THE WORK OF LEAVES 185 



are several inches high. The plants grow very tali 

 and slender and the leaves are small and yellow, re- 

 sembling the seed-leaves in color (Fig. 103). Put the 

 plants in the light for a day or so. Test them for 

 starch. Leave them in the light until they turn green. 

 Then test again for starch. Does the result indicate that 

 the green substance is necessary for making starch?^ 

 It is this substance, called leaf-green, or chlorophyll, 

 which is extracted by the alcohol when you test the 

 leaves for starch. Usually it is not formed in darkness. 

 Do you now see why the seed-leaves are not green ? 

 Eemove some of the earth so as to expose them to the 

 light. Do they turn green ? 



The seed-leaves and foliage -leaves are different be- 

 cause they have different tasks to perform, and their 

 structure must be adapted to the special kind of work 

 they have to do. We may sum this up by saying, 

 function determines structure. 



A good illustration of this is seen in the history of 

 the Castor- bean seed-leaves. At first they are absorb- 

 ing organs, pure and simple, and their structure is 

 admirably adapted to the work they have in hand. 

 Later, when they come to do the work of foliage-leaves, 

 they grow much larger, thicker, tougher and more 

 fibrous. They spread out above ground in a position to 

 catch the light (Fig. 104), and their internal structure 



1 Find out whether leaves variegated with white spots make starch in the 

 colorless portions. 



