Land and Sea

Difficult transitions

My drawing of kelp blades detached from intertidal rocks on which they were anchored. Mendocino Coast, California

            Here's a news flash-- life on land is really different than life in the sea.  It is not an accident that life evolved in the nurturing sea, for the land is actually quite hostile to life.  With the loss of the support of buoyancy, creatures had to evolve new ways to support the body and to move, the oxygen-rich air required different ways to exchange gases, reproduction by spawning needed to be replaced with direct transfer of sperm, excretion of ammonia carried a high cost in water loss, salts to regulate body fluids were not readily available and in most terrestrial habitats water was precious and needed to be conserved. In view of the problems, it is no wonder that of the 25 phyla of animals only two, the vertebrates and the arthropods, have truly and completely terrestrial representatives.  Among plants, there is really only one, the flowering plants.

            As a consequence of these demanding adaptations, there are creatures that we take for granted on land that are simply absent from the sea, for example, insects (with the exception of a few water striders) because insects are unrepentant air breathers.  But also flowering plants because how would they keep their flowers from drowning in the sloshing sea, and what is there out there to pollinate them? So in the place of flowering plants, the continental margins are home to the kelps, aka brown algae (we will ignore the bazillions of single-celled and other algae here).

            A stroll on a California beach, especially after a storm, will yield a rich variety of kelps piled in drifts, an oceanic gift to the terrestrial kelp flies, ghost crabs and beach fleas.  Although kelps have no vascular system and no roots, they attach themselves to the bottom by tangled holdfasts, the various species differing greatly in form, size, and location. Some large species form undersea forests up to 80 meters tall, like a drowned redwood forest.

A bull kelp forest. Photo: Tavish Campbell

  During my first time on a Pacific beach as a young college student, I found photographing the beauty of this beached kelp irresistible.  Evicting hundreds of beach fleas, I floated the kelps in water to capture their intricate forms.  At the time, I knew little of kelps other than their smooth, rubbery blades, their shiny floats, and their holdfasts, piled in large drifts after storms.  It was many years before I gave thought to the reproductive lives of kelps. No doubt, I shared that lack of thought with a vast swath of people.       

One of my first photographs of kelp, in water in a white tray. WRT 1961.

But eventually, I understood that comparing reproduction in kelps and seed plants reveals how the ancestral mode of reproduction still seen in kelps was modified to make a go of it on land.  When a kelp’s “time is upon it,” it produces little organs somewhere on its blades that shed haploid (having only a single set of chromosomes) male and female spores (the kelp is diploid, i.e. with two sets of chromosomes) into the water. These settle nearby and grow into tiny male and female gametophytes whose jobs are to produce haploid sperm and eggs, respectively. The sperm from the male gametophyte swim to the female gametophyte to fertilize the egg (which is then diploid again), which then overgrows the female gametophyte to produce the kind of kelp blade we started with. Only an expert could find the tiny gametophytes, but finding the sporophytes is like falling off a log. You don’t even need to enter the sea because you can find piles of them on Pacific beaches. This reproductive system shares an ancestry with that of terrestrial plants, but because it can take place only in a water medium, for truly terrestrial plants, it is not possible.

            So how did this aquatic system morph to become water-independent for life on land?  To begin with, among land plants, mosses and ferns are not water-independent, forming tiny gametophytes in moist places so that sperm can swim or be carried in films of water. That is why they mostly occur in moist places.

Male gametophytes of the liverwort, Marchantia sp.

From this semi-terrestrial state, flowering (seed) plants have done away with even this need for water by converting the entire male gametophyte into a desiccation-resistant pollen grain and retaining the female gametophyte within the sporophyte (the plant we see) as the ovary.

Schematic of the relationship of male and female ancestral gametophytes to flowering plants. The female gametophyte has been subsumed as the ovary and the male as the pollen grains.

Flowering plants then cleverly engaged animals (indeed, flowering plant evolution is intimately coupled to the evolution of insects) or wind to carry the pollen (the male gametophyte) to the female gametophyte, after adding some machinery to capture and recognize the pollen and induce it to deliver the haploid sperm nucleus to the egg. OK, sure, there are many other refinements, but that is the essence of it. Some flowering plants have opted to rely on the wind (a less viscous version of water) to carry their gametes, and in a sense have reverted to a fluid transport medium. A visual demonstration of this process is the annual spring layer of oak and pine pollen that gives cars and all exposed surfaces a yellow tint, with dozens of pollen grains in every square millimeter. Below is a magnified sample of my car windshield one day after I cleaned it. The outlined area of 100 square millimeters provides the curious an opportunity to count the grains for themselves.

Close-up of my car windshield during oak and pine pollination season, one day after washing the windshield. Count the grains in a hundred square millimeters if you have nothing more important to do.

Evolution always acts on raw material that is already present, but contemplating the raw material that is kelp, who could have predicted that sexual reproduction in plants would become water-independent? Who could have predicted how it would come about? One can but marvel!

My drawing of a kelp with a very complex surface sculpture. San Juan Id., Washington, 2018.