68 INTRODUCTION TO CYTOLOGY 



tion spectra of chlorophyll mixtures it has been suggested that the chloro- 

 phyll may be in the colloidal state or adsorbed at the surface of colloidal 

 particles in the stroma. ^^ K. Stern (1921), however, showed that chloro- 

 phyll exhibits its characteristic fluorescence only when molecularly 

 dispersed, as in a true solution; moreover, he reported that chlorophyll 

 mixtures are fluorescent only when lipide is present. ^- Such facts indicate 

 that chlorophyll, itself in the molecular state, exists in and perhaps on 

 colloidal lipide droplets in the stroma. 



A special structural feature of some plastids is the pyrenoid. Pyre- 

 noids are small refractive masses of some proteinaceous substance lying 

 in or on the chromatophores of many green algse, red algae, flagellates, and 



Fig. 31. Fig. 32. 



Fig. 31. — Chloroplasts in maize leaf , showing starch-containing regions. {Photograph 

 by Weier.) 



Fig. 32. — Diagram of cross section of chloroplast of Elodea as interpreted by Zirkle 

 (1926). a, cytoplasmic granule adhering to the cytoplasmic sheath (6); c, pore in stroma; 

 d, stroma; e, starch inclusion as it appears in polarized light;/, central cavity. 



certain liverworts. ^^ They have been reported to originate de novo,^^ 

 by division, 1^ or by both methods. ^^ During the reproductive phases 

 they may vanish and reappear. Pyrenoids may be simple (Protococ- 

 caceae, probably), double (Ulothrix), or multiple (Conjugatse, Anthoceros). 

 It seems clear that the pyrenoid is associated in some way with the 

 synthetic processes carried on by plastids, although the precise nature of 

 this association is by no means clear. When the product of synthesis is 

 starch, as is commonly the case in Conjugatse and Chlorophycese, it often 

 appears as a series of grains about the pyrenoid (Fig. 26, G), but in other 

 cases {e.g., certain Rhodophycese) the starch appears outside the plastid 

 so that the pyrenoid is surrounded by no such "starch sheath." Some 

 algae with pyrenoids, for example certain diatoms and Chrysophycese, 



" Tswett (1906), Willstatter and Stoll (1913, 1918), Zirkle (1926). 



1^ See Lloyd (1924) on the fluorescent colors of plants. 



1^ See the general account by Geitler (1926) and the paper by Czurda (1928). 



1^ Schmitz (1882, 1884), Schimper (1885), G. M. Smith (1913, 1914) on Tetradesmus 

 and Scenedesmus, W. Zimmermann (1921) on Volvox, Hazen (1922) on Brachiomonas, 

 Kater (1929a) on Chlamydomonas. 



1* M. Hartmann (1921) on Eudorina, Ishikawa (1921a) and Ikari (1923) on 

 Porphyra, Bold (1931) on Chlorococcum. 



i« McAllister (1914, 1927) on Anthoceros, Peterschilka (1922) on Mougeotia, 

 N. Carter (1926) on Ulva, Czurda (1928, 1929) on Spirogyra, Geitler (1926). 



