100 INTRODUCTION TO CYTOLOGY 



grains of starch, while in waxy-sweet maize they contain similar grains 

 of the red-staining "starch" (Fig. 51, c).^^ 



Cellulose is the chief constituent of the cell wall in most groups of 

 plants. It is not often found in the pure state in the wall, other sub- 

 stances, notably lignin, ordinarily being present in physical or chemical 

 combination with it (p. 176). The proportion of pure cellulose may be 

 as high as 90 per cent in cotton fiber, but in beech and oak wood it is as 

 low as 35 per cent. Although chiefly supporting in function, cellulose is 

 sometimes used as a reserve product; the hemicelluloses more often func- 

 tion in this capacity. Closely allied to the celluloses are the pectins. It 

 is a striking fact, as remarked by Sponsler (1923), that starch, cellulose, 

 and pectic substances, which are about the only solid materials deposited 

 directly and in quantity by plant protoplasm, have the same proportional 

 formula CeHioOs. 



Glycogen, a substance of great importance in animals, is found also 

 in the Cyanophycese, myxomycetes, fungi, and bacteria. It may exist in 

 the form of viscous or solid masses in the cytoplasm, or in colloidal 

 solution in the vacuoles. In these plants it appears to function much 

 as starch does in higher plants. The many mucilages and gums which 

 occur so widely in plant tissues are composed chiefly of carbohydrate 

 materials, the former being condensation products of various sugars and 

 the latter these products together with complex acids (Onslow). Plant 

 slimes may apparently arise in different cases as modifications of cell-wall 

 substance, within the cytoplasm, or at the boundary between the cyto- 

 plasm and the vacuoles (Czapek; E. L. Smith, 1923). Sugars of several 

 types are of special importance in the metabolism of plants and are fre- 

 quently present as storage products. 



Proteins. — Ergastic protein bodies are constantly being encountered 

 in cytological study. These may be either crystalline or non-crystalline 

 and may lie in the cytoplasm, plastids, nucleus, or vacuoles. In animal 

 eggs the storage materials commonly occur in the form of yolk globules, 

 or "deutoplasm spheres," which consist for the most part of relatively 

 complex protein compounds; globules of fat or oil are usually associated 



" Lampe and Meyers (1925), Lampe (1931). For the composition of starch, see 

 Meyer (1895, 1913, 1920), Czapek (1913), Abderhalden (1923), Reichert (1913), 

 and Brink and Abegg (1926). For the structure of the starch granule, see papers of 

 Nageli, Schimper, Meyer, Kabsch, Binz, Dodel, Salter, Kramer, and Sponsler. For 

 the occurrence of starch, see Meyer (1895) and Winkler (1898). Belzung (1887) and 

 Eberdt (1891) deal with the origin of starch in chloroplasts and amyloplasts, respec- 

 tively. For accounts of "Floridean starch," see Schmitz (1882), Bruns (1894), 

 O. Darbishire (1896), Henckel (1901), Kylin (1913), and Mangenot (1923a). The 

 development of maize endosperm and its storage products is described by Lampe 

 (1931). The relation of genes to the production of such products in endosperm or 

 pollen is discussed by Brink and MacGillivray (1924), Demerec (1924), Longley (1925), 

 Brink and Burnham (1927a6), Brink and Abegg (1926), and Brink (1927ce, 1928, 

 1929a6). 



