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- sieve elements and
- companion cells.
Sieve elements are so-named because their end walls are perforated. This allows cytoplasmic connections between vertically-stacked cells. The result is a sieve tube that conducts the products of photosynthesis --- sugars and amino acids --- from the place where they are manufactured (a "source"), e.g leaves, to the places ("sinks") where they are consumed or stored; such as
- roots
- growing tips of stems and leaves
- flowers
- fruits, tubers, corms, etc.
Sieve elements have no nucleus and only a sparse collection of other organelles. They depend on the adjacent companion cells for many functions.
Companion cells move sugars and amino acids into and out of the sieve elements. In "source" tissue, such as a leaf, the companion cells use transmembrane proteins to take up --- by active transport --- sugars and amino acids from the cells manufacturing them. Water follows by osmosis. These materials then move into adjacent sieve elements by diffusion through plasmodesmata. The pressure created by osmosis drives the flow of materials through the sieve tubes.
In "sink" tissue, the sugars and amino acids leave the sieve tubes by diffusion through plasmodesmata connecting the sieve elements to the cells of their destination. Again, water follows by osmosis where it may
- leave the plant by transpiration or
- increase the volume of the cells or
- move into the xylem for recycling through the plant.
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