140 



All Food Is Made by Green Plants unit hi 



Fig. 166 Miles of green plants -a-ith tl:eir chloroplasts zvorkhig actively. Wl.iat are the 

 results of their work? How does the work of the chloroplasts benefit the plants? How 

 does it change the atmosphere around them? (department of conservation, Michigan) 



each made up of the elements carbon, 

 hydrogen, oxygen, nitrogen, and mag- 

 nesium. The only way you can obtain it 

 in the laboratory is to extract it from the 

 chloroplasts by means of alcohol. To- 

 gether with the chlorophyll, but hidden 

 by it, are yellow substances; one of these 

 is carotene, a substance very important 

 to you (see p. 178). It may be present in 

 the chloroplast in large amounts. It shows 

 clearly in carrots, apricots, sweet pota- 

 toes, and yellow corn where there is no 

 chlorophyll present to hide it. 



Usually chlorophyll forms only in the 

 presence of light although the yellow 

 substances may be made either in light 

 or dark. If a plant sprouts in the dark it 

 will not be green. On the other hand, 

 strong light causes the chlorophyll to 

 decompose or disappear, and a leaf ex- 

 posed to strong light is green only be- 

 cause its cells are acti\'e and new chloro- 



phyll is constantly being formed. In the 

 fall, as the weather becomes cooler, the 

 leaves form chlorophyll at a slower rate. 

 It is then that the yellow coloring, which 

 has been hidden by the green, shows up, 

 giving the brilliant yellow tints to some 

 autumn foliage. In some leaves, as less 

 chlorophyll forms a new red coloring 

 appears. 



The work of chloroplasts. A vast 

 amount of work may be done within a 

 green leaf. Do Exercise 3 to learn that 

 green leaves make starch in the pres- 

 ence of light. In the presence of light 

 each chloroplast is working actively. It 

 is combining two simple compounds; 

 water (H.O), which has risen to the 

 leaf from the roots, and carbon dioxide 

 (CO^,) which has entered the leaves 

 from the air through the stomata. And 

 what is the result of this combining, or 

 synthesis as chemists commonly call it? 



