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.

Fern2

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