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The two teenagers are walking across the planet to warn their parents of an evil plan by a major company.

They are wearing pressure suits. They are running out of oxygen. The boys spend the night sealed inside a huge alien plant that seals up overnight.

One boy cannot bring himself to turn off their flashlight even though he feels he is wasting the batteries.

The flashlight being on saves them! The inside of the leaves they are sealed inside undergoes photosynthesis in the artificial light. This produces enough oxygen to save their lives.

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  • Either massive deja vu or this has been asked here before. I re-read it as a result of seeing a very similar question
    – Chris H
    Commented Mar 26 at 21:34

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Red Planet by Robert Heinlein

When Jim and Frank sneak into Howe's office and rescue Willis, the bouncer repeats two overheard conversations between Howe and Beecher, the unscrupulous colonial administrator of Mars, detailing Beecher's plans for Willis and for the colony. When Beecher learns Howe has a bouncer, he is ecstatic; the London Zoo is willing to pay a hefty price for a specimen. Worse, Beecher has secretly planned to prevent the annual migration of the colonists (to avoid 12 months of severe winter weather) in order to save money. The boys run away from school to warn their parents and the colony.

The boys set out to skate the thousands of miles on the frozen Martian canals to their homes. During the trip, Frank gets sick. On the third night, they are forced to take shelter inside a giant Martian cabbage-plant (nearly suffocating when it folds up at night).

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    No problem! Come back with more questions
    – Andrew
    Commented Mar 25 at 12:24
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    Years after reading it, I thought to do the calculation. Optimistically, two people keeping very still will have 50W metabolism each, so 100W. 100W light output is ridiculous for a flashlight, perhaps 3 orders of magnitude too high. And then, photosynthesis is <5% efficient, the flashlight isn't perfect either, and much of the light is absorbed by the boys' clothing and equipment. Maybe a 10 kW flashlight would be enough, except it would cook cabbage and boys.
    – John Doty
    Commented Mar 26 at 21:54
  • @JohnDoty, remember that this was written in the era of incandescent bulbs. 10W would not be unreasonable for such a bulb; a lantern battery could power it all night. Assuming Martian photosynthesis is more efficient (and broader-spectrum) to compensate for the lower light level, it's borderline possible.
    – Mark
    Commented Mar 27 at 1:16
  • @Mark Even if Martian photosynthesis and the flashlight are both 100% efficient, 10W is much less than 100W.
    – John Doty
    Commented Mar 27 at 1:29
  • I think the OP is misremembering. The plant didn't give them oxygen - their flashlight caused it to close around them which kept them warm. Oxygen wasn't the issue
    – Andrew
    Commented Mar 27 at 1:46
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Red Planet by Robert A Heinlein.

From Wikipedia:

Mars has been colonized by humans, but is governed by an administrator appointed by an Earth-based company - the colonists have no political power. Colonial teenagers Jim Marlowe and Frank Sutton travel to the Lowell Academy boarding school for the start of the academic year.

Beecher, the unscrupulous colonial administrator of Mars... has secretly planned to prevent the annual migration of the colonists (to avoid 12 months of severe winter weather) in order to save money.

The boys set out to skate the thousands of miles on the frozen Martian canals to their homes. During the trip, Frank gets sick. On the third night, they are forced to take shelter inside a giant Martian cabbage-plant (nearly suffocating when it folds up at night)

...The flashlight saves them from suffocation...

There's also a major plotline revolving around Jim's pet - a native Martian lifeform known as a "bouncer"

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  • Thank you. This is correct. I accepted the other answer.
    – Judith Jones
    Commented Mar 25 at 12:40

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