7IO THE AMERICAN NATURALIST. [Vol. XXXIX. 



thallophytes one notices at once certain features of their distri- 

 bution in various groups. The large chromatophores are charac- 

 teristic of the cells of simpler and more primitive groups and the 

 small plastids, numerous in the cells, are generally present in 

 types which are at a fairly high evolutionary level. There are 

 exceptions of course to this general statement but some of 

 these are probably significant of phylogenetic relations. 



The evidence all indicates that the primitive type of chroma- 

 tophore was a large structure in the peripheral region of the 

 protoplast and with an ill defined boundary or occupying the 

 entire surface of the cell. This type of structure is at present 

 characteristic of chromatophores of the Cyanophycese and is also 

 present in numbers of the lower groups of green algae. Thus 

 we may find many types in the Pleurococcaceas whose cells con- 

 tain a pigment so diffused that it is impossible to establish 

 definite limits and similar conditions often appear in the cells of 

 some of the higher algae as in Hydrodictyon and certain simple 

 forms of the Ulothricaceae. 



The simple diffused types of chromatophores of the lower 

 algae become replaced in higher groups either by sharply differ- 

 entiated structures of definite form and often showing internal 

 organization in the form of pyrenoids or by numerous plastids. 

 There is considerable evidence that the plastids have arisen by 

 the successive splitting or division of large organized chromato- 

 phores. The most highly differentiated chromatophores are 

 found in the Conjugales and the remarkable size and symmetry 

 of these cells is emphasized by the same peculiarities of the 

 chromatophores. They are generally so placed in the cells as to 

 give an almost perfect balance of protoplasmic structure. This 

 principle is especially clearly illustrated among the desmids and 

 in such forms as Zygnema and Mougeotia while even Spirogyra 

 illustrates the principle strikingly in the distribution of its spi- 

 rally wound chromatophores. 



Plastids are characteristic of the Siphonales, Charales, most 

 of the Rhodophycese, the higher Phaeophyceae, and all groups 

 generally above the thallophytes. It seems to be the type of 

 structure best suited to cell activities since with few exceptions 

 it is found in groups in the highest lines of plant evolution in 



