IX. LEAVES AND THEIR WORK 



Problem XVIII. A study of leaves in relation to their 

 environment. (Laboratory Manual, Prob. XVIII.} 

 (.a) Reactions of stems and leaves to light. 



(b) Structure. 



(c) Important functions. 



(-0 Absorption and respiration. 

 (#) Food-making and //* J>i/-product. 

 (?) Evaporation of excess water. 

 (4} The leaf as a mill (optional). 



(d) Means of protection (optional). 



(e) Some leaf modifications (optional). 



(f) Importance to inan. 



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. 



Effect of Light on Plants. 

 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 



, , , A pocket garden which has been kept in com- 



long and more or less re- plete darkness for Kvertl weeks . Notice the 

 dining. We can explain bleached condition of stems and leaves. 



115 



