OR ' PHOTOSYNTHESIS ' 213 



The chemical nature of chlorophyll is unknown : its production 

 is, however, in some way dependent upon the presence of iron 

 in plants although it does not appear to contain this element. 



The chloroplasts of plants grown in the dark or covered up for 

 a time, lose their green colour and become colourless or pale 

 yellow. With the exception of the chlorophyll of the chloroplasts 

 present in the embryos of certain plants, the production of this 

 green pigment is dependent upon light : the cotyledons and first 

 leaves of most seedlings and the leaves from underground buds of 

 perennial plants only become green when they reach the surface 

 of the soil. Moreover, the formation of chlorophyll is influenced 

 by heat ; the plastids of many plants grown in the dark do 

 not develop a green tint even when exposed to light when 

 the temperature is below freezing-point, but do so at higher 

 temperatures. 



Chlorophyll, perhaps in a more or less altered form, can be 

 extracted by means of alcohol : its solutions are fluorescent, 

 appearing blood-red when seen by reflected light, and green 

 when viewed by transmitted light. When acted upon by acids 

 it changes to a dirty brownish-green colour. After death of 

 the cytoplasm of the cells, the acid cell-sap, which is confined 

 within the vacuole of the cells when the plant is living, diffuses 

 through the cytoplasm to the chloroplasts, causing them to 

 change to the brownish-green tint so characteristic of dead leaves. 



Light is not only essential for the formation of chlorophyll, 

 but it is also directly necessary for the process of ' carbon- 

 fixation,' as it is from the energy of the sun's rays that the 

 energy required to effect the decomposition of the carbon 

 dioxide and water used in the process is derived. 



In darkness, green plants are unable to effect the synthesis of 

 carbohydrates from carbon dioxide and water, and under such 

 conditions they decrease in dry weight owing to the loss caused 

 by respiration, which goes on at all times (see chap. xix.). 



In shady places, in badly-lighted rooms, and in greenhouses 



