Poinsettia plants are native to Central America, especially an area of southern Mexico known as 'Taxco del Alarcon' where they flower during the winter. The ancient Aztecs called them 'cuetlaxochitl'. The Aztecs had many uses for them including using the flowers (actually special types of leaves known as bracts rather than being flowers) to make a purple dye for clothes and cosmetics and the milky white sap was made into a medicine to treat fevers. (Today we call the sap latex!)
The poinsettia was made widely known because of a man called Joel Roberts Poinsett (that's why we call them Poinsettia!). He was the first Ambassador from the USA to Mexico in 1825. Poinsett had some greenhouses on his plantations in South Carolina, and while visiting the Taxco area in 1828, he became very interested in the plants. He immediately sent some of the plants back to South Carolina, where he began growing the plants and sending them to friends and botanical gardens.
One of the friends he sent plants to was John Bartram of Philadelphia. At the first Philadelphia flower show, Robert Buist, a plants-man from Pennsylvania saw the flower and he was probably the first person to have sold the poinsettias under their botanical, or latin name, name 'Euphorbia pulcherrima' (it means, 'the most beautiful Euphorbia'). They were first sold as cut flowers. It was only in the early 1900s that they were sold as whole plants for landscaping and pot plants. The Ecke family from Southern California were one of, if not, the first to sell them as whole plants and they're still the main producer of the plants in the USA. It is thought that they became known as Poinsettia in the mid 1830s when people found out who had first brought them to America from Mexico.
There is an old Mexican legend about how Poinsettias and Christmas come together, it goes like this:
There was once a poor Mexican girl called Pepita who had no present to give the the baby Jesus at the Christmas Eve Services. As Pepita walked to the chapel, sadly, her cousin Pedro tried to cheer her up.
'Pepita', he said "I'm sure that even the smallest gift, given by someone who loves him will make Jesus Happy."
Pepita didn't know what she could give, so she picked a small handful of weeds from the roadside and made them into a a small bouquet. She felt embarrassed because she could only give this small present to Jesus. As she walked through the chapel to the altar, she remembered what Pedro had said. She began to feel better, knelt down and put the bouquet at the bottom of the nativity scene. Suddenly, the bouquet of weeds burst into bright red flowers, and everyone who saw them were sure they had seen a miracle. From that day on, the bright red flowers were known as the 'Flores de Noche Buena', or 'Flowers of the Holy Night'.
The shape of the poinsettia flower and leaves are sometimes thought as a symbol of the Star of Bethlehem which led the Wise Men to Jesus. The red colored leaves symbolize the blood of Christ. The white leaves represent his purity.
The Poinsettia is also the national emblem of Madagascar.