1. Plants: Biologists can study plants, including their structure, function, growth, and development. They can also study the interactions between plants and their environment, such as how plants respond to changes in light, temperature, and water availability.

2. Animals: Biologists can study animals, including their behavior, ecology, evolution, and physiology. They can also study the interactions between animals and their environment, such as how animals adapt to different habitats and how they interact with other species.