The Fine Structure of Senescing Cotyledons of Cucumber

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Abstract
Changes in fine structure have been followed in mesophyll cells of attached and detached cucumber cotyledons from maturity at 16–20 days to an advanced stage of senescence at 50 days. From about 25 days in the attached cotyledons, the chloroplast thylakoids gradually disappear and large osmiophilic globules accumulate in the stroma, which itself is reduced in density. At the same time the ribosomes are lost, first those free in the ground plasm, followed by those attached to the endoplasmic reticulum (E.R.). The E.R. later vesiculates and breaks down. The mitochondria become smaller and ultimately contain few cristae, but they remain morphologically intact until a very late stage. The tonoplast breaks down from 35 days and the plasma membrane from 40 days, the latter probably resulting in cell death. The pattern of changes in cotyledons excised at 16 days and subsequently floated or stood in water is similar to that in attached cotyledons except that the chloroplast breakdown is more rapid and the ribosome loss is less rapid and complete. Breakdown of the tonoplast is delayed to 40 days and the plasma membrane remains intact. These observations are discussed in relation to possible senescence mechanisms.