Leaves and Stem
Wood sorrel is a perennial plant, that can spread by means of horizontal thin rhizomes (underground stems with small scale leaves). The leaves, like the flowers, arise directly from the soil. They may be described as trefoil, that is, they are composed of three equal and heart-shaped leaflets (see below) - which may droop and close up at night.
The three leaflets are held at the top of a long stalk / petiole (about 4 inches / 10 cm). The lower or under surface of the leaf may have a purple tinge, whilst the upper surface is a 'pale' or lighter green compared to the leaves of some woodland species.
Heart shaped leaflet :-
Flowers and Fruits
The flowers are solitary, white and cup-shaped carried up on leafless stalks (petioles). These arise directly from underground bulb and can be about 4 inches / 10 cm tall. The flowers have 5 white petals that have mauve / pink veins. Within the flower are 10 stamens and 5 styles. The flower is approx +/- an inch across.
The flowers are cleistogamous, that is, they undergo pollination and fertilisation before the flower opens.
Ecology and other notes
Wood sorrel is widely distributed throughout the U.K. except for the East of England, for example, around the Wash. It is found in woodlands and hedge banks; it is the only native sorrel and is sometimes 'associated' with ancient woodland.
There are many other species of sorrel to be found but they are mostly introduced.
Images courtesy of Rob Starbuck.