Humans need to eat plants in order to survive. This is not inconsistent, however, with treating plants respectfully. Some steps that can be taken are to avoid wasting food of any kind, and to consider that it's possible to eat some plant parts without killing the entire being (such as by eating foods that are seeds, fruits, buds, flowers, and leaves).
Feeding plants to animals for later human consumption relies on many more plants than does our eating plants directly, since production of animal products can be relatively inefficient depending on how it is done.. Vegetarianism still makes a great deal of sense even for those who want to treat plants more ethically since it is a more efficient use of plant products for humans to eat these directly rather than eating the animals that ate the plants.
Ethical treatment of other beings is not a zero sum game. It's possible and desirable to look at all beings through an ethical lens. Decent treatment of one group of beings does not preclude decent treatment of other groups.
Recent scientific research has confirmed that plants are extremely complex beings, even though their bodies are not organized in the same ways as animals. While plants don't have brains and nerves like many animals, they are nevertheless extremely sensitive to environmental factors and they rely on many alternative processes and mechanisms for constantly making the complex decisions needed in the course of living their lives.
In many cultures past and present it is commonly accepted that plants can interact meaningfully with humans in ways that we can understand and learn from. Modern Western culture represents more of the exception rather than the rule regarding what is a long held acceptance of plants as intelligent and communicative beings.
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