Unlike humans, why do plants not have something blood-like to transport $O_2$ throughout their body?
In plant cells, O2 enters through stomata or lenticels. Then, the O2 molecules are transported to the cells through the process of diffusion. Stomata and lenticels are found all over the surface of plant body, i.e., leaves, stem and root. Because of this, plants don't need blood like connective tissue to transport O2 and CO2.