SYNTHESIS, STORAGE AND BkKAKIX )\V\ n; 



It should be mentioned that two slightly different forms of chlorophyll 



(a and b) are always present in the chloroplasts, the significance of this being 

 unknown. Accompanying the green pigments are two yellow pigment" 

 Carotin and Xanthophyll, whose function in the chloroplasts is also uncertain! 

 It may be noted that the carotin of plants is of great importance to animal 

 nutrition, since it is from that substance that Vitamin A is produced in the 

 animal body. These yellow pigments also occur separately from chlorophyll 

 in yellow and red flowers and fruits and other organs, for example the carrot. 



Fig. 78. 



Results of experiment to show that Photosynthesis only proceeds in parts of the 

 plant that are exposed to light. See Text. 



Since light-energy is utilised in photosynthesis, it is not to be 

 expected that the process should continue in plants or parts of plants 

 that are deprived of light. If a potted plant is placed in darkness, 

 application of the iodine test will show that the amount of starch 

 present in the leaves gradually decreases until a negative result is 

 finally obtained. This usually requires from twenty-four to forty- 

 eight hours. The explanation is that the starch originally present 

 has been used up for respiration (see p. 133), or transported in soluble 

 form to other parts of the plant : and owing to the absence of light 

 no new starch has been photosynthesised. If the plant is returned 

 to light, starch formation is resumed. If, in a particular leaf, light 



