Euphorbia pulcherrim is a shrub with attractive red colour bracts which are commonly mistaken as petals of the flowers. The flowers are very small and inconspicuous, grouped within small yellow structures found in the centre of each bract bunch, and are called cyathia.

The poinsettia is native to Mexico and also found in the wild in deciduous tropical forest at moderate elevations in Central America. In the native language, the plant was called ‘Cuetlaxochitl’ meaning “flower that grows in residues”.

The plant’s association with Christmas began in 16th-century Mexico, where legend tells of a girl, commonly called Pepita, who was too poor to provide a gift for the celebration of Christmas and was inspired by an angel to gather weeds from the roadside and place them in front of the church altar. Crimson blossoms sprouted from the weeds and became beautiful Cuetlaxochitl. From the 17th century, this flower was widely used in the Church in Christmas celebrations. The star-shaped leaf pattern is said to symbolize the Star of Bethlehem and the red colour represents the blood sacrifice through the crucifixion of Jesus Christ.

The blood red coloured bracts of
The blood red-coloured bracts of Cuetlaxochitl
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The star shaped leaves of Cuetlaxochitl
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But some variety does not have star-shaped leaves. They have leaves with smooth margins.

Cuetlaxochitl was brought from Mexico to Egypt around 1860. It was loved by the U.S. ambassador Joel Poinsett. The plant was later brought to USA in 1825 and since then the plant was named Poinsettias after Joel Poinsett.

Albert Ecke and his son Paul, immigrants from Germany to Los Angeles in 1900 grew and sold poinsettias initially in street stands. Paul developed the grafting technique and successfully cultivated a variety suitable for pot planting. The grandson Paul Jr promoted and marketed the plant successfully into an enterprise taking up major market shares of poinsettias in USA and overseas. Poinsettias then becomes widely used as a decorative plant for Christmas in North America and Europe.

Poinsettias is known as a short day plant as the plant requires long nights and short days to induce flowering. Long night means about 12 hours in darkness for at least a few days, eg 5 days. At the same time, the plants require abundant light during the day for the brightest bract colour development. Poinsettias flowers in winter around December in the Northern Hemisphere. The same plant flowers around June, the Winter month in the Southern Hemisphere.

A huge amount of crossing has been done on Poinsettias and many varieties with different colours of the bract have been created from red to orange, pale green, cream, pink, or white.

Poinsettias produces sap and white latex which can be harmful to human and animals with low toxicity. It can also cause allergy to some people.

 

Floral Structures

 

Taiwan
From Peng, Ching-I (2015), The Chinese-English Illustrated Botany Glossary, Owl 貓頭鷹, ISBN 978-986-262-243-8 Involucre: One or more whorls of small leaves or bracts standing close underneath a flower or flower cluster

From the above diagram, two different types of flowers- the male and female flowers can be seen. Male flower is a flower with only the stamen remaining. Female flower is a flower with only the pistil remaining.

 

However in the Poinsettias variety I have been studying, it seems that the female and male flowers stay in the same cyathium. Later on the female flower protrudes out from the cyathium. What I observed is similar to the following description in Professor L.H. Bailey, Manual of cultivated plant, The Macmillan Company, 1949

Euphorbia pulcherrima: Aa, flowering branch, x 0.25; Ab, cyathium, x 1; Ac, same, vertical section, x 2; Ad, staminate flower, x 6; Ae, pistillate flower, vertical section, x 3; Af, ovary, cross-section, x 5, (From L.H. Bailey, Manual of cultivated plant. The Macmillan Company, 1949)
Euphorbia pulcherrima: Aa, flowering branch, x 0.25; Ab, cyathium, x 1; Ac, same, vertical section, x 2; Ad, staminate flower, x 6; Ae, pistillate flower, vertical section, x 3; Af, ovary, cross-section, x 5, (From L.H. Bailey, Manual of cultivated plant. The Macmillan Company, 1949)

Cyathium: a type of inflorescence characteristic of Euphorbia; the unisexual flowers condensed and congested within a bracteate envelope from which they emerge as anthesis.

Bracteate: bearing bracts, a much-reduced leaf, particularly the small or scalelike leaves in a flower cluster.

Anthesis: Flowering; strictly the time of expansion of a flower when pollination takes place.

 

Staminate (the male ‘flower’)

2 (3)
Staminate flower (From L.H. Bailey, Manual of cultivated plant, The Macmillan Company, 1949)

 

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The staminate flower
The relative position of the same staminate (right in the centre of the image) with other parts of the flower.
The relative position of the same staminate flower (in the centre of the image) with other parts of the flower. Next to the staminate, there are stamens that look like typical stamens.

 

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Another staminate flower
A staminate flower at a late stage after maturation
A staminate flower at a late stage after maturation

 

Besides staminates, some ‘typical stamens’ are also present in the cyathium. More studies will be conducted to find out the relationship between those two structures.

 

The pistillate (the female flower)

 

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The 5 stigmas of the pistillate are conspicuous. In front of the stigma is a stamen that does not look like the staminate, but looks like a typical stamen.

 

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The flower is cut longitudinally into halves to show the internal parts

 

2 (8)
The cyathium, external view (From L.H. Bailey, Manual of cultivated plant. The Macmillan Company, 1949)
2 (5)
The pistillate flower, vertical section (From L.H. Bailey, Manual of cultivated plant. The Macmillan Company, 1949)

 

The following images show the pistillate grows out from the cyathium
The pistillate flower has protruded out from the cyathium
The pistillate flower has protruded out from the cyathium

 

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Another pistillate flower has protruded out from the cyathium
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The 6 stigmas of the pistillate flower
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The 6 stigmas as seen under the microscope

 

The ovary superior, 3-carpelled, placentation axile

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The transverse section of the ovary showing 3 carpels inside
2 (6)
Ovary,  cross-section (From L.H. Bailey, Manual of cultivated plant. The Macmillan Company, 1949)
2 (4)
Ovary, longitudinal-section (From L.H. Bailey, Manual of cultivated plant. The Macmillan Company, 1949)

The nectary (the nectar gland)

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The side view of the nectary (yellow structure) of the cyathium

 

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The side view of the nectary (yellow structure) of another cyathium under the microscope

 

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The top view of the nectary (yellow structure) of the cyathium

 

 

There are gaps in my knowledge, I welcome advice and guidance from botanists. Please email me directly :

patrick.i.siu@gmail.com

Thank you very much.

 

 

Bibliography :

https://en.wikipedia.org/wiki/Poinsettia

Bailey, L.H. (1949) Manual of cultivated plant, The Macmillan Company

Lawrence, G.H.M. (1951) Taxonomy of Vascular Plants, The Macmillan Company

Peng, Ching-I (2015) The Chinese-English Illustrated Botany Glossary Owl 貓頭鷹, ISBN 978-986-262-243-8