Take, for example, this pea variety I photographed in the walled vegetable garden at the National Botanic Gardens in Dublin.
I thought it looked a bit odd, so looked up the variety - Sancho. It's a semi-leafless type, which is apparently better for commercial growers as the harvesting is easier than with leafy varieties. It seems the harvesting machinery copes better without so much foliage. So, assuming Mother Nature did not provide this Sancho variety herself, plant breeders have, over the years, selectively crossed pea varieties with smaller amounts of foliage in order to breed out the leafy tendencies. Now, on my RHS course we learnt about leaf adaptations - and a pea tendril is one such leaf adaptation. Basically it's a leaf that has adapted (in an evolutionary sense) to serve a different purpose - in this case, it helps the plant to support itself, to grow vertically, thus maximising exposure to sunlight and gaining a competitive advantage over those pea plants that cannot climb so high.
So, in the process of breeding peas to decrease the amount of leafy growth on the plant, the breeders have selected pea varieties that produce more tendrils. This has resulted in the Sancho pea that not only has less foliage and is easier to harvest for commercial and allotment growers alike, it also produces more tendrils, leading to a better in-built support system and thus a smaller need for managed support such as pea sticks, frames and nets. A most welcome development in my book.