(More technically, Poikilohydric Species play possum)
This week’s fieldtrip is tomorrow, so with no crystal ball, so I’m taking a little pre- trip literally to the back yard. Today we’re talkin’ plants with no roots and no internal plumbing, or almost none. All water goes in through the foliage as it rains, but when dry the plants look and act like death, until resurrection when soggy returns. Although there is a spectrum of possibilities and variation in mechanisms, the interesting cases are not mere wilting or closing up shop, but rather extreme deeply suspended animation, essentially lifeless. You wonder how long a plant in that state can rise from the grave. Answer: long.
Remember the term poikilohydric (= plant zombie).
Florida is a great place to encounter these resurrecting oddballs. We have a lot of epiphytes, some of which behave poikilohydrically, most famously our Resurrection Fern (Pleopeltis polypodioides) as photographed by John Bradford:
Spanish-Moss and its cousin Ball-Moss (Tillandsia recurvata) may not be full-blown dry-and-diers, but they get mighty thirsty hung out to dry.
These species use umbrella-shaped scales (think micro-toadstools for second analogy in one sentence) to capture and distribute rainfall.
One drop can irrigate an entire scaly leaf as we will see momentarily. The scales have intricate structure, and flip down in the presence of water. Water moves from scale to scale like an electric current traversing a wire. Watch this video of one drop going a long way. CLICK TO SEE!
What is so astonishing is the depths of the Snow White slumber. Hard-core cases go through profound internal biochemical and structural reduction and disassembly. Internal membranes collapse or disappear. Special protective proteins, enzymes, and electrolytes form. Internal cellular components vanish. Some go so far as to dismantle their photosynthetic apparatus, and reconstruct it upon remoistening. Seen with a microscope the leaf cells look deceased. Then it all comes back.
Below is a series of photos of a liverwort from a tree in my yard. After the photo showing the liverwort on the hot dry bark are three microscope views of the same leaf at the same magnification, the first snapshot while dry, the 2nd photo 15 minutes after re-moistening, and the third shot about an hour later. Notice that in the dry condition the cells look hollow with no apparent chloroplasts (the green disks in the later photos). The chloroplasts are either gone or collapsed into a thin film against the outer walls of the cell. Yet just minutes post-moistening chloroplasts appear magically, and after an hour the previously deadish cells look perky.
The reappearance of chloroplasts is a jiffy. What is truly mind-boggling after wetting, however, is the instant return of respiration, intake of oxygen and expulsion of carbon dioxide, such as our breathing. In their dry state poikilohydric plants show no detectable, biological activity. Within seconds of rewetting carbon dioxide exhales. (The very first carbon dioxide given off is probably not from metabolism, but merely forced out of nooks and crannies by the wetting, then metabolic carbon dioxide from life renewed follows in a few more seconds.) The graph below shows warp-speed carbon dioxide release by a remoistened moss. So fast it’ll make your head spin.
Why that burst happens is not well understood. I mean, why would the plant start respiring, burning sugars, before it is ready to replensih its own sugars by photosynthesis. One line of thought is metabolism revs up like “gentlemen start your engines” before the transmission engages, that is, before all the plant’s membranes and internal structures return to working capacity. The slumbering plant needs to work fast to exploit a fleeting cloudburst.
The distribution of plants with poikilohydric superpowers is broad and spotty. The “lower” plants with no roots or veins need it, having no ability to take lift water from the soil and no pipes to move it internally: some algae, lichens, mosses, and liverworts. Rootless rain dependence may seem like a drag, but it is a plus for living rootless on tombstones, telephone wires, and tree trunks. Some plants with roots and veins, “vascular plants,” have poikilohydric representatives, including fern allies (our local Selaginella), ferns, and assorted Dicots and Monocots, including a few grasses and sedges.
You might ask, how could so many different unrelated plants come up with the same bag of tricks? Well, convergent evolution is not rare, but more tantalizing, the ability of mature plants to go into quasi-death-dormancy is mirrored in something all seed plants have—seeds. The genetic mechanisms that allow seeds to lie dormant and dry for centuries might extend sometimes in some species into adult life (and death, and life).