IX. LEAVES AND THEIR WORK 



Problem XVIII. A study of leaves in relation to their 

 environment-. (Laboratory Mamtal, Proh. .YVIII.) 

 (a) Eedctions of stems and leaves to light, 

 (jb) Structure. 



(c) Important functions. 



{!) Absorption and resjnrabion. 

 (~) Food-making and its by-product. 

 {3) Evaporation of excess water. 

 (4) Tlie leaf as a mill (optional). 



(d) Jfeans of protection (optional). 



(e) Some leaf modifications (optional). 

 (/) Imnortaitce to man. 



Differences between Roots and Stems. — A comparison of the 

 young root and developing stem of a bean seedling show that sev- 

 eral marked differences exist : (1) the color of the stem is greenish, 

 while the roots are gray or whitish ; (2) the stem has leaves and 

 branches leaving it in a more or less regular manner, while the 

 smaller roots are extremely irregular in their method of growth ; 

 (3) the stem grows up- 

 ward, while the general 



direction taken by the 



roots is downward. 

 I Effect of LightonPlants. 



■ — In young plants which 



have been grown in total 



darkness, no green color is 



found in either stems or 



leaves, the latter often 



being reduced to mere 



scales. The stems are 



long and more or less re- 



chning. We can explain 



A pocket garden which has been kept in com- 

 plete darkness for several weeks. Notice the 

 bleached condition of stems and leaves. 



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