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During division and development there is within the cell an organized sequence of synthesis and movement of materials and organelles. The position of the organelles is related both to the formation of the material and to its organized deposition or transport to particular sites at particular times within and outside the cell membrane. Plant tissues offer unique opportunities for the study of development since each cell in a column of cells which lies between the meristematic and the mature regions of the tissue represents a stage in the sequence of the growth and these can be studied either separately or relative to one another. The tissues are also very suitable for microscopical techniques such as radioautography and freeze-etch that are of especial importance for studying the sites of synthesis and the movement of material within the cell.

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The membrane system is made up of the nuclear envelopes, rough and smooth endoplasmic reticulum, Golgi apparatus and plasmalemma. Interconnexions between the various parts of the system are shown and these probably represent a flow of membrane from the endoplasmic reticulum through the Golgi apparatus to the plasmalemma. Membrane fractions have been isolated from broken cells and their function in the synthesis of polysaccharides established. It has been shown that the matrix polysaccharides of the wall (pectic substances and hemicelluloses) are formed within the membranes and that the pattern of synthesis of these polymers changes during differentiation of the cells. Cellulose microfibrils are probably synthesized at the plasmalemma which is formed by incorporation of membrane bounded vesicles from the Golgi apparatus. Thus the assembly of the polymers takes place either when the membrane is within the cytoplasm or when it is incorporated as the plasmalemma of the cell.