DAILY MOVEMENT IN POTATO. 



31 



the stomata of both surfaces of the leaves were wide open. At 1 

 p. m. the lower had closed to 30 per cent, although the upper were 

 still open. At 2 p. m. the lower stomata were closed and the upper 

 were but 40 per cent open. By 3 p. m. the stomata of both surfaces 

 were closed and so remained until 9 p. m. when the upper stomata 

 showed a slight amount of opening. At 10 p. m. the lower stomata 

 also started to open; the upper had then opened to 30 per cent of 

 maximum. At 11 p. m. the upper stomata were 60 per cent open and 

 the lower 15 per cent. At midnight the upper stomata were wide 

 open and the lower were 40 per cent open. At 1 a. m. the lower 

 stomata were 80 per cent open and fully open at 2 a. m. The stomata 

 in both surfaces were open until 11 a. m., when the lower stomata 

 suddenly began to close. At 12 noon the lower were but 10 per cent 



100 

 90 

 60 

 70 

 60 

 SO 

 4C 

 3O 



eo 

 to 



'B 



NOOW IZ3456789IO 



MT. I Z 3 * 6 6 7 8 9 10 II NOOH I 



FIG. 14. Series 20, showing movement in upper (A) and lower (B) stomata 



of potato, plot 7. 



open and the upper stomata started to close. At 1 p. m., when the 

 series ended, the lower epiderm showed all stomata closed and the 

 upper but 15 per cent open. The stomatal movement in the upper 

 epiderm of alfalfa for the same time shows that potato is not so 

 susceptible to evaporation as alfalfa. Nevertheless, the stomata of 

 potato, like those of alfalfa, are influenced by these factors. 



Less available water tends to increase the day closure. When it 

 is very much less, the stomata close very early and the entire plant 

 shows signs of wilting. During the night such a plant will recover 

 and its stomata again open. When the small reserve of water gained 

 overnight begins to disappear, the stomata again close and the 

 plant assumes once more a semi-wilted appearance. Hence, as in 

 alfalfa, the stomata of potato react to excessive water-loss by closure. 

 This cuts down the period for the absorption of carbon dioxide and 

 naturally the amount of photosynthesis which can occur in such a 

 plant, without regard to other effects due to water-loss. 



The stomata of potato show other interesting differences as com- 

 pared with those of alfalfa. One variety, the name of which was un- 

 known, had hypostomatal leaves. The variety used in these experi- 



