78. 



When this takes place, the cells of the lower epidermis 

 again increase in size v;hile those of the roesophyll 

 tissue decrease and ultimately draw water out of the palisade 

 cells. But the palisade cells soon begin to decrease in size 

 and they ultimately drav/ water out of the cells of the 

 upper epidermis causing them to decrease in size, As the 

 water is continually "being drawn out of the cells of the 

 lower epidermis by evaporation, these keep on replacing 

 the loss with water Obtained by the process of osmosis 

 from the mesophyll cellslUnd the latter in turn continue 

 to replace t-i-v.""/^ ?res by water obtained from the palisade 

 cells which likewise continue to absorb water from the 

 cells of the upper epidermis, This process continues until 

 a certain limit is reached beyond which the leaf does not 

 give off any water .When that stage is reached the cells of 

 the upper epidei-rais have shrunk the most, since there was 

 no reservoir from v/hich they could draw their supply of v/ater 

 to replace the loss, Those of the palisade tissue shrink about 

 as ramch as those of the upper epidermis, while those of the 

 lower epidermis having had a continual supply during the 

 entire process of evaporation shrink considerably less than 

 those of the upper tissues so that the cells of the least 

 size appear' on the upper epidermis and those of the greatest 

 size are found in the lower epidermis. For this reason the 

 lower epidermis is exposed during curling,v/hile the upper 

 one is concealed, This explanation seems plausible when v;e 

 consider the increase in size of the cells during expansion 

 (as shown in Table VII ) as an index of the shrinkage during 

 evaporation. 



