Kingdom: Plantae
(unranked): Angiosperms 
(unranked): Eudicots
(unranked): Asterids
Order: Ericales
Family: Myrsinaceae
Genus: Anagallis
Species: A. arvensis
Binomial name: Anagallis arvensis
Common names: Scarlet pimpernel, Red pimpernel, Red chickweed, Poor man's barometer, Shepherd's weatherglass, Shepherd's clock. 

The barometer or weather glass common names have their origin in the fact that the flowers close when atmospheric pressure decreases and bad weather is approaching. It is most well known for being the emblem of the fictional hero The Scarlet Pimpernel.
This is a relatively small, scrambling annual weed.   It can germinate and grow at any time of the year and is commonly found in flower gardens and crops. 
Scarlet pimpernel has prostrate stems that clamber across the ground and can grow up over surrounding objects and plants. It has a square, hairless stem, with occasional branches that are perpendicular to the stems and in opposite pairs. 
Leaves have no stalks (known as "sessile" leaves) and grow in opposite pairs. On the underneath of the leaves, there are often small black glands present, which look like dots. 
The flowers are not really scarlet as suggested by the name, but more an orange or orange-red colour.

Anagallis arvensis Scarlet Pimpernel .JPG

Anagallis arvensis Scarlet Pimpernel -001.JPG
Anagallis arvensis Scarlet pimpernel .JPG

Anagallis arvensis Scarlet pimpernel 1 .JPG

Anagallis arvensis Scarlet pimpernel 2 .JPG

Anagallis arvensis Scarlet pimpernel 4 .JPG 

Thanks to Wikipedia for text and information: