THE PALISADE CELL 



143 



Light 



synthesis begins at once in the light and ceases as suddenly in 

 darkness. After these preliminary observations a small spec- 

 troscope adapted to the purpose is placed beneath the micro- 

 scope stage. Light is thrown by the mirror through the spec- 

 troscope and the preparation is so adjusted 

 that the colors of the resulting minute spec- 

 trum succeed each other along the length of 

 the alga. It is then seen that the bacteria 

 flock about the alga where it is illumined by 

 the red light, gradually thinning out in the 

 orange and yellow until relatively very few 

 hover in the regions of the green, blue, and 

 violet. There seems to be no other inference 

 from this than that the food-constructing or 

 photosynthetic efficiency of each part of the 

 spectrum is measured by the relative number 

 of bacteria collecting in its domain. Fig. 75 

 shows how the red portion surpasses the 

 others in this respect. It has been found 

 that light that has passed through one leaf 

 is too weak in the red to energize the chloro- 

 plasts in another. 



The Palisade Cell the Chief Photosynthetic Unit. — Most 

 of the work of food-construction is done in the palisade cells, 

 since they occupy the upper half of the leaf where the incom- 

 ing light is strongest, and they have four to five times as many 

 chloroplasts as the cells of the spongy parenchyma have. When 

 food synthesis is going on a palisade cell is the scene of great 

 activity. Fig. 74 is a diagrammatic expression of this fact. 

 Here for simplicity a single chloroplast in a palisade cell is made 

 to represent the many smaller ones that actually occur. Light 

 penetrates the chloroplast from above, carbon dioxide flows in 

 from the intercellular spaces, and water enters from the vein 

 below the cell. The food product in the form of sugar diffuses 

 from the chloroplast into the cell-sap and thence into the food- 

 conducting cells in the vein. Oxygen as a by-product of food- 



FiG. 74. — Diagram 

 to show the activities 

 going on in a palisade 

 cell. The arrows from 

 the chloroplasts into 

 the cell cavity indicate 

 movement of photo- 

 synthesized product. 

 The thickness of the 

 arrow shafts under 

 "Light" indicate the 

 relative light inten- 

 sities. 



