Mary Anderson Grierson (September 27, 1912-January 30, 2012) was a Welsh-born Scottish illustrator and longtime staff artist at the Royal Botanic Gardens, Kew.
Biography on Wikipedia: https://en.wikipedia.org/wiki/Mary_Grierson

Epigeneium Coelogyne by Mary Grierson
ND / Watercolor on paper / 15-3/4″x11-2/5″ / Shirley Sherwood Gallery at Kew, London, UK

Mary Grierson on Artnet: http://www.artnet.com/artists/mary-grierson/

Further reading:
https://www.huntbotanical.org/exhibitions/show.php?85
https://suffolkartists.co.uk/index.cgi?choice=painter&pid=4222
https://www.botanicalartandartists.com/about-mary-grierson.html