Describe the exchange of gases which takes place in the leaves of a plant: $(a)$ During daytime. $(b)$ At night.

(N/A) During daytime, photosynthesis occurs, producing $O_2$. The leaves use some $O_2$ for respiration, and the excess $O_2$ diffuses out into the air. The $CO_2$ produced by respiration is entirely consumed in photosynthesis, and additional $CO_2$ is taken in from the air. Thus, the net gas exchange is: $O_2$ diffuses out, $CO_2$ diffuses in.
$(b)$ At night, photosynthesis does not occur, so no $O_2$ is produced. $O_2$ from the air diffuses into the leaves for respiration, and the $CO_2$ produced by respiration diffuses out into the air. Thus, the net gas exchange is: $O_2$ diffuses in, $CO_2$ diffuses out.