The ultrastrutures of the root protophleom sieveelement at different developmental stages of Arabidopsis thaliana. L. were investigated using the technique of high pressure freezing and freeze substitution fixing specimen. The results show that in the development of hte sieve elements, the nuclei undergo typical characteristics of the programmed cell death (PGD): the nuclear envelopes form emboli, the chromation condenses and aggregates towards the nuclear envelope, which degrades and fully disappears later. Before the nucleus degradatin, neither the nuclear envelope undulation, nucleus lobe nor marked dilation (or bleb) of perinuclear space could be observed. In the cytoplasm of the mature sieve element, there are starch-like granules separately sheathed with a layer of membrane and usually iwth mitochondria around. These granules seem to provide substrates to mitochondria in their function. Small vacuoles orginate from endoplasmic reticulum (ER), and no bigger vacuole was found.