208 



EXPERIMENTS WITH PLANTS 



of the stomata, as we may observe on very hot days, 

 and especially in hot dry winds. Place the apparatus 



out of doors in 

 the wind (the hot- 

 ter and drier the 

 wind the better) , 

 and observe the 

 effect on the rate 

 of evaporation. 



On the other 

 hand, the sto- 

 mata open as 

 soon as favorable 

 conditions for 

 starch- making (i. 

 e., the requisite 

 supply of water, 

 sunlight and 

 warmth) return. 

 The plant has in the stomata and guard -cells an 

 automatic apparatus of great efficiency, yet wonder- 

 fully simple, on whose proper working its very life 

 depends. If we examine a stoma carefully, we see 

 the two sausage -shaped guard -cells filled with green 

 chlorophyll granules, while the surrounding epidermal 

 cells have none. It is on this that their opening 

 and closing depends. Under conditions favorable for 

 starch -making, the guard- cells produce sugar, which 



120. Diagram showing in section and surface view the 

 form and position of the guard-cells when closed 

 (heavy lines) and open (light lines). 



