Saturday, July 16, 2016

What's that structure: Iris versicolor and derived floral anatomy

I was reminded today (by facebook) that three years ago Iris versicolor was blooming when I was visiting a friend's cottage and that I had taken a number of very nice photos and shared them. Seeing these photos brought to mind the particularly interesting anatomy of this flower, so I felt inspired to write a post about it today.

Iris versicolor on the bank of a river
I have posted about this flower, and shared these particular photos on this blog before, but back when this blog was quite a bit lighter on the science. I briefly talk about the unusual floral anatomy of Iris versicolor (blue flag iris, fr: clajeux), but I did not go into much detail, nor did I put the information into any context about floral anatomy generally.

Today's post is a sort of remedy to that previous post.

Let us begin with a basic understanding of floral anatomy. All flowers develop along the same fundamental plan, with assorted modifications. The more a flower deviates from the basic or foundational plan, the more 'derived' it is considered to be. Generally, more derived traits indicate greater evolutionary change over time relative the ancestral condition or trait.

Trillium erectum (red trillium) - example of basic floral structure: the three green, pointed things on the outside are the sepals, the three red ones are the petals, then the six next ring are the stamens, and finally in the centre there is a visible stigma, which is attached to a style and ovary below. Note that these structures come in a specific order from bottom to top
It is simplest to conceptualize floral anatomy as divided into a set of ordered rings, from outermost to innermost; these will be easier to understand if you follow along with the photo of Trillium erectum above. These layers are derived from leaves (i.e. they are modified leaves, if we go back far enough in the evolutionary history of flowers). The outermost ring contains the sepals, which usually serve primarily as a layer to protect the developing flower in the bud stage, and sometimes also serving as structural support for petals. The next ring in contains the petals, which are of course the primary visual attractant structure. Assorted derivations of the basic petal plan can also help manipulate the orientation of a pollinator approaching a flower, thereby increasing precision of pollen transfer, or to restrict access of pollinators to various parts of the flower, thereby reducing resource loss to robbers or ineffective pollinators. In some families of flowers (notably, Lilaceae, the lily family, and Asparagaceae, the asparagus family), sepals serve similar attractive and structural functions to petals and are not immediately distinguishable from them visually. In these cases, we refer to both sepals and petals as tepals. Below, a photo of Lilium philadelphicum (wood lily) shows a great example of tepals. Notice the lack of any visible sepal, and also if you look closely where the tepals attach to the stem you can see that there are three attached lower and three attached higher; those attached lowers are derived from sepals and those attached higher are the 'original' petals.

Lilium philadelphicum, example of tepals
The next layers are the ones which produce reproductive cells: first, the stamens (composed of filament, a structural element, basically a stalk to, and anther, the portion of the plant that contains cells that create male gametes, i.e. pollen). This layer is responsible for the generation and presentation (exception: secondary pollen presentation in some families, a matter for another post entirely) of male reproductive cells. The innermost ring of cells is the female reproductive portion of the flower, containing some form of ovary with ovules inside, style(s) (a raised portion to receive pollen), and stigma(s), the receptive surface on this style which receives and germinates pollen for fertilization of the ovules. Together, a stigma-style-ovary set is called a pistil, and one flower may have many of these. The particular anatomy of this portion of a flower has a lot of variation I won't get into here, as the general notion presented here is sufficient for the purpose of understanding what's so cool about I. versicolor.

So, back to I. versicolor. Now that we have a reasonable understanding of floral anatomy, something seems odd about this flower.

Iris versicolor, top view
You may now be wondering -- where are the stamens, the pistils? I just spent a fair bit of time writing about all these rings of structures, but everything looks like a petal here.

The flowers of I. versicolor are highly derived; irises are of sufficient anatomical interest that there are actually special names for all the structures for these irises, but they are all analogous to the layers described above. I'll take you guys through these layers again from top to bottom and point them out with photos.

Iris versicolor
The outermost layer is supposed to be the sepals. This remains true in the irises. The outermost layer in this case, the sepals of this iris, are the three largest structures, the ones that broaden out in a kind of spoon-like fashion at the tips. The spoon-like portions are referred to as "falls" and the yellow patch as a "signal".

The petals are actually the three things sticking up in the middle, referred to as "standards" (somebody was way too enthusiastic about the quasi-military and flag-based metaphors in naming the parts of an iris).

Iris versicolor - style crest, falls, signal
This is where things get interesting now. We're still trying to find the stamens and pistils, right? They're found above the sepals; the pistils are that smooth-looking structure that curves down over the top of the sepals, while the anthers are curved and tucked underneath. The arching portion of this fused structure is called the style arm, and the raised portion at the very end that curls upward is called the style crest. Under the style arm, the stamens are arching along the top of the tube-like constriction made by the sepal and pistil. Finally, the stigma, the receptive part of the pistil, is a ridge of hard tissue at the intersection between the style arm and style crest, where the structure seems to 'fold' upward.

Iris versicolor - stigma, style crest, falls, signal
And that pretty much covers the awesome, highly derived anatomy of irises.

This particular species of iris, I. versicolor, is native to eastern North America (range map here). It is an obligate wetland species [1], found exclusively where there is sufficient water (lakesides, marshes, ponds, streams, etc). It is the provincial flower of Quebec.

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