142 



CONSTRUCTION OF PLANT's FOOD 



The Sun's Energy. — The amount of the sun's energy that 

 comes to the earth every day is too vast for us to reaHze. A 

 part of that which falls upon the leaves of plants is reflected, 

 but the greater part enters and, since the epidermis is trans- 

 parent, penetrates to the chloroplasts. Fig. 74 shows in a dia- 

 grammatic way what happens to it there. It is a matter of 

 common observation that very little light penetrates through a 



leaf even in full sunlight; and when the 

 little that comes through is analyzed 

 with a spectroscope it is found that it 

 consists of some red, orange, and 

 yellow, and a relatively large amount 

 of green, while the blue and violet 

 parts of the spectrum have been 

 almost entirely absorbed by the leaf. 

 Less than 2 per cent, of the absorbed 

 light, we know from experiment, the 

 chloroplasts employ in food-building, 

 and approximately 98 per cent, is 

 used in the evaporation of water. 

 By a very ingenious method, known as Engelmann's bac- 

 terium method, it is demonstrated that the red Kght rays are far 

 more potent in photosynthesis than the others, although the 

 orange and yellow rays are much employed in this work; while 

 the green, blue, and violet rays are of all the least useful. The 

 Engelmann demonstration, based on the fact that oxygen is 

 evolved during photosynthesis, is carried out as follows: Motile 

 bacteria that are known to be attracted by oxygen, together 

 with a filamentous alga, are placed on a glass slip in a drop of 

 water and a coverglass is placed on and sealed around the edges 

 air-tight with vaseline. Examining the preparation with a 

 high power objective it is observed that when it is exposed to the 

 light the bacteria flock together along the alga, showing that 

 oxygen is being evolved by it. If the light is now screened off 

 the bacteria scatter, only to assemble along the alga again im- 

 mediately the screen is removed. This shows that photo- 



FiG. 73. — Diagram showing how 

 the position of the chloroplasts 

 against the vertical walls of the 

 palisade cells exposes them to 

 good advantage to light from all 

 quarters of the sky. 



