POINSETTIA FLOWER ANATOMY
The Poinsettias possibly blooming in your house right now are herbaceous greenhouse productions of what in the wild grows to be a woody shrub ten feet tall and higher. You can see the thick, woody, flowering branch-tip of a 10-ft-high tree near my place above at the right. In that picture the red things are not flower petals but rather modified leaves called bracts. Because actual Poinsettia blossoms are very small and inconspicuous, the red bracts take over the job of attracting pollinators. Now take a look at the close-up below and to the left.
So far there's little similarity between Castor-Beans and Poinsettias. But now we do come to a similarity, and that is that each plant bears strictly male flowers and strictly female flowers; both Poinsettias and Castor-Beans are monoecious. Unlike the Castor- Bean with its female flowers positioned above its male ones, however, Poinsettias cram their male and female flowers together inside the cyathium. The flowers are much reduced, each male one consisting of a single stamen while each female flower consists of a single pistil; neither flower type bears a calyx or corolla. In each cyathium there are usually several male flowers but only one female one, and that female is attached in the cyathium's center. Moreover, the female flower does something extraordinary: It sits atop a stemlike pedicel that grows so long that it bears the female completely outside the cyathium, as shown in the above picture, where the female flower is labeled as the pistillate flower, because it consists of nothing but the pistil (stigma, style & ovary). As in Castor-Bean flowers, the red starfish thingy atop the upside-down ovary is the feathery style. In the above picture you also see stamens and immature styles poking from other less-developed cyathia.
Do you enjoy as much as I the esthetic experience of knowing to look for such things as unisexual flowers and waxy, yellow, goblet-like glands -- because of the family relationship -- but then finding the anticipated flowers and glands distributed in surprising places and with completely unexpected configurations? |
Our native-Mexican Poinsettias, which are flowering now,
belong to the same family as Castor-Beans, the Spurge or Euphorbia Family. Therefore, it
can be interesting to compare Poinsettia flowers with those of the Castor-Bean. 
Neither are the yellow-green, cuplike things shown in the picture at the base
of the large red bracts flowers. The cuplike thing labeled as a "cyathium" is a
structures unique to the genus Euphorbia, of which the Poinsettia is one. Poinsettias are
EUPHORBIA PULCHERIMA. The cyathium grows around several small individual flowers, somewhat
similar to how a fig's flowers develop inside the fleshy fig. 
At the right you see yet another similarity between Castor-Bean and Poinsettia
flowers: They both produce waxy looking, yellow, goblet-like glands. It's just that
Castor-Beans put their glands at the top of their petioles, while Poinsettias stick them
on their cyathia.