Plants lack specialized excretory organs, so all metabolic wastes are removed exclusively through stomata by diffusion. 

Plants assimilate ammonia into amino acids and can store waste in leaves, bark, and vacuoles. 

In mammals, the primary nitrogenous waste is uric acid, which requires minimal water for excretion compared to ammonia. 

Animals and plants both rely mainly on transpiration as their primary mechanism of excretion.