Question

why me Chander and plastids are said to be semi autonomous

Answer 1

plastids are one of the cell organelles that possess their own DNA. this DNA helps them do all/most of their functions on their own without the help of the main cell's DNA. therefore they are called semi - atonomous aka semi automatic

Answer 2

chloroplasts has their own genetic system and complete protein synthesis machinery, enzyme for photosynthesis are synthesised by both the genes of chloroplast and nucleus thus chloroplast are called as semi autonomous organelle of cell