Final answer:
Plants excrete metabolic waste primarily through transpiration and the release of oxygen during photosynthesis. Transpiration involves the release of water vapor through leaf stomata, and photosynthesis releases oxygen as a byproduct.
Step-by-step explanation:
Plants excrete metabolic waste through two primary processes: transpiration and the release of oxygen during photosynthesis. Transpiration is the process by which water vapor is released to the atmosphere through tiny pores called stomata on the plant leaves.
This is a passive process that does not require metabolic energy in the form of ATP. The second process is the release of oxygen as a byproduct of photosynthesis. While photosynthesis is not traditionally viewed as an excretory process, it does involve removing a byproduct of the plant's metabolic activity, thus fitting the broader concept of excretion.