Cyathium dissection and explanation

Today, I decided to delve into the morphology of the poinsettia cyathium (Euphorbia pulcherrima). For those unfamiliar with a cyathium, you should probably read this introduction to poinsettia morphology first. The observation and full set of photos for this dissection can be found here.

First off, here is a poinsettia cyathium and just like all other members of the genus Euphorbia, it has white milky sap. The sap can be seen when a cyathium is cut off.

The stalk that the cyathium sits on is called the peduncle. The big yellow part is called the gland. The small reddish things coming out of the tip are the staminate flowers. The fringed parts under the staminate flowers are involucral lobes and rudimentary cyathial glands (more on these later).

The involucre
The involucre, or cyathial wall, is thought to be made up of five fused bracts (though each gland has two prominent vascular bundles) which each bare an apex and a gland. The apex of the leaf is called an involucral lobe. In poinsettia, it is difficult to show all this because the involucral lobe is laciniate (fringed to the point of having several narrow lobes) and there is only one gland. The reason poinsettia has only one gland instead of the more typical five is that the four other glands were reduced into lobe-like outgrowths that look similar to the involucral lobes. To explain this, I'll first show a photo of Euphorbia missurica.

Source observation: https://www.inaturalist.org/observations/2369030
Note the triangular lobes between the glands and the single linear lobe at the far left (the involucral lobe at the far right was destroyed but was where the large cut out is now between the right two glands). The single linear lobe is the rudimentary "5th" gland. Now, here is a photo of the poinsettia involucre (left: outside; right: inside).

Note how on the photo of the internal view, there are alternating areas below the apex. There are areas with extra tissue (white) which extend forward into the center of the cyathium. These areas represent the bases of the glands. The areas that look more leaf-like represent the bases of the leaf area.

Bractioles and fascicles
The light-colored area mentioned above is special. The tissue that comes out here separates the staminate flowers into five clusters known as fascicles. These partitions become fringed towards the middle or tips and have been called bracteoles. If each can truly be called a bracteole, they represent the bracts of the staminate flowers. Time for photos. The one on the left represents two staminate fascicles surrounded by bracteoles (connate below; divided above). The one on the right represents a cross-section of a cyathium showing the five fascicles and the bracteolar tissue separating each one. Note how it is a fused structure completely surrounding all of the fascicles except where it touches the involucre. It also surrounds the pedicel of the pistillate flower (the gynophore) which is the round structure in the center.

There are also bracteole-like structures that originate at the bases of androphores instead of the bases of fascicles.

It isn't completely clear how these structures should be interpreted, but likely represent another set of bracteoles (rudimentary bracts).

Flowers
The staminate flowers sit atop pedicels called andophores or andropeds. There is a clear line where the androphore ends and the flower begins.

The pistillate flower is abortive in the plant I have.

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How to make cyathium dissections

When doing a cyathium dissection, it is important to either ensure that the cyathium/plant is not turgid (i.e., wilting), has been left sitting for a while so that it isn't turgid when cut, or has been rehydrated after drying (by putting in water with a small dollop of soap). If one of these steps aren't taken, the sap will obscure some of the structures, will stick to your hands, and can cause a safety problem should you rub your eyes after getting the sap on your fingers.

There are several pieces of information that are good to obtain while doing a dissection. For identification and description, it is useful to get the number of staminate flowers per cyathium, number of staminate flowers per fascicle, involucral internal vestiture, description of involucral lobes, and description of glands and rudimentary glands. Gynophore length (and sometimes androphore length) can be useful too but are usually very easy to obtain with very simple dissection.

The most informative shot is a photo of the inside of the cyathium from the center with and without staminate fascicles in the way (left: with fascicles; right: without fascicles).

The reason for this is that it accurately represents the morphology of the glands that can't be observed from the outside, the shape of rudimentary glands, the shape of the involucral lobes, and the hairs (or absence of the hairs) within the involucre. This should be done first to maximize the use of the cyathium.
To make the cut, one should first cut off the peduncle a little above the base of the cyathial wall (as shown at right above) so that the flowers (staminate and pistillate) are all free. After that, a cut must be made in the cyathial wall. It is usually best to cut just next to a non-rudimentary gland if possible and cut to the base (I have cut in different places in this photo series for showing other parts of the morphology). The cyathium can be spread out, put under a coverslip and photographed (both sides are ideal. After getting the shot with the fascicles, gently pull the fascicles out from the cyathium wall, put under a coverslip and shoot again.

The fascicles that are pulled away from the involucre can then be dissected. It is simple enough to just pull and pinch it apart. The staminate flowers can then be counted. The most accurate count will come from old cyathia that have few or no anthers. Under a microscope, the androphores can be differentiated from the bracteoles by being generally wider, their consistent cylindric shape, and prominent vascular bundles.

In case anyone is wondering why I'm writing this and using Euphorbia pulcherrima for the example, I am doing so for three reasons. 1. It is popular and well known by the general public which facilitates educational opportunities. 2. It has large cyathia which make dissection easy. 3. It is easily obtainable. Though E. pulcherrima is a bit over commercialized for my taste, the educational opportunity it brings with it is great. I hope that this gives you a new perspective on the species that facilitates learning more about Euphorbia and even botany in general.

Posted by nathantaylor nathantaylor, January 14, 2019 03:08

Comments

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Thanks, these are very helpful!

Posted by janetwright 9 months ago (Flag)
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Glad you like them!

Posted by nathantaylor 9 months ago (Flag)

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