Observations on the development of the sieve-plate pores of Acer pseudoplatanus have enabled five stages in the differentiation of the cell to be recognized. The functional relationships of the endoplasmic reticulum to the formation of the sieve plate and its close association with other organelles of the cell during development have been described in detail. During differentiation slime bodies are formed and these disaggregate to form the fibrillar material present in the lumen of the mature cell. It is probable that the fibres found in the slime body arise from granular bodies which may consist of ribosomes, seen within the organelles. The granules are thought to form the fibrils which are linear subunits of the fibres of the slime body and the sieve-plate pores. An account is given of the degeneration of the nucleus and the changes in the fine structure of the mitochondria and plastids which accompany the differentiation of a cambial initial into a mature sieve element. The formation of laminar material and other inclusions found in the mature cell is also described. Autoradiographic techniques have been used to show the function of the Golgi bodies and their associated vesicles in the development of the cell wall and also to indicate that some of the callose formed at the sieve plate is deposited after the general formation of wall material. This latter observation lends support to the view that callose is deposited at the sieve plate in response to wounding of the tissue.