The inconstant weed

       Life is Juicy

Life begins in the waters—
Not the deep, but the borders of land:
The stagnants that nourish the sterile earth
Like a juicy gland.

Life is the seed of the marriage
Of liquid and solid events.
In the coves, in the swamps, in mysterious pools …

–Leonard Bernstein 1947

 

Sluggish currents and lazy winds mold a soft carpet into fantastical forms…

A great part of the surface water feeding Farrar Pond enters through what some call Beaver Brook (or Beaver Dam Brook), which drains the wide marshes south and east of Baker Bridge, skirts the Fields of St. Anne’s and Mount Misery, and empties due south of Fairhaven Bay. En route, it rests in two small ponds excavated and dammed by the DeNormandie family in the 1950s. As relatively (and sometimes very) fast-moving water

slows into sunlight,

a seasonal dusting of duckweed (of which there are several genera and species) emerges. Where nutrients are concentrated (as here, draining eutrophic marshes and fertile farmland), wind or water may pile the tiny plants into a dense mat inches deep on the surface,

from which dipping branch-tips emerge festooned:

Nutrients are the key. Just fifty steps away, this isolated pool—wet most of most years, but perhaps still best described as “vernal”—exhibits  peripheral and submerged mosses, several kinds of algae and other aquatic or water-loving plants, but no duckweed:

Seeding (via actual seeds or, more commonly, vegetative propagation) by one or more duckweed species seems inevitable; though the pool may be too small and tree-bound for ducks to land, despite vigilant oversight

diverse mammals, reptiles and amphibians undoubtedly traverse the intervening ridge. But this is a glacial outwash plain, highly permeable and only lightly capped with fertile soil even ten millennia post-glacier. So absent a nutrient-rich inflow, soluble minerals needed for plant life are quickly lost to sub-surface outflows, and replaced only slowly by rotting vegetation and occasional imports:

The visible life-cycle of duckweeds differs from that of most pond plants. This rootless drifter overwinters at the bottom as seeds or free dormant buds (turions). Developing sprouts from either contain air pockets (aerenchyma) that loft them to the surface for the duration of the growing season. As winter looms, new seeds and turions are sent below for safekeeping.

In between though, propagation largely takes a somewhat unusual path. Each tiny plant has a region of meristematic tissue, which generates daughter cells asexually. These grow and detach to create new adult plants. As with aphids that are born pregnant, this enables rapid and efficient reproduction; numbers can double in just a couple of days. In a small artificial pool, inoculated from the big pond years ago by both bucket and carapace,

the first new plants begin to float up amid the tangled algae of a desiccated mid-spring:

In a few weeks, larger patches

spread and thicken

enough to provide open concealment for amphibians,

while clearings persist to encourage bottom-rooted plants to emerge:

The carpet continues to expand sideways until coverage is complete, except where disturbed by rain gathered into larger drops by overhanging branches (left side of this image),

sweeping by lilies when their stems or pads are disturbed by wind and indelicate swimmers,

or the plashing entry of frogs from their perches on pad or bank, whether chasing airborne bugs, startled by approaching humans or just moving efficiently from place to place,

(detail)

while lighter

(note pendant roots)

or more lissome creatures leave smaller footprints:

When the single layer  can expand no further laterally, and subsurface light remains adequate, the mat thickens, still floating high enough not to clog the intake of this important piece of hydraulic equipment

and spilling over to the disadvantage of other floating plants (some of which shed it more easily than others):

Yet as the tiny hordes poise to overcome all, their glory begins to dim. First in occasional spots,

then in larger patches, the plants bleach and die:

Parallel decline of the larger genera

bespeaks temporary challenge to the pond’s health, occasioned by intense light and heat around solstice-time, and lack of rainfall to cool and stir the water, bringing needed nutrients down and up to the vital surface. Some of the corpses sink or decay; others serve as softened food for still-herbivorous tadpoles

until almost none remain:

No unnatural catastrophe, this disappearance is the natural result of genetic programming. Unlike species (periodic cicadas and synchronous bamboos, e.g.) that flourish and disappear on a precise clock to achieve sequential predator satiation and starvation and ensure their own survival, duckweeds simply have a limited potential for asexual budding—in the range of a dozen generations. (Our own somatic cells, by contrast, are about four times as durable.) The heavy mats in the Beaver Brook feeder pond represent not only multiple populations that may have evolved divergently over hundreds or thousands of years, but also multiple species. So they will expand in overlapping cycles and without visible interruption. But like rare blood types on Pacific Islands settled by single canoe voyages—or, more dramatically, achromatopsia on Pingelap—the duckweeds in this one little pool have suffered a genetic bottleneck: just a handful of cultivars to start, most died off or eaten or otherwise extinguished over the years, leaving just a few lineages with limited coverage. A salutary warning to proponents of crop, livestock or human monoculture…

But not all those who wander are lost, and in just a couple of weeks, the cycle begins again, to be repeated perhaps twice more before deep autumn

—and again, with variations, in seasons to follow.