88 



BIOLOGY FOR BEGINNERS 



light and air, on both of which all the activities of the leaf 

 depend. 



Arrangement. Not only are leaves adapted by their shape for 

 this exposure, but by their arrangement on the stem. Look at a 

 tree from above or at a house plant from the " window side " and 

 observe that the branches and leaf stems (petioles) have so ex- 

 tended and twisted them- 

 selves, that each leaf is 

 exposed and very few cast 

 any shade upon their 

 neighbors. 



Heliotropism. Another 

 adaptation for leaf ex- 

 posure is their ability to 

 constantly turn them- 

 selves toward the light. 

 This is an every day 

 observation, but no one 

 can explain just how they 

 do it. The process is called 

 heliotropism (which means 

 sun turning), and is very 

 essential to the work of 



the leaves. Roots turn from light (negative heliotropism) while 

 this response made by leaves toward the light is termed positive 

 heliotropism. 



Modified Leaves. Like roots, leaves are often modified to 

 perform special functions: They may be reduced to mere ten- 

 drils for climbing (pea) or they may develop as thorns for protec- 

 tion (barberry). They may thicken up with stored nourishment 

 and even reproduce the plant (live-f or-e ver) , or most curious of 

 all, may develop into traps for insects (sundew and pitcher-plant) . 

 Fall of Leaves. Most plants of temperate climates shed their 

 leaves, either all at once in autumn (maples, elms) or a few at a 

 time the year round (pines and spruces). They do this so they 

 may rid themselves of waste mineral matter that has accumulated 



FIG. 22. Sunflower with young head turned 

 to the morning sun. From Atkinson. 



