TEE WORK OF LEAVES 217 



of Iris, Gladiolus, Eucalyptus, Acacia, etc. Observe 

 the behavior of Grasses in this respect. Many young 

 leaves remain vertical until their epidermis becomes 

 thick enough to prevent evaporation, when they 

 change to the horizontal position. Note as mciny cases 

 of this as you can. Many adult leaves change their 

 horizontal position to the vertical one in the middle of 

 hot summer days, when the light and heat are too in- 

 tense. Study the familiar plants about you and find as 

 many cases of this sort as you can. 



We have already learned (page 124) that plants 

 may have their roots submerged in water and yet suf- 

 fer from lack of it. This is because there are sub- 

 stances in the water which attract it away from the 

 plant. Try the experiment of placing leaves (or whole 

 plants) with their stalks dipping in strong salt solu- 

 tion. Plants in alkali lr.nds and along the seashore, 

 where the water is brackish, suffer in this respect and 

 consequently show the same devices to diminish evapo- 

 ration as we find in plants of desert or semi- arid 

 regions. 



While it is an advantage to have the leaf placed 

 vertically when the sun's rays pour straight down, 

 making the light and heat excessive, it will, under 

 ordinary circumstances, be far better to have the leaf 

 placed at right angles to the light, since in this posi- 

 tion the leaf can absorb the maximum amount. The 

 same leaves which in the excessive heat of midday 



