198 Our Field and Forest Trees 



through which air and moisture can pass freely. 

 Some leaves have such pores on their upper sur- 

 faces also, and we find similar tiny openings on 

 green stems, on young fruit, and on branches less 

 than a year old. These openings are " stomata," 

 or mouths. They open into little spaces among 

 the green cells of the leaf, and their use is 



best described by the 

 word " transpiration." 

 Thanks to them, 

 leaves can breathe 

 away any moisture 

 which the plant does 

 not need. 



Each mouth opens 

 between two cells 

 (Fig. 50), somewhat 

 like a pair of lips. 

 Like all their neigh- 

 bor cells, these lips become swollen in moist 

 weather, when water is rising fast from the roots 

 into the foliage, and they shrink and droop in 

 times of drought. When they swell they stand 

 apart so that the little mouth looks as if its lips 

 were whistling, and moisture can pass out between 

 them. Then as soon as the leaf has parted with 

 the moisture it does not need, all its cells become 

 somewhat limp, and the stomata lips droop 

 together so that the mouths are closed (Fig. 51). 

 If leaves get thoroughly soaked, so that water 



Fig. 50. Structure of the epider- 

 mis of a leaf, showing the stomata, 

 or little mouths. 



