1. In Massachusetts, cambial division in white pine starts as early as February in some years, and is rapid during May. Phloem differentiation may lag until late summer or early fall, and daughter cells may remain in various stages of maturity through the winter. 2. Sieve tube differentiation starts with a localization of the nucleus in a protoplasmic bridge in the center of the cell. Rapid turgor expansion follows, bulging the pit areas and enlarging the cell. Protoplasmic streaming and vital stain accumulation indicate a high state of cell activity. As the sieve tube matures the nucleus disintegrates; the cytoplasm ceases streaming, becomes fibroid in consistency, develops an affinity for stains, and assumes a parietal position in the cell. The cell walls thicken, secondary walls are formed, and the connecting protoplasmic threads of the sieve areas stain heavily. Cessation of streaming, loss of the power to accumulate vital stains, and absence of plasmolysis in the presence of hypertonic solutions indicate a low activity state and a high permeability of mature sieve tube cytoplasm. 3. Denaturing of the protoplasm of the connecting threads is attended by callus formation in their immediate vicinity. Starch grains in the sieve tube plastids are released into the lumen and float free in the liquid contained there. 4. Senility of the sieve tube is attended by increase in the volume of callus on the sieve areas, a thinning down and loss of staining affinity by the cytoplasm, and a reduction in the numbers of starch grains. 5. Death of the sieve tube results in loss of the cytoplasm and callus, invasion of the lumen by air from the outer bark, and finally collapse of the elements. 6. White pine phloem contains no fibers; secondary wall thickening occurs in sieve tubes. Other genera of conifers have phloem fibers, and secondary wall thickening is confined to these elements, which are lignified. 7. Protoplasmic connections in the sieve areas of white pine are slender, solid, and fibroid in structure throughout the functioning period. Those of redwood are even more slender, consisting of anastomosing strands. 8. The picture of the functioning sieve tube of the conifers developed during these studies offers little support to theories of phloem transport requiring protoplasmic streaming or a high activity state in the mature sieve tube cytoplasm.