Creation is light

Plants give us the love, the nourishing power of the sun, which is the same energy of all the stars, of all light. These cosmic energies emanated by plants thus nourish, sustain and make grow our own astral body. In this way the existence of plants is a great offering, a sacrifice. They offer us not only their own nutritive value, but the very light and love from the stars, from the cosmos whose messengers they are. They bring to us the universal light so that we can enter the universal life. They exist for psycological, as well physical nourishment.
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Plants exist to transmute light into life. Human being exist to transmute life into consciousness, love. [...] Plants exist to transmute light into life trough photosynthesis. the human being transmutes life into consciousness through perception.
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The proper usage of a plant or herb, during which its true power is released, implies a communion with it. The plant, when we are one with it, will vitalize our nervous system and invigorate our perception. This means giving value to a plant as something sacred, as a means of communion with all nature.

From "The Yoga of Herbs" David Frawley and Vasant Lad.