174 



PLANT PHYSIOLOGY 



The necessity of iron for the formation of chlorophyll was for 

 a long time regarded as a strong proof that iron is one of the 

 constituents of chlorophyll. The exact analyses performed by 

 Willstatter have shown that this view is not correct. The iron 

 must be regarded, therefore, as a specific catalyst without which 

 there cannot go on some of the preliminary stages of the produc- 



Fig. 50. — Seedlings 

 of broad beans grown 

 in light {from Nathan- 

 sohn). 



Fig. 51. — Seedlings 

 of broad beans grown 

 in darkness {after 

 Nathansohn). 



tion of green pigment, like the formation of leucophyll or other 

 precursors. 



It is always possible by means of microchemical reactions to 

 detect in the chloroplasts considerable amounts of iron, which, 

 according to Warburg, plays a very important role in the process 

 of photosynthesis. This theory w^ll be discussed later on. 



Chlorosis is sometimes observed under natural conditions, 

 most frequently on soils rich in lime and giving a somewhat 



