THE ROLE OF GREEN PLANTS 



259 



Relation of Artificial Light to Food Making 



We have already noted that there are great differences in the 

 amount of sunhght required by plants. As a matter of fact, very 

 strong sunlight may cause harm since it overheats the protoplasm, 

 thus endangering the life of the plant. Moreover, it increases the 

 rate of transpiration so that water is evaporated too rapidly. Experi- 

 mental evidence with growing plants shows also that too much sun- 

 light may retard growth. Some plants are shade loving, as may be 



Shade loving plants on a forest floor. Note the leaf arrangement with reference 



to light. 



seen in any field trip to a forest. The differences in illumination are 

 correlated with differences in the structure of the leaf, the ])lants 

 which are exposed to bright sunlight having a well developed palisade 

 layer, while the spongy parenchyma is not so well developed. The 

 reverse is true in shade-loving plants. In addition, plants that live 

 in the shade are apt to have a very thin epidermis and usually ha\-e 

 dull leaf surfaces which do not reflect the light as reatlily. 



Contrary to common belief, it is possible to grow i:)lants without 

 sunlight as pro^'ed by recent experiments (Harvey) with a large 

 number of different crop plants such as grains, tomatoes, squash, 

 peas, potatoes, and others. Plants exposed continuously to the light 



