Histological Effects of Absorbed Radioisotopes upon the Callus of Sequoia sempervirens

Abstract

1. Callus cultures of Sequoia sempervirens were grown for 85 days on agar media containing phosphorus 32 (P32), sulfur 35 (S35), and carbon 14 (C14). Histological studies were made on paraffin sections of them. 2. Calluses grown on media containing S35 showed the most pronounced histological effects. Thickening resulted from formation of new cells at right angles to the surface. Internal parenchyma was largely disintegrated. Finally, the marginal meristems underwent hypertrophy and ceased formation of new cells. All calluses died after transfer to new media. 3. Calluses grown on media containing P32 showed hypertrophy of all cells, including the marginal meristems. This was essentially a maturation, and one result was cessation of division in the marginal meristems. Groups of these cells died and left large necrotic areas. Internal parenchymatous cells were not destroyed. All calluses died after transfer to new media. 4. Calluses grown on media containing C14 showed limited histological responses. There was some hypertrophy of internal parenchymatous cells and greater-thannormal formation of new cells at right angles to the flat surface. These changes caused considerable thickening of the calluses. The marginal meristems and the cambiums around the tracheid groups continued to form new cells. All calluses continued growth after transfer to new media. 5. The possibility is considered that part of the effect of radioactive sulfur was brought about by its disintegration into chlorine; thus, that there was a chemical effect on the tissues in addition to radiation. 6. A discussion is given of the probable susceptibility to radiation of cells of the calluses, using the previously gained knowledge that accumulation of these radioisotopes is greatest in the marginal meristems and cambiums. 7. The dilution effect is utilized in attempting to explain the wide divergences between the histological effects of S35 and of P32, on the one hand, and of C14, on the other.