710 THE AMERICAN NATURALIST. [Vor. 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 Cyanophyceæ and is also 
present in numbers of the lower groups of green algæ. Thus 
we may find many types in the Pleurococcaceze 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 algæ as in Hydrodictyon and certain simple 
forms of the Ulothricaceze. 
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 Rhodophycez, the higher Phzeophycez, and all groups 
generally above the thallophytes. It seems to be the type of 
structure best suited to cell activities since with few exceptions 
itis found in groups in the highest lines of plant evolution in 
