how do the grain weighting some grammes grow up to weight hundeds of kg?

Answer 1

through photosynthesis and metabolic activity like cellular respiration the plant gets the energy it needs to grow, then growth is primarily a function of cells splitting and the vacuoles in them getting bigger

Answer 2

They keep pulling in water and nutrients that they use to build more and more mass. Some (like sunflowers) do it amazingly quickly. Others (like redwood trees) take thousands of years. However they all do it a microgram at a time.