidly, however, only when the delicate lining of the air cells 

 in the lungs is kept moist ; and so the chief function of the 

 water escaping from the lungs is to maintain their inner surface 

 continually wet. Let the lung lining once become dry, and the 

 rate at which oxygen could enter and carbonic acid gas escape 

 from the blood would be so slow that life could not be main- 

 tained ; and in order that this fatal accident shall not occur, the 

 lung surface is placed on the inside of the chest, where the rate 

 of evaporation is very greatly impeded. 



When we turn to the breathing of plants, we find that they, 

 too, are only able to accomplish that very important work as 

 rapidly as it needs to be done by having a very broad surface 

 against which the air may come, but so placed that it shall be 

 kept always wet ; and, just as in our case, it would never do to 

 have this surface exposed to the open air, so the real breathing 

 surface of plants is spread out on the inside of their structure, 

 where hot, strong winds can never reach it. 



In Fig. 5 is represented a piece of a barley leaf, partly dis- 

 sected and much magnified, which shows the breathing surface of 

 this plant, and how it is protected from excessive evaporation, 



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