Sex and the sensitive fern

My academic training is in zoology. I have interests in invertebrate zoology, and in molluscan physiology in particular. Given those facts it may not surprise you that, many years ago as a newly-minted college professor, I always found myself one very small step ahead of my students when I had to lecture on the life cycles of what some refer to as the primitive plants. I was especially out of my element when discussing the bryophtes (non-vascular plants like mosses) and the group of vascular plants known as the pterophytes (the ferns). As animals we are used to thinking about life cycles that involve the fusion of gametes (sex cells, egg and sperm) to form organisms (which form gametes, which fuse to form organisms … and so on). The life cycles of many primitive plants, however, differ from those we see among the animals in that they pass through two different body types which differ in size and in showiness, the sporophyte and the gametophyte. The cells of sporophytic plants have two sets of chromosomes while the cells of gametophytic tissues have only one set. Parallels aren’t necessarily appropriate, but consider that the cells of our tissues have two sets of chromosomes while our gametes have only one set. The thing to remember is that in ferns, for example, sporophytic and gametophytic tissues both form visible organisms (although of quite different sizes), plants, while in organisms like you and me it is only the life cycle stage with two sets of chromosomes in each cell that we actually see. Animal sperm and egg (or, forgive me, the structures that produce them) are, for the purpose of this argument, invisible. During its life, a fern will cycle through the different body types. There will be separate (small and quite cryptic) male and female gametophytic plants. These will form sperm and egg, respectively. The sperm and egg will fuse to form the sporophytic stage. A large plant. The cells of sporphytes have two sets of chromosomes. These are the plants we commonly see as we stroll through the woods. Certain cells of the sporophyte will undergo that same process which, in animals, forms gametes, but in this case the process results in spores (with a single set of chromosomes). These are broadcast and then germinate as gametophytes once more. This is called Alternation of Generations. So, why have I felt it necessary to drag you through all of this? I have done so because I wanted to be able to tell you that the image below is of the sporophyte, the spore-producing phase, of Sensitive Fern also, and for obvious reasons, called Bead Fern. The spore-producing structures (sori) look like little beads. I came across many of these plants while walking Charles Brown Brook just yesterday. You may also be interested to know that the species is said to be sensitive because it is often among the first of the woodland species to succumb to the effects of frost, come autumn.


21 thoughts on “Sex and the sensitive fern

  1. Although no botanist either, I was aware of the sperm and egg mode of reproduction in the plant kingdom. But like many, until learning of that, it never occurred to me and came as a bit of a surprise … as did learning that some, although not most, fungi rely on sexual reproduction as well. While mammals, birds and many reptiles reproduce in similar manner, the insect world is filled with varying strategies some of which are beyond the average human’s imagination. Very nice close up of the fertile frond, BTW.

    • Your comment has given me an idea for a future post. Perhaps one without images … one which discusses the evolutionary benefits (origins) of sexual reproduction. D

  2. This really is fascinating. I have, to some degree, moved from the, “Ohhhh … look at the pretty flower” stage to the “Oh, look … Clematis drummondii stage. Still, I’ve never had a class in botany, and I’m still working on things like petals, sepals, stamens, and pistils. Still, this finally made sense to me. I copied the entry, broke it down into six paragraphs, and didn’t move from one to the next until I understood what you were describing. I’ll say this. It made me appreciate for your photo far greater than I would have otherwise.

    • Your comment made me laugh. Joanna is the botanist in the family and, even after more than 33 years of marriage to her, I still haven’t moved beyond the ‘Ohhh … look at the pretty flower’ stage that you so described so well! I’m such a disappointment to her! D

    • Also … thanks for your supportive words concerning the science content. I often hesitate to write about such things for fear of a lack of interest on the part of my readers. I’m so glad you enjoyed the writing and even went so far as to print out and work your way through the tangled discussion of sporophytes and gametophytes. You’ve given me reason to persevere. Thanks so much.

  3. Perhaps we, too, come in five body types: baby, child, adolescent, adult and geriatric. Of course I’m only having a bit of fun for myself here 🙂 I, too, share your love for exposed rock – might as well, given that we have so much of it where I live!

  4. The frost is very well captured. I just love your introduction, botany was my favourite subject at university. I remember studying the different life cycles of different groups … and often mixing them up 😉

    • Your comment made me laugh this morning. I wonder how many of us have had headaches from these difficult-to-remember life cycles? It would appear that I have not been alone!

    • Good question Elke. Each little (spore-producing) bead is approximately 2mm in diameter. I’m glad you liked the up-close view of one of our goats … it was taken quite a few years ago.

  5. LOL! I love parhelia7’s reference to “biology ramblings”! Are you sure you’re still not teaching? I remember seeing these grape colored berries … little did I know they were sori! Gorgeous colors. Beautiful in its simplicity. Did you ever come across these in PA? Are they a regional variety found only in the Northeast?

  6. Beautiful! I missed your photos and biology ramblings 🙂 I’m pleased Alternation of Generations was not a natural idea to you either; when I taught Botany I remember scrambling to come up with an explanation that made sense to the students for this concept! It took me a while to get it myself.

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