Club Mosses

and the evolution of land plants

Common club moss. Photo by Bob Armstrong

On a recent walk near Echo Cove, I noticed a lovely patch of a club moss sporting dozens of erect spore-bearing ‘cones’. We have several types of club moss here, but the only one I recognize (so far) is Lycopodium clavatum or running club moss. It often has long stems that are covered with short leaves, and they ‘run’ over the ground before making erect branches that bear cones on stalks. Despite their common name, club mosses are not mosses at all; they are on a different branch of the evolutionary tree.

I knew that they originated a long time ago, although they were not the first plants to live on land. But seeing this modern specimen made me think about the evolution of early land plants and the problems that attend a change from an aquatic to a terrestrial environment.

The first land plants belonged to a group of green algae (called Charophyta), some of which became terrestrial perhaps 500 million years ago. It is not clear why they did so, although some researchers suggest that land-living allowed escape from various alga-eaters in the water. But land-living meant that these early colonists risked desiccation: both the plant and its spores had to be protected from drying. This could be done in two basic ways: avoid the problem by growing and producing spores only in wet conditions, or develop water-impervious layers around the plant and its spores. Furthermore, although they were already able to photosynthesize carbohydrates (from carbon dioxide and water), now they had to get the necessary carbon dioxide in a gaseous form, from air. Most of the early land plants were very thin, often only one cell thick, so gases could readily diffuse in and out.

The first non-algal land plants were liverworts and mosses, appearing roughly 450 million years ago. These plants grow close to the ground or other surface, seldom extending upward more than a few centimeters. Although they live on land, they need at least a film of water for reproduction: sperm have to swim to reach eggs to fertilize. They occupy two branches of the evolutionary tree that are adjacent to each other but totally separate from the branch that leads to all other living land plants.

Around 430 million years ago, there was a new development that changed everything. It is not well understood how it happened, but some presumably moss-like land plants developed vascular tissues that conducted fluids from one part of the plant to another, so they were no longer dependent on diffusion. That made a big difference! Thus were born xylem, for conducting water mainly upward, and phloem, for conducting carbohydrates from the green leaves to the rest of the plant. This made possible the development of root systems that both anchored the plants in the ground and allowed the uptake of water from the soil. It also made possible the vertical development of woody stems that raised the leaves well above the ground surface, reaching more light and air, and eventually developing large trees.

One of the first offshoots of the major lineage called vascular plants–which now could exploit both the soil and the aerial space above ground–was a cluster of minor lineages that included the club mosses and quillworts.  Among the fossils of the early forms of club moss was a tree that sometimes grew to be 100 feet tall. Unlike the trees we are familiar with, the trunk of these tall club mosses was not stiffened mainly by wood but mostly by its bark. Some researchers suggest that these trees grew for a number of years but died after reproducing once. They were once widespread on different continents but faded away by about 300 million years ago. That left the un-treelike forms to continue and they are with us yet.

Still to come were the ferns (there were now-extinct types that made seeds) and other familiar modern plants. These invented seeds by covering embryos with maternal tissue. The added tissue provided more protection from drying and often developed structures adapted for dispersal on land. In most cases, the enclosed embryo was also endowed with a packet of nutrition for seedling growth. They developed pollen grains to carry sperm cells through the air so they were not dependent on water for reproduction. Eventually the developmental pattern of some leaves changed, producing flowers for attracting pollinators.

The world changed entirely with the arrival of an array of vascular plants with seeds and pollen. Now there were grasslands and forests providing habitat as well as more food and more ways to get it for more kinds of animals, which embarked on their own various evolutionary trajectories.

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Apical dominance

which leader will lead?

Did you ever wonder how it happens that spruce trees typically have such nice, conical tops? The uppermost shoot, called the leader, produces a particular hormone that suppresses growth in the branches below, most effectively in the branches nearest the leader. The effect dwindles to negligible on the lowermost branches. Voilá! –a conical top to the tree.

If the leader is damaged—chewed by a porcupine, or invaded by an insect, or cut off, the next-lower branches rapidly start to grow, and the damaged tree-top may now display two or three new leaders, until one of them might eventually take over. The dominance of the leader at the apex is disrupted, allowing lower branches to grow more. A similar but usually smaller effect may occur on the end of branches.

The effect of that suppressive hormone is countered by another hormone, one that encourages growth. The balance between the two hormones differs among species of plants, so not all plants grow with conical tops. But one can see the same phenomenon in other trees: for example, look at the cottonwood trees that have been decapitated to prevent them from growing up into the power lines. The remaining branches grow well and begin to reach upward more than before the top was removed.

The strength of apical dominance and the effects of release from that dominance vary among species, although very few general patterns have been discerned (such as effects of habitat, geography, climate, or life history). And the evolutionary pressures that govern both the strength of apical dominance and the effects of its release have been little studied. So I’ll just present a few examples here, to illustrate some of the variation and complexities.

Herbivory, by grazing and browsing animals, often crops off the tops of a plant, ending the dominance effect and leading to branching, which frequently increases the production of flowers and fruits (this is why gardeners commonly pinch off the tops and ends of branches, making a plant bushier and potentially more fruitful). But this begs the obvious question: if branchi-ness is good for the reproductive output of the plant, why was apical dominance so strong, suppressing the branches? Presumably, there are other advantages associated with apical dominance. For example, if the plant allocates resources not to branches but to good vertical growth, this can be advantageous by reaching more light and increasing the survival of the plant, which would eventually lead to greater lifetime reproduction. That option might over-ride the advantages of short-term increased fruit production in most circumstances. Nevertheless, the ability to respond to release, if herbivory occurs, is itself a useful trait. In other words, the evolutionary fitness of a plant may be better with strong apical dominance in the absence of herbivory, but better with release when herbivory occurs.

The club moss (Lycopodium) grows along the ground, sending up short vertical shoots. If the main upright shoot dies, dominance is reduced, and lateral spread increases. The horizontal growth then eventually reaches a new location and a new upright shoot forms there. A living, dominating shoot is successfully exploiting a good site, but when it fails and releases lateral growth, the individual plant as a whole may survive by reaching a new site. In short, the dominance is advantageous in certain circumstances but not in all.

In a species of fireweed (Epilobium in some taxonomies), strong apical dominance leads to good seed production and good seed dispersal (by wind) in areas where competition for light is severe, giving an advantage to good vertical growth and reduced branching. However, this only works where soil nutrients are sufficient to support such growth; in poor sites, which cannot support good growth, the potential advantages of apical dominance may be negated. Again, the advantage of apical dominance is seen in certain circumstances, in this case related to resources.

The relationship between resource availability and the effects of apical dominance are seen also in bearberry (Arctostaphylos uva-ursi). Here (in contrast to fireweed), apical dominance is weak in rich habitats, the plants are much-branched, and the plant is exploiting its present location to the fullest possible extent. In resource-poor habitats, apical dominance is strong, as the plant allocates its limited nutrients to growing in situ: release from apical dominance, perhaps by herbivory, leads to increased branching, increasing the chances of the plant stretching out to find a better site.

These few examples serve to emphasize that multiple factors interact to determine the strength of apical dominance and the consequences of release from that dominance. No wonder that researchers have yet to find general patterns in the ecology of apical dominance. The physiology is well understood; now to learn the whys and wherefores!