Thus, stomata can close during the day, the plant conserves water, and there is still an ample supply of CO 2 for photosynthesis. The transport of the products of photosynthesis by phloem to the rest of the plant. In angiosperms, sieve-tube members are the specialized cells of phloem that function in translocation. Porous cross walls called sieve plates are in between the members and allow phloem to move freely along the sieve tubes. Phloem sap contains primarily sucrose, but also minerals, amino acids and hormones.
Sugar flows from source to sink.
Source and sink depend on season. A tuber is the sink when stockpiling in the summer, but is the source in the spring. Minerals may also be transported to sinks. The sink is usually supplied by the nearest source. Direction of flow within a phloem element can change, depending on locations of the source and sink.