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PLANT PHYSIOLOGY 



chyma cells by which it is surrounded are what expand or 

 close, raise or lower, the leaflets and the whole leaf by 

 changes in the amount of water which they contain, in 

 other words, by changes in their turgescence. These par- 

 enchyma cells act as the cushion on which the blade 

 of the leaf rests. As has been repeatedly described in the 

 text-books, and in numberless works on "the wonders of 

 Nature,"* the position of the leaves and leaflets varies, 

 or may be made quickly to change, according to the con- 

 ditions surrounding the plant. The accompanying figures 

 illustrate the periodic changes occurring in the position 



Figure 20. " Sensitive Plant " (Mimosa pudjca) by day (a) , by night ( 6) , 

 and In light and air of excessive brightness and dryness (e)-. From 

 MacDougal. 



of the leaves and leaflets of Mimosa pudica under normal 

 conditions. Figure 20 a is the ordinary " day position," when 

 the light is fairly strong, the air warm, and water suffi- 

 ciently abundant to allow rapid transpiration without harm. 

 Figure 20 b is the ordinary "night or sleep position," when 

 the light is weak, the air cool and moist so that dew will 

 form. Figure 20 c is the position taken when the illumina- 

 tion is so intense and transpiration so rapid that there 

 is danger of excessive loss of water.t 



* For example, see " Living Plants and their Properties," by Arthur and 

 MacDougal, Chapter IV., New York, 1898, from which the following de- 

 scription is mainly drawn. 



t For a discussion of these movements with relation to transpiration 

 see pp. 141-2. 



