PL AST IDS AND CHONDRIOSOMES 105 



irregular ends; in Mdogonimn, an irregular parietal net; in Spirogyra 

 (Fig. 37, B), a spirally coiled ribbon; and in the desmids, a series of 

 radiating plates (Carter 1919, 1920). The chromatophore of Antho- 

 ceros (Fig. 37, C) is spindle-shaped, becoming chain-like in the elongated 

 columella cells (Scherrer 1914). The chromatophore of Selaginella 

 may also assume this form (Haberlandt 1888). The chromoplasts of 

 Ariscema (Fig. 37, D) are frequently sharply angular. In the Clado- 

 phoracese (Carter 1919) the cell is completely lined by a thin chromato- 

 phore which may be entire or fenestrated. In many cells irregular strands 

 pass inward through the cell cavity. Indeed it seems not improbable 

 that in some such cases the plastid may be not at all sharply distinct 

 from the rest of the cytoplasm, the two grading one into the other, and 

 the chlorophyll at certain stages permeating all parts of the cytoplasm. 

 The observations of Timberlake and Harper appear to show that such is 

 the condition in the young cells of Hydrodictyon. Thus the physiological 

 processes show various degrees of localization in the cell, causing manifold 

 degrees of structural transformation and delimitation of the cytoplasmic 

 regions involved (Harper). 



Of all chromatophores the chloroplasts stand first in importance, for 

 they bear the green pigment, chlorophyll, which, in the presence of 

 light, enables them to combine water from the soil or other surrounding 

 medium with carbon dioxide from the atmosphere to form carbohydrates, 

 the first visible product being starch. The chloroplasts are therefore 

 the world's ultimate food producers. In addition to chlorophyll other 

 pigments, notably xanthophyll, are usually present. Although the body 

 of the chloroplast can be developed in darkness, the chlorophyll will 

 usually not be elaborated unless light is present. Most young seedlings 

 grown in the absence of light show a pale yellowish color, which is due to a 

 substance known as chlorophyllogen, contained in the plastids. When 

 such "etiolated" plants are placed in the light the plastids become green, 

 apparently through an alteration of the chlorophyllogen to chlorophjdl 

 (Monteverde and Lubimenko 1911). Other conditions necessary for the 

 development of chlorophyll are a favorable temperature and the presence 

 of iron, oxygen, and certain carbohydrates. 



The structure of the chloroplast is an extremely difficult matter to 

 determine, and has been the subject of some controversy. It is generally 

 thought that the body of the plastid is composed of a finely fibrillar 

 meshwork, the stroma, which may be somewhat denser at the periphery, 

 and that the coloring matters are held in the meshes of the stroma in the 

 form of minute droplets. No limiting membrane is definitely known. 

 The included droplets are apparently not composed of the pigments alone : 

 it is probable that they are rather globules of some oily or fatty material 

 containing the pigments in solution. The pigments may easily be dis- 

 solved out with alcohol and other reagents. On the other hand, it has 



