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How do plants like green onions seem to grow forever when put in jar of water? I understand that plants get a lot of their carbon from the air (as per this question) but surely they need more than just carbon to grow new leaves.

The answer in the linked question mentions that plants in general get their mass from sources like water and nutrients like nitrogen and phosphorous, and ions.

However, if a plant is growing directly in [distilled] water where will it get those nutrients (like phosphorous and the ions) from?

Where does that extra mass come from?

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  • $\begingroup$ Right there in the first answer to the question you linked to is a statement that shows that over half the mass added by the plant comes from a particular source other than air. $\endgroup$
    – mgkrebbs
    Jun 4, 2018 at 21:30
  • $\begingroup$ The sources it mentions are water and then specific nutrients like nitrogen and phosphorous, and ions. However, other than the water where do those other items come from $\endgroup$
    – Zain Rizvi
    Jun 7, 2018 at 18:31
  • $\begingroup$ If you put the onion in distilled water only, the plant will not get any more nitrogen, phosphorous, or ions. The onion has some stores of these atoms, though, and will continue growing a good while since the bulk of needed molecules (cellulose, sugars) are formed only from carbon dioxide and water. $\endgroup$
    – mgkrebbs
    Jun 7, 2018 at 19:39

2 Answers 2

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Short answer
Plants with storage compartments can grow for extended periods on water alone.

Background
It indeed seems the question is indeed different from the suspected dupe after all.

According to Turesson (2014)

Seeds, tubers and roots are the most common sites for [...] energy storage and the forms in which energy is stored are predominantly oil, starch and sugars. Underground storage organs mainly store starch and sugars, while seeds from different plant varieties accumulate high levels of starch and also oils and proteins.

So storage compounds like "seeds, tubers and roots" can sustain prolonged growth in the absence of pretty much anything, except water; just consider the familiar seedlings of cress on cotton wool (Fig. 1). There's basically nothing there, just water.

cress
Fig. 1. Cress grown on cotton wool and water. source:Dreamstime

Reference
- Turesson, Carbon Allocation in Underground Storage Organs, Doctoral Doctoral Thesis (2014), Swedish University of Agricultural Sciences Alnarp

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The most common elements in organic chemistry are carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus.

Water provides hydrogen and oxygen; air (on Earth) provides oxygen, carbon (mostly CO2), and nitrogen. This leaves out only sulfur and phosphorus.

Therefore, any plant that needs only trace amounts of sulfur and phosphorus (or already comes with all that it needs) could grow reasonably on air, water, and light. Bear in mind that another limiting factor is trace amounts of magnesium in chlorophyll which can't be replenished on air and water.

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    $\begingroup$ I'm failing to see how this answers the question. Further, an answer needs to be backed up by sources. $\endgroup$
    – AliceD ♦
    Oct 25, 2018 at 8:13

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