MOVEMENTS OF WATER IN THE PLANT. 2$ 



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

 of the most effective is a covering of branching 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 300 times. 



(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. 



