376 THE AMERICAN NATURALIST. [VOL. XXXVIIL 



pyrenoid. The detailed structure of chromatophores was first 

 described by Schmitz ('82) and of plastids by Meyer ('83). 

 The most complete study of plastids however is that of Schimper 

 ('85). The body of the plastid is always denser than the sur- 

 rounding cytoplasm. It has a porous structure that is only 

 visible under high magnification and there are sometimes present 

 very delicate fibrils. The coloring matter, oily in consistency, 

 is held in the pores as minute globules. The plastid may 

 therefore be compared to a very fine-textured sponge saturated 

 with pigment. All of the coloring matter of the plastid may 

 be readily extracted with alcohol leaving the colorless proteid 

 matrix. 



The pigments of plastids are then in the nature of secretions 

 held in these specialized regions of protoplasm. Chlorophyll is 

 the principal substance and, as has before been said, is almost 

 always present, but the amount is sometimes so small that its 

 green is completely hidden by the color of other pigments. 

 Chlorophyll itself contains greater or less amounts of two other 

 coloring matters that may be readily separated from the pure 

 green, a yellow xanthophyll and an orange red carotin, both 

 substances closely related to chlorophyll. The other pigments, 

 characteristic of the chromatophores in some groups of algae, 

 are however quite distinct from chlorophyll. There is phycocyan, 

 found in the blue* green algae (Cyanophyceae), phycophaein and 

 phycoxanthin, characteristic of the brown (Phaeophyceae) and 

 phycoerythrin of the red (Rhodophyceae). 



Chloroplasts are found almost universally in green plants 

 above the Thallophytes and are also present in the large group 

 of algae the Siphonales and in the Charales. They are some- 

 times formed very numerously in the cell, reproducing rapidly 

 by fission (see Fig. 2 a 2, 3) and lie in the layer of protoplasm 

 just inside of the plasma membrane. They are sensitive to light 

 and readily shift their position in the cell. Strong illumination 

 results in their retreat from exposed positions to the sidewalls 

 and bottom of the cell where the light is less intense. If the 

 illumination be weak they may all gather on the side most favor- 

 able for the reception of light. These facts are well illustrated 

 by the behavior of the plastids in some of the Siphonales (e. g. 



