46 PLANT PHYSIOLOGY 



colloidal state; but the former hypothesis would give more the 

 impression of a suspension, while the latter seems to convey the 

 idea of an emulsion. The fluorescence so characteristic of chloro- 

 phyll is seen only when in solution; and when chlorophyll is 

 shaken with a lipoid substance (oleic acid, lecithin, cholesterin) a 

 marked fluorescence occurs, resembling that seen in the living 

 plant. Also the absorption bands of chlorophyll in the cell are 

 very similar to those of chlorophyll in lecithin. 



Priestley and Irving (1907) have found the chlorophyll dis- 

 tributed only at the surface of the plastid where it can absorb as 

 much light and expose as much surface as possible. Here it seems 

 to be in a very thin layer, adsorbed to the protein of the plastid. 

 Zirkle (1926), however, finds the chlorophyll evenly distributed 

 throughout the protein ground-substance of the plastid. The 

 conditions necessary for the formation of the chlorophyll in the 

 plastid have been stated in the previous chapter. 



Chlorophyll. — Chlorophyll (Gr., green leaf) which received 

 its name from Caventou and Pelletier (1819) was not isolated 

 until nearly a century later (1912) when Willstatter, as a result 

 of experiments already classic, obtained pure chlorophyll and 

 determined its chemical composition. To him and his stu- 

 dents (especially Stoll) most of our knowledge of chlorophyll 

 is due. 



Willstatter showed that chlorophyll as ordinarily obtained is a 

 mixture of two substances which he has called chlorophyll " A" and 

 chlorophyll "B." The former has the formula CssH^Os^Mg 

 and the latter CssHyoOe^Mg. While some plants are richer in 

 chlorophyll than others, the average total yield from 1 kg. of 

 fresh leaves is 1-2 g. and of dried leaves 5-10 g. The proportions 

 of A to B are as 31 to 11 in land plants; in the brown algae, 16 to 1 ; 

 and in the green algae, 31 to 22. Both these chlorophylls are amor- 

 phous in the plant but when extracted with ethyl alcohol they 

 form green crystals. They have different solubilities in organic 

 solvents, however, and are separated from each other because of 

 this property. Chlorophyll A is blue-green in transmitted light 

 and blood-red by reflected light, while chlorophyll B is yellow- 

 green by transmitted light and brownish-red by reflected light. 

 Both show this peculiar property of fluorescence. 



Chlorophyll is an ester, that is, it is a combination of an acid 

 with an alcohol. If an alcohol with the formula ROH is mixed 



