The bone marrow(which is found in bones) produces cells called lymphocytes which is an essential part to the immune system. The lymphocytes circulate in your blood, and there are 2 types of lymphocytes: T and B cells. The B cells produce molecules that destroy viruses or bacteria
Well, the leaves contain stomates, which let in CO2 and release O2. The roots collect water, which travel up the xylem, which are contained in the stem.