THE ROLE OF GREEN PLANTS 



253 



woody tissue. At one point on the petiole, usually close to the main 

 stalk, a little time before the leaves drop from deciduous trees in the 

 fall, a layer of delicate, thin-walled cells is formed which extends 

 completely across the petiole. This is called the separation or ab- 

 scission layer, and it is at this point that the leaf is cast off. 



How Green Plants Make Food 



The general biologist is concerned not so much with the structure 

 of the organism or with detailed minutiae as with the general 

 metabolism of an organism as a whole. He wants to know how plants 

 and animals act as living things, both alone and in relation to each 

 other. We have examined the green plant from the standpoint of 

 structure and are ready to consider it as an organic whole, as a living 

 organism that releases en- 



leof on live plant 

 + light- 



boilecL 



■— r^ in -^oodi 

 ■\>^ alcoViol 



positive reaction 

 "where sta.r-cVi was 

 locctte<:C 



ergy, respires, feeds, repro- 

 duces, and in time dies. 

 But we must remember that 

 in addition, the green plant 

 makes food, and it is this 

 process upon which we will 

 now focus our attention. 



It is a relatively simple 

 matter to prove that sun- 

 light is necessary for starch 

 making in a leaf. Place a 

 healthy green plant in dark- 

 ness for a couple of clays. 

 Then pin strips of black 

 cloth over parts of some of the leaves and expose the plant to bright 

 sunlight for a few hours. Later, remove the leaves and boil them to 

 soften the tissues, adding alcohol to extract the chlorophyll, and 

 finally, place them in a solution of iodine. A blue color will appear in 

 those parts of the leaves exposed to sunlight, while the covered areas 

 will be colorless. The appearance of the blue color in the presence of 

 iodine is the regular test for starch, thus showing clearly that sunlight 

 is necessary for starch making. 



Another simple experiment may be performed to show that air is 

 also a necessary factor. Place a healthy green plant in darkness for 

 two or three days, then carefully smear vaselin(> on th(> ui)i)(>r and 

 lower surface of two or three l(?aves, leaving the others uiitoiiclicd. 



Proof that light is necessary for starch forma- 

 tion in green leaves. 



