I don't think I can go into all the science behind it...but I can tell you that it doesn't necessarily have to be buried. For example, if you don't support the tomato plants and let them sprawl, any vine that contacts the ground can root into the ground. I had that happen last year with a beefmaster that overcame its inadequate support structure.

Below I linked to a Google search which should answer your question.

The specialized structures are called Root primordia aka adventious roots.

Can also be seen on the lower part of the main stem, and in one case has been ided with a specific tomato disease, which I forget right now.

So, no real mechanism, just specialized structures and IMO are most useful when a person is forced to plant plants horizontally for one reason or another, which does give rise to a more elaborate fibrous root system.

Hope that helps,

Carolyn

Here is a link that might be useful: Root Primordia

OK - thanks. I knew that in the wild the tomato is a sprawling plant which roots when it touches or even approaches the ground and I have seen the bumps ie root primordia (thanks Carolyn for the term) myself and that is one reason I was dubious about the statement that the roots grew from the trichomes. It seems they are specialised structures and are not trichomes after some sort of transformation. It is as I suspected but there are frequent repetitions on the web of the idea that the hairs on the stems somehow 'turn into roots' when they contact the soil. All clear to me now. Thanks again.

The hair on the stem absorbs moisture. And if buried in moist soil will develop roots but not every hair will become a root. Based on that principal you plant a small young short branch and it will root and become a plant. Many vegetables do that like basils, shiso, mint.. So planting deeper and/or hilling around the stem of tomato can helt it to develop more roots and be more stable too. Same goes for squash and cucumber.

Thanks seysonn, but I was really looking for some science, that I could understand, as to how the roots are formed and which cells/structures on the stems have the potential to form roots. The ability of many plants to root from stems is well known and exploited by gardeners. I use it myself when I plant Brassicas deeper when transplanting. Carolyn's info about the root primordia was what I was after. I was trying to find out whether it was indeed the trichomes (hairs) which became roots and it seems that it is not. The growth of adventitious roots appears to be caused by a flow of auxins towards preformed root initials (primordia) in a response to excess moisture and is a defence against oxygen deprivation in flood conditions. At least I think that's it, if I have understood it.

Here is a link that might be useful: Tomato stem rooting