Giant kelp (Macrocystis pyrifera) is the world's largest marine plant, growing as much as 50-60 cm (20-25 inches) per day! During favorable oceanic conditions, when water temperatures are a cool 50° - 65° F and nutrients are plentiful, the giant kelp can reach lengths of up to 30.5 meters (100 feet) long. The giant kelp uses gas filled floats to keep the plant suspended, allowing the kelp blades near the ocean surface to capture light for photosynthesis. (photo: Shane Anderson)