Guilliermond - Atkinson 



54 — 



Cytoplasm 



layer. This supposed submicroscopic structure makes it possible 

 to form new hypotheses for the mechanism of chlorophyll assimila- 

 tion (Baas Becking and Hanson, 1937). 



As for the pyrenoids which are found in the chloroplasts of 

 many algae, they are still variously considered and opinions as to 

 their significance are not yet well fixed. SCHMITZ, Chmielevsky, 

 and LUTMAN consider them permanent organelles, multiplying by 

 division. Schimper, Klebs and Strasburger saw them disappear 

 and maintain that they are bodies which form de novo, an opinion 

 which today seems demonstrated. It was thought that the pyren- 

 oids constituted a reserve protein elaborated by the plastids. Other 

 authors, on the contrary, think they play an important role in the 

 foiTnation of starch which always arises in their interior (Lutman, 

 McAllister) or in contact with them (Chadefaud). 



In terminating this discussion as to the structure and chemical 

 nature of chloroplasts, attention is called to the ability of chloro- 

 plasts to reduce silver salts, especialFy in a 1% solution of silver 



A B C 



F'iG. 20. — Positions taken by chloroplasts in cells of seedlings of Saccorhiza bulhosa in 

 intense (A) and weak (C) illumination. B, intermediate position. (After Sauvageau). 



nitrate. This property, discovered by MOLISCH and called the Mo- 

 LISCH reaction, is connected neither with chlorophyll nor the caro- 

 tinoid pigments, is produced only in living tissues and seemed to 

 MOLISCH to be independent of light. MOLISCH attributed it to the 

 presence of formaldehyde in the chloroplasts. Investigations of 

 Gautheret have led to important information about the conditions 

 under which the MOLISCH reaction is carried out. This research 

 showed that light plays a role in the reduction of silver nitrate by 

 chloroplasts: the reduction begun by the action of light may con- 

 tinue subsequently in the dark. Gautheret's work proved finally 

 that this reaction is not due to the presence of formaldehyde in the 

 chloroplast but to reducing substances still unknown. 



More recently GiROUD and his collaborators claimed that the 

 Molisch reaction is to be attributed to the presence of ascorbic acid 

 within the chloroplasts which, therefore, these workers believe 

 to be the source of this substance. 



Movement of chloroplasts:- It has been known for some time 

 that chloroplasts are capable of moving from place to place and, 

 depending upon the intensity of the light, are capable of placing 

 themselves on one side or the other of a cell. But for a long time 

 it has been said that they play only a passive role in these move- 



