Many investigators have studied the light line, and different theories have been advanced as to its function, physical properties, and chemical nature. Schleiden and Vogel (35, p. 26) in describing the mature testa of _Schizolobium excelsum_ in 1838 undoubtedly referred to the light line when they stated that the walls of the Malpighian cells were not equally thickened. Mettenius (26), in 1846, was probably the first definitely to describe the light line. This author believed it was composed of pore canals, all appearing at the same height in the cells, but he was unable to prove this by cross sections. Lohde (20) studied the light line in seeds of _Hibiscus trionum_ and found it cutinized. Hanstein (8) states that the Malpighian cells are composed of two cell layers and the light line is produced by the adjoining walls of the ends of the cells. Later, this same author (9), according to Harz (12), refers to the light line as a perforated disk composed of tissue of strong refracting power. Russow (34) concludes that the light line is produced by neither chemical nor mechanical changes but is caused by a modified molecular structure containing less water than the remainder of the cell wall. Hiltner (13) agrees with Russow's explanation. Harz (12, p. 561) also agrees with Russow and adds that he has observed that the light line disappeared in a number of cases after applications of nitric acid. Wigand and Dennert (43) suggested that the light line is due to a series of erect fissures, while Tietz (37, p. 32) believes it is due to a chemical modification and that the phenomenon results from the exceptionally extreme density of parts of the cellulose membrane. Junowicz (16) found evidence of cellulose material. The cell wall at this point was strongly refractive and had a different molecular structure. After studying _Phaseolus vulgaris_, Haberlandt (7, p. 38) agrees with the Russow explanation. In the seed of this plant the light line colored blue after being treated with chloriodid of zinc. Sempolowski (36), who investigated the light line in _Lupinus angustifolius_, states that there is not only a difference in the molecular structure but also a chemical modification of the cell wall at this point, since with iodin and sulphuric acid the cell wall colored blue, whereas the light line colored yellow. Wettstein (41), who studied seeds of Nelumbo, agrees with Russow (34) and Sempolowski (36) that chemical and physical modifications occur. He found that iodin and sulphuric acid colored the Malpighian cells intensely blue, the light line at first yellowish, and then later it gradually became blue. This reaction may be accelerated by heat. Iodin produced the same effect, and the light line colored blue more rapidly. When treated with a water-withdrawing medium the light line was not altered for some time, but finally disappeared with continued application. Cooking for a long time in caustic potash or standing in cold caustic potash caused