Every fact in the three plants lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Simplifications to be aware of: the younger lessons say plants 'make their own food'. More exactly, they make sugar (glucose) from carbon dioxide and water using light energy, and they also need minerals from the soil to build new tissue. Plants also respire, day and night, like animals do; photosynthesis only runs in the light. The figure 'more than half of Earth's oxygen comes from the ocean' is an estimate that scientists give as a range; it comes from tiny ocean organisms, not from seaweed alone. In the pondweed experiment the bubbles are mostly, but not purely, oxygen.
All sources
[1] Biology 2e, 8.1 Overview of Photosynthesis
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Gives the overall equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ driven by light energy; in plants photosynthesis generally takes place in the leaves, inside chloroplasts, where the pigment chlorophyll captures light; small regulated openings called stomata let carbon dioxide in and oxygen out; nearly all ecosystems depend on photosynthesis for energy, and it supplies the oxygen many organisms need.
[2] Concepts of Biology, 5.1 Overview of Photosynthesis
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Plants are autotrophs: they make their own food from sunlight, carbon dioxide and water. Heterotrophs, such as animals, cannot photosynthesise and depend on autotrophs directly or through the food chain. The main product is glucose, which cells break down to release energy. 'Chlorophyll is responsible for the green color of plants.'
[3] Biology 2e, 30.5 Transport of Water and Solutes in Plants
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Water is absorbed by the roots and moves up through the xylem; evaporation of water from the leaves through the stomata (transpiration) creates the pull that draws it upward; the phloem carries sugars made in the leaves to growing parts and storage sites.
[4] Biology 2e, 30.6 Plant Sensory Systems and Responses
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Phototropism is a directional response that lets plants grow towards light; gravitropism makes roots grow into the soil and shoots grow up towards sunlight.
Woods Hole Oceanographic Institution · Reliability: High
Type: Oceanographic research institute
More than half of the planet's oxygen comes from the ocean, produced in the sunlit top 200 m or so, mainly by phytoplankton (tiny algae and bacteria that photosynthesise); one kind, Prochlorococcus, alone produces an estimated 5–10%.