MOVEMENTS OF WATER IN THE PLANT. 25 



piration may be lessened or controlled, are very numerous. One 

 of the most effective is a covering of bristly hairs. Much the 

 greater part of the water thrown off by the plant is transpired 

 from the thin-walled cells in the interior of the leaf into the inter- 



Fig. 23. 



Air-chamber and opening of Marchantia polymorpha : magnified 300timesv 



(Kerner.) 



cellular spaces which communicate with the open air through 

 the stomata. The stomata (Figs. 22 and 23) are openings in 

 the epidermis, which are controlled by guard-cells. When 

 more water is transpired from the leaf than is furnished by the 

 roots, the guard-cells become flaccid, and the walls are thick- 

 ened in such manner that in this condition these cells change 

 their form and close the openings of the stomata entirely. 

 When the necessary water-supply is at hand the guard-cells are 

 turgid and the stoma remains open. The action of the guard- 

 cells is also influenced by light, wind, and other factors. 

 Transpiration is increased by heat, light, dryness, high pres- 

 sure, and movements of the air, and lessened by the opposite 

 conditions. 



EXPERIMENT 29. 



INFLUENCE OF HUMIDITY ON THE AMOUNT OF TRANSPIRATION. 



Place a well-leaved Begonia grown in a pot, on one pan of a drug- 

 gist's balance. Cover the soil by means of two glass plates, or tie a 

 piece of oiled cloth around the entire pot, to prevent evaporation. 



