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changes in the guard-cells are not clear. The chloroplasts 

 in them are thought to have something to do with the 

 matter, but no one has been able to show just what. 



The guard-cells are attached to each other at the ends. 

 As a consequence of this, when they are turgid (swollen 

 with water) , they bow out ; they curve away from each 

 other and the stomate thereby is opened. On the other 

 hand, when the guard-cells are flaccid (shrunken through 

 loss of water) they straighten elastically and practically 

 close the slit between them. Hence the stomates tend to 

 be open when the water supply is abundant and closed 

 when the water supply is scanty. A natural conclusion 

 from this is that guard-cells regulate transpiration. To 

 some extent they probably do regulate it, but it has also 

 been noted that guard-cells tend to open the stomates in 

 the light and close them in the dark. This evidence can- 

 not be interpreted like the other, for the reason that danger 

 from loss of water is greater when the sun is shining than 

 it is at night ; that is, the tendency of light to open the 

 guard-cells would conflict with the tendency of water short- 

 age to close them. 



In view of the responses of guard-cells to light, and in 

 view of their possession of chlorophyll, it has been held 

 that their behavior is related to photosynthesis rather 

 than to transpiration. In any case, whatever it is that 

 causes their opening or their closure, it is evident that the 

 principal advantage to the plant in possessing stomates is 

 that thereby those gas exchanges which are necessary for 

 photosynthesis are facilitated. Whether there is much 

 advantage in guard-cells as checkers of transpiration 

 is doubtful; if a plant is in real danger of drying out, 

 it is probable that the straightening of the guard-cells 



