Pyrenoids and starch 131 



Pyrenoids sometimes divide equally or unequally on the division of the 

 cell and they may multiply within the limits of any one chloroplast. They 

 often disappear completely, and they can arise spontaneously in the chloro- 

 plasts of many of the filamentous and unicellular Green Alga3. They can be 

 caused to disappear by starvation and they are often conspicuous in well 

 nourished cells. Although there are many Green Alga^ in which pyrenoids 

 never occur, there is much evidence to show that in others their presence or 

 absence is largely a question of nutrition. Under certain circumstances they 

 may be entirely absent from the chloroplasts of Desmids or even those of 

 Spirogyra. 



Chmielewski's hypothesis that the pyrenoid was a permanent cell-organ 

 always arising by the division of pre-existing pyrenoids is incorrect, as pyre- 

 noids often arise de novo, especially in young cells. Also, Meyer's view that 

 pyrenoids have no function other than that of a protein-reserve is not entirely 

 correct, as Timberlake has shown very clearly in the case of Hydrodictyon 

 that the pyrenoid is actually the seat of those processes resulting in starch- 

 formation, and that a portion of it becomes converted into starch, presumably 

 by other complex chemical processes. Timberlake takes the view that ' the 



"fl^a 



Fig. 90. A and B, optical sections of two pyrenoids (with associated starch grains) of Hydro- 

 dictyon reticulatum (Linn.) Lagerh. x 2250 (after Timberlake). C and D, pyrenoids (with 

 associated starch grains) of Closterium Ehrenbergii Menegh. x 1250 (after Lutruan) ; C is an 

 optical section, and D the surface view ; py, pyrenoid ; s, starch grains. 



pyrenoid is an active body, differentiated in the chlorophyll-bearing cyto- 

 plasm, which in co-operation with the latter acts as the base of the process 

 of starch-formation.' In Hydrodictyon all the starch grains originate from 

 the pyrenoids and gradually get pushed out into the cytoplasm. All the 

 starch is therefore ' pyrenoid-starch.' This has been shown by Lutman ('10) 

 to be the case in Closterium Ehrenbergii. But starch -formation does, of 

 course, occur in the chloroplasts of Green Algae which have no pyrenoids, and 

 this starch might very well be regarded as ' stroma-starch,' since its method 

 of formation is presumably different. Thus, although Klebs was wrong in 

 attempting to discriminate between ' pyrenoid-starch ' and ' stroma-starch ' 

 in Hydrodictyon, in which Alga Timberlake has shown that the difference 

 does not exist, yet ' stroma-starch ' occurs in those chloroplasts without 

 pyrenoids, and there is no reason why it should not occur in some chloro- 

 plasts which possess pyrenoids. 



In Dicranoch&te reniformis, one of the setigerous members of the 

 Protococcales, Hieronymus ('92) states that not only does the central portion 



92 



