THE AMERICAN NATURALIST. [Vol. XXXVIII. 



for the proper balance of protoplasmic elements in narrow 

 confines, a division prompted by the activities of the cytoplasm 

 rather than emanating from within the plastid. 



The view of the permanence of the plastid as a cell organ has 

 received its strongest support from the classical work of 

 Schimper ('85). We are not prepared to deny it and to assert 

 that the plastid may arise de novo. Yet those who study the 

 cells of embryonic tissues and reproductive phases know that it 

 is extremely difficult to follow the plastids and that these 

 structures require other than the usual methods of cell research 

 to establish their presence. Several writers (Eberdt, Dangeard, 

 Husek and others) have expressed their belief that plastids may 

 arise de novo but no one has thoroughly traced the appearance 

 or disappearance of these structures in any cells. 



The plastid in phylogeny has never received the attention that 

 it deserves. Beginning with the conditions among the Cyano- 

 phyceae and the lowest Chlorophyceae (which will be further 

 discussed in Section VI) we find the pigment distributed so 

 generally throughout the cell that it is doubtful if the term 

 chromatophore should ever be applied to regions so indefinite in 

 outline. Above these groups the pigment is confined to propor- 

 tionally smaller areas in the cytoplasm and these become 

 chromatophores when their form is clear. The primitive chro- 

 matophores were solitary and filled a large part of the cell. 

 The pyrenoids arose in the chromatophores probably as the 

 result of the influence of metabolic centers upon the protoplasm. 

 It is scarcely possible that a large chromatophore should be 

 absolutely homogeneous throughout; there would develop one 

 or more centers of metabolic activity and such would exert some 

 influence on the form of the protoplasm. 



But the large single chromatophore does not seem to be the 

 form best adapted to the work of a cell perhaps, if for no other 

 reason, because it requires a mechanical adjustment of other 

 cell organs to itself and would interfere with the quick circula- 

 tion of material and the general balance of cell activities. 

 It seems possible that mechanical difficulties may have led to 

 the division of large chromatophores and the substitution o 

 numerous small plastids. This change was instituted in the 



