Growth rings on trees can measure time, allowing scientists to date things from the deep past. But, paleobotantist Dr. Kirk Johnson explains why, in the tropics, some trees have no rings.

For more, check out the <a href="https://www.wgbh.org/podcasts/particles-of-thought/extended-interview-fossil-hunting-sinkholes-and-paleobotany-with-kirk-johnson">extended interview</a> with Kirk Johnson.

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