How to transport organic matter from the green inclusions below poinsettia to the red bracts above through the sieve tube?

Your understanding of "sieve tube conveying organic matter from top to bottom" is not accurate. For more information, please read Baidu Encyclopedia on the transport of substances in plants. The following are some excerpts:

The long-distance transportation of substances between organs of higher plants is carried out in the vascular system. Water, mineral elements and organic matter all have their own main transport channels, such as conduits and screens. Water and mineral elements are mainly transported through xylem ducts. Catheter is the main component of apoplast, which is connected by a series of cell cavities without protoplasm and end wall of cell wall. After the roots absorb water and mineral elements from the soil, they transport them to the aboveground parts through transpiration tension generated by root pressure and leaf transpiration loss. The diameter of the catheter cavity is large, so the resistance to liquid flow is small, and the sweating flow speed can be as high as tens of meters per hour. Organic matter is mainly transported from the "source" to the "storage" through the sieve tube of phloem. "Source" usually refers to leaves that can carry out photosynthesis; "sink" refers to the organ that consumes photosynthetic products, including the top of the growing vegetative body and the forming fruits and tubers. Protoplasm still exists in the cells that make up the sieve tube, and the end wall is specialized into a sieve plate with sieve holes. There are usually some fibrous structures (P- protein) in the sieve pores. When organic matter is transported through the fibrous structure of sieve pores, the resistance is great, which may be driven by the swelling pressure difference between the source and sink, or it may need to be pushed by the living cells near the transport tissue. The conveying speed of sieve tube is much lower than that of conduit, only a few tens of centimeters per hour. Whether an organ is a "source" or a "sink" varies with the growth and development of plants.