260 



THE MAINTENANCE OF THE INDIVIDUAL 



of nitrogen-filled tungsten lamps of from 200 to 1000 watts produced 

 both viable fruits and seeds. The bearing of this experiment upon 

 growing crops in areas where the days are short and the intensity of 

 sunlight not great is readily seen. Lamps have been put on the 

 market for use in the home which provide space directly underneath 

 the bulb for stimulating plant growth during the winter season. 



What Goes On in the Green Leaf in Sunlight 



When we examine the green leaf to see how it is adapted to use the 

 energy of sunlight, several interesting facts are discovered. One is 

 that a plant places its leaves so that they get the largest possible 

 amount of sunlight, in a given period. Petioles and even stems of 

 plants turn with the sun so that a maximum amount of green surface 

 is exposed to its rays. Looking at a tree from above as the bird sees 



Diagram to show the cells of the palisade layer of a leaf at two different times 

 during the day. Which of the two receives full sunlight ? 



it, leaves are found to be so arranged that there is a minimum amount 

 of overshading, the leaves forming a sort of mosaic or pavement on 

 which the sunlight falls. Examination of the internal structure of the 

 leaf also shows that the palisade layers which contain the greatest 

 number of chloroplasts per cell are massed close under the upper part 

 of the epidermis. It is this layer of palisade cells wdiere most of the 

 work of starch or sugar making takes place. In the cells themselves, 

 the green chloroplasts are so placed that a maximum amount of light 

 falls upon them. When the sun's rays are slanting during the morning 

 and afternoon, light can reach all of them readily, while at the period 

 of greatest illumination, when the sun's rays are direct, less light 

 reaches them as they lie one above the other. Their position may be 

 changed in the protoplasm, their movement being controlled by the 



