DNA microdensitometry and autoradiography after treatment with 3H-thymidine were used to study the phase of dispersion of chromocenters (Z phase) in parallel with chromocentric nuclei in Phaseolus coccineus. In all materials studied, two types of chromocentric nuclei were present. In radicle apices of dry seeds, two classes of nuclear DNA contents were measured, 2 C (G 1) and 4 C (G 2). The 2 C DNA class comprised all chromocentric type I nuclei, the 4 C class included Z phases and chromocentric type II nuclei. The 4 C (G 2) condition of Z phases implies that Z phases maintain their nuclear structure for some time after the end of DNA replication. Shoot apices also contain 2 C (G 1) and 4 C (G 2) nuclei but 4 C nuclei (Z phases and chromocentric type II nuclei) are rare. In seedling root apices, Z phases are from 1.02 to 4.08 times as frequent as prophases. This excludes that Z phase is a very early prophase. DNA microdensitometry shows that the chromocentric type I includes 2 C (G 1) nuclei and nuclei in the first part of the S phase, Z phases include 4 C (G 2) nuclei and nuclei in the last stage of the S phase and chromocentric type II includes mainly 4 C (G 2) nuclei and nuclei in the second part of S. After 90 minutes of treatment with 3H-thymidine all Z phase nuclei are labeled. This result and the microdensitometric data unequivocally demonstrate that Z phase is located at the end of S. The present results and those of previous authors on Z phase are discussed in relation to Geitler's concept of Angiosperm endomitosis. It is concluded that the term "Angiosperm endomitosis" must be abandoned and substituted by the term "chromosome endoreduplication".
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