Wood Sorrel (Oxalis acetosella)
A walk through woodland at this time of year will probably reveal Wood Sorrel. It is a small white flower that one casts an eye to, says "It's Wood Sorrel" and you walk on.
Closer inspection, however, reveals more detail and especially the violet veins in the petals. I believe insects, especially bees, can see ultra-violet light and these veins lead to the centre of the plant and so guide any visiting insect to the nectar source and so to the pollen on the stamens. If the bee has already visited a previous plant of the species then accumulated pollen may be acquired by the stigma (the tube in the centre) from where it finds its way down the tube to the ovaries where the seeds are.
I was also interested to see the yellow at the base of the petals. I assumed at first that this was pollen that had stained them but looking in the book it seems that the inside of the petals are naturally yellow even though on the outside they are pure white.
Wood Sorrel is not related to other plants bearing the name sorrel. It is a member of the Oxalis family and I have heard it called Wood Oxalis. Common in Oak and Beech woodland, especially in dryer areas. They flower in March and April and will soon be over for another year.