104 



INTRODUCTION TO CYTOLOGY 



physiological work being done, the various pigments being associated in 

 an intimate manner with different reactions occurring within the plastids. 



Leucoplasts. Leucoplasts are relatively small and colorless. They 

 are found commonly in the cells of meristematic tissue, and may be 

 retained in some kinds of differentiated cells, such as the glandular hairs of 

 Pelargonium. K lister (1911) states that the leucoplasts of Orchis are 

 very fluid in consistency, undergoing amoeboid changes of shape and 

 multiplying by irregular fission. The smaller leucoplasts appear to 

 represent juvenile stages in the development of plastids of more highly 

 differentiated types, for under certain conditions they develop into the 

 larger and more highly specialized leucoplasts known as amyloplasts, and 

 into the various kinds of chromatophores mentioned below. 



Chromatophores. Chromatophores, or chromoplasts, are plastids bear- 

 ing one or more pigments, and having thus a more or less decided 

 color. In green plants the most important of these pigments are chlo- 



FIG. 37. Various forms of plastids. 



A, Draparnaldia. B, Spirogyra. C, Anthoceros. D, chromoplasts of Arisoema. E, 

 cell of Selaginella, showing position assumed by plastid in response to light (direction shown 

 by arrow). A, B, and C show pyrenoids. (E After Haberlandt.) 



rophyll, carotin, and xanthophyll. Chlorophyll is apparently a combi- 

 nation of two simpler pigments, chlorophyll a and chlorophyll 6. The 

 cells of the Phaeophyceae, Cyanophyceae, and Rhodophyceae are character- 

 ized'respectively by the presence of yellow carotin, blue phycocyanin, 

 and red phycoerythrin, in addition to chlorophyll. The Cyanophyceae 

 exhibit an especially rich variety of pigments, which in many cases do not 

 appear to be held within definite chromatophores. 1 



Chromatophores are usually spherical, ovoid, or discoid in shape, but 

 many peculiar forms are known, particularly among the green algae. In 

 Ulothrix the chloroplast has the form of a complete or incomplete hollow 

 cylinder; in Draparnaldia (Fig. 37, A), a hollow cylinder with very 



1 For the literature pertaining to plant pigments see Palladin 1918. See also 

 Haas and Hill (1913), Willstatter and Stoll (1913), Jorgensen and Stiles (1917), 

 Wheldale (1916), and Beauverie (1919). The distribution of carotin is discussed in 

 an earlier paper by Tammes (1900). 



