CYTOKINESIS AND THE CELL WALL 177 



alterations in fatty substances which are produced in the protoplasts and 

 then migrate into and along the cell walls to the outer surface of the 

 epidermis. D. B. Anderson (1928) finds the outer wall of the epidermal 

 cell of Clivia to consist of (1) an inner layer of cellulose and pectin; 

 (2) a thin layer consisting mostly or wholly of pectin; (3) a cuticular 

 layer composed of (a) a region of cellulose and pectin lamellae containing 

 cutin and (6) a region of cutin and cellulose with little or no pectin; 

 and (4) a cuticle of pure cutin. In Aloe new cutin is secreted below the 

 old cuticle (Ziegenspeck, 1928). 



A variety of mineral substances, such as silica, calcium carbonate, 

 and calcium oxalate, as well as more complex organic compounds, includ- 

 ing tannins, oils, and resins, are often deposited in the walls of old cells. 

 The heartwood of trees owes many of its qualities, such as color and 

 density, to the presence of these additional materials in the walls and 

 cell cavities. 



Among the thallophytes many species have walls essentially like those 

 of higher plants, cellulose being the chief constituent. Others are 

 characterized by the predominance of other compounds, notably chitin in 

 zygomycetes, ascomycetes, basidiomycetes, and certain algae; pectins in 

 the bacteria; and keratin in myxomycetes. In many specific cases the 

 results of various workers do not agree. ^^ 



That the cell wall is not wholly an ergastic secretion, but contains 

 protoplasm in some form, is a view which has frequently been main- 

 tained.^^ Wiesner supposed the growing wall to be made up of regularly 

 arranged particles ("dermatosomes") connected by protoplasmic fibrils, 

 growth involving an intercalation of new particles between the old ones. 

 Hansteen-Cranner believes the wall to be a colloidal network of celluloses 

 and hemicelluloses with extensions of the plasma membrane in the 

 meshes. In the cellulose walls of a number of meristems Wood (1926) 

 found no more than 0.001 per cent of protein matter, which is less than 

 would be expected if the wall contained significant amounts of protoplasm. 



Ultramicroscopic Structure of the Wall. — In connection with this 

 subject, which has recently come into prominence anew, it is of interest 

 to note the prophetic nature of certain early theories.'^ Von Mohl 

 advanced the view that the cell wall grows by the deposition of material 

 in successive laminae ("apposition theory"). On the other hand, Niigeli, 

 as a result of his researches on starch grains and the wall, concluded that 

 the wall is composed of molecular complexes called viicellce surrounded by 

 water films and that growth involves the intercalation of new micellae 



'^ See van Wisselingh (1924) and literature there cited; also Mameli (1920), 

 Tiffany (1923), Wurdack (1923), and Thomas (1928). 



18 Wiesner (1886 et seq.), Molisch (1888), Hansteen-Cranner (1919, 1922, 1926). 



"Von Mohl (1853, 1858), Nageli (1858, 1863, 1864), Dippel (1879), Krabbe 

 (1887), Strasburger (1882, 1889), Wiesner (1886, 1890, 1892), Correns (1889, 1892, 

 1893). 



