378 THE AMERICAN NATURALIST. [VOL. XXXVIII. 



examples are found in the colored cells of certain floral parts 

 and fruits (Fig. 2, b i). These pigments are generally either 

 xanthophyll (yellowish) or carotin (orange red). Chloroplasts 

 may also turn brown especially in older cells that are losing their 

 contents. The colors of some leaves and flower parts are due 

 not to the plastids but to substances dissolved or otherwise held 

 in the cell sap of the vacuoles. The brilliant coloration of 

 autumn foliage is of this character as well as some of the tints 

 of petals, hairs and other structures. The chromatophores of 

 the higher brown Algae (Phaeophyceae) and most of the red 

 (Rhodophyceae) have the discoid form characteristic of chloro- 

 plasts (Fig. 2 b 2, 3). They might be called phasoplasts and 

 rhodoplasts if one wished to classify plastids according to their 

 color. 



The structure of chromatophores is frequently complicated by 

 the presence of pyrenoids which may be quite numerous in the 

 body. These structures are denser regions of the chromatophore 

 with a definite boundary. They are proteid in character and 

 are known to vary in size with nutritive conditions and may 

 completely disappear if the cell is starved. They have been 

 regarded as masses of reserve oroteid material but certain func- 

 tions of great importance are also associated with them. The 

 arrangement of starch grains in the chromatophores of many 

 algae is clearly around the pyrenoids as centers. For this reason 

 they have been called amylum centers. Timberlake (:oi) has 

 recently shown in Hydrodictyon that segments are split off 

 from the pyrenoids (see Fig. 2, c 2) and changed directly into 

 starch grains which naturally lie for a time close to the source 

 of their formation and only later become distributed throughout 

 the chromatophore. It is probable that similar conditions will 

 be found in other algae (Conjugales, Protococcales, etc.) and we 

 may soon have a much clearer understanding of the pyrenoid. 

 The indications are that the pyrenoid will prove to be a region 

 of the chromatophore differentiated as a metabolic center, more 

 or less prominent according to conditions of nutrition, and that 

 its most conspicuous activity is the formation of starch by the 

 -direct transformation of portions of its substance. 



Some other structures besides the pyrenoids have been 



