Below is what I wrote and cut and pasted from another site as I mentioned above. The person asked about it but also said it was for hardening off, which it isn't, and so that's why the last sentence re hardening off being a different issue.
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The initial observation was that in commercial greenhouses, which have narrow aisles, the plants along the aisles were bushier and stronger. And it was due to the workers brushing up against them as they went down those narrow aisles.
The phenomenon is known as thigmotropism, or response to touch.
The Cornell Coop ext in my area suggested that commercial tomato growers brush the tops of the plants with a broom handle a couple of times a day, but as my commercial friend Charlie noted, it takes him and his workers sometimes up to 6 hrs a day just to water what's in the greenhouses, so there's really no time for that.
Besides, all greenhouses have huge exhaust fans and they can help as well. So home growers can stroke their plants and/or run a fan on low speed over the tomato seedlings as they grow,
Hardening off is a totally separate issue. ( smile)
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Carolyn
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