Ageing in Plant Embryos

Summary Although no cell division was detected in the radicle during the first 24 hours of imbibition by the caryopses of Zea mays, fine structural changes showed the re‐establishment of a developmental pattern during this period. Such changes included mitochondrial differentiation, the appearance of dictyosomes and a complex system of ER, and the association of ribosomes to form polysome‐like structures. Association between ER and lysosome‐like bodies was seen during this stage, together with an intensification of the acid phosphatase reaction within the lysosomes. By the 48‐hour stage, easily‐separated, senescent cells had been produced and cell division had begun. Vesicles produced by the hypersecretory activity of the dictyosomes were either incorporated into the developing lysosomes or passed through the cell membrane to the wall. In the outermost cells the lysosomes finally ruptured and acid phosphatase activity became detectable in the ground cytoplasm. The development of the lysosomes and the hypersecretory activity of the dictyosomes are discussed as evidence of constructive changes implying genetic control of cellular senescence in the root cap.