Kicking the plastic habit 2 - What about Silicone or Rubber?

 Kicking the plastic habit - What about Silicone or Rubber?

In my previous post, I neglected to mention two other materials that might replace some plastic use - silicone and rubber. Are they any better?

The short answer is yes, but with some important qualifications. Neither is made from fossil fuels, and so that is a plus.  Both are less toxic to us and our environment, and neither breaks down into fine particles like microplastics. 'll deal with silicone first.

Silicone is a rubber-like material that is actually made from plain old silicate sand (SO2), as is glass. There are many silicone products that could replace fossil fuel-based plastics, including multiple-use stretchable bowl covers that can replace plastic wrap for saving dinner leftovers. There are also silicone baby bottles, water bottles, mugs, etc. not to mention spatulas and other utensils that take advantage of silicone's high heat resistance. So, sounds like a pretty good deal, right?

Maybe. A significant factor in evaluating any technology is the degree of processing involved. A longer, multi-step manufacturing process is likely to require more energy and result in more pollution. Cutting a piece of bamboo culm to use as a drinking cup is far more environmentally friendly than extracting the cellulose from the bamboo and then using it to make socks (see the process for making viscose in my previous post. 

Glass is essentially sand that has been melted and re-shaped. It takes a little heat, but is a simple process that has been used for centuries. Silicone, however, is a much more radical transformation of sand. Like carbon, silicon atoms have four bonding sites, so have a similar ability to form long polymers with varied side chains.  Intense heat (around 1000 degrees Celsius) is required, however, to remove the oxygen from silica molecules, transforming it into pure silicon.  The silicon is further processed, with more heat, to make it form plastic-like polymers.  The heat required is generally derived from fossil fuels. 

The amount of energy required to make one kilogram of silicone is roughly11 kilowatt hours, the equivalent of 5 kg of coal, or nearly a gallon of petroleum. Roughly the same amount of fossil fuel is needed to make a kilogram of plastic. So the consumption of fossil fuels is about the same for silicone as it is for plastics. Making silicone puts carbon dioxide into the air, while making plastics solidifies it into the stuff that litters the oceans and our water supply. Both problems can be addressed, but the track record so far is not good. Once again, one must look for responsible producers when considering silicone products.

The starting ingredient for both glass and silicone is abundant, but the mining of sand, potentially defacing the landscape and disrupting local ecosystems, is a factor to be considered in the eco-friendly equation. Glass is a better use of sand, and can be recycled over and over. It appears that silicone is non-biodegradable, or at least takes a very long time to break down. So, like plastics, it must be safely buried at the end of its useful life. In the end, silicone has an edge over plastics in that it is less toxic and does not break down into microplastics. 

In the rubber tree, Hevea brasiliensis, latex
drips freely from cuts in the bark, a process
called tapping, which does not harm the tree. 
Photo by Vis M, CC BY-SA 4.0.

What about real rubber? Natural rubber has the potential to do a lot of what silicone and plastics do, but is plant-based and biodegradable. Rubber comes from the sticky latex formed by certain trees and herbaceous plants.
The function of latex for the plants themselves is generally said to be as defense against insect damage. 

The primary commercial source  of latex is the rubber tree, Hevea brasiliense, in the Euphorbiaceae (Spurge or Poinsettia family) but many  members of the Fig Family, (Moraceae), also produce latex, as do members of the Milkweed Family, and others. Dandelions (Asteraceae, or the Sunflower Family) were grown during World War II as an alternate source of latex, when America and its allies lost access to the commercial rubber plantations of Southeast Asia. The cultivation of dandelions for latex is being revived as a more environmentally-friendly alternative to rubber plantations.

Rubber from natural latex sources can replace plastics and silicone in many applications, and is biodegradable, though depending on how it has been processed it may take many years to decompose. Rubber that has been vulcanized to make it more durable, breaks down much more slowly than raw latex or minimally processed rubber.  There is experimentation with bacteria that can speed up the decomposition of rubber. Of course, synthetic rubber is a different story altogether, as it is made, like plastic, from fossil fuels. 

Processing latex in the field and conversion to rubber products in factories are, of course, potential sources of pollution. There are ways to minimize such problems, and consumers can choose products that have been responsibly manufactured.

The biggest potential problem with rubber is that the rubber trees are generally grown in massive plantations, resulting in the usual disease and pest problems associated with monocultures, along with   deforestation and the loss of biodiversity. Similar problems plague palm oil plantations, and indeed most modern agriculture. The cutting down of biodiverse rain forests for rubber or oil production, seems somehow more egregious than replacing grasslands with wheat or corn. 

A better way to grow rubber is through mixed plantings with other crops. It is compatible with growing other tree species for wood, cellulose, fruits or nuts, or for intermixing with low-growing shade-tolerant crops. Such farming can be as profitable as monoculture plantations. It is similar to how indigenous peoples originally harvested rubber directly from wild-growing trees in the forest. 

Bottom line, silicone, and even better, natural rubber, can be considered options for limiting our use of plastics derived from petrochemicals, if the energy and environmental impacts can be mitigated. Once again, with so many people on the planet, there is a cost to whatever technology we use. We can only choose the lesser of evils. 

Kicking the plastic habit with a little help from plants

 I was recently on the expedition class Viking Octantis for a cruise along the Chilean coast. The ship staff included a team of scientists and naturalists who were actively investigating environmental changes in the oceans while engaging and educating the ships passengers.  One lecture dealt with the issue of plastics in the oceans, which now pollute even Antarctic waters and coastlines. That sobering lecture was the inspiration for this essay.

We live in a plastic world. Plastics are durable, inexpensive, and convenient. From plastic water bottles, packaging, and shopping bags to durable kitchen gadgets and automobile parts, plastic products derived from fossil fuels have made life more convenient and less expensive. HOWEVER, from start to finish - from the extraction of fossil fuels to the manufacture of plastic products to their eventual disposal -  the use of plastics poses serious risks to the environment, wildlife, and our own health. 

A sea turtle entangled in a fishing 
net. Photo by Doug Helton.

The primary problem with plastics is that they never go away. Plastic production accounts for about 6% of fossil fuel use, a small amount compared to what is consumed for automobile, aircraft, and industrial fuels, but because of their near indestructibility, the volume of plastics on our planet builds up year after year.  According to a recent study (Li et al., 2023) the amount of plastics produced annually (as of 2019) was a staggering  460 million tons, of which only 9% is recycled. 

The remains of an albatross with stomach full
of plastic debris. photo by Forest & Kim Starr (USGS)

Regrettably, much of this plastic ends up in the oceans, where it can persist for hundreds of years. In 2014 there was one pound of plastics for every five pounds of fish in the oceans worldwide. Plastic water bottles and conventional disposable diapers can last up to 450 years in the sea. Marine mammals, sea turtles and birds get tangled in the debris and drown, or starve to death when their guts get clogged with plastic debris. 

Microplastics are everywhere.

But that's only part of it. While chemically indestructible, much of the plastic debris eventually breaks down into finer and finer particles, what are referred to as microplastics. These fine particles can be taken up by filter-feeding plankton, shrimp, and krill, and then move up the food chain.  Ongoing research is showing that microplastics in our food and water can harm human health (Li et al., 2023).

The major synthetic fabrics used today, polyester, nylon, and acrylic, are highly valued for their smooth feel, resistance to wrinkling, and quick-drying properties,  In addition to their eventual arrival at a landfill, every time such fabrics are washed in your home washing machine, tiny bits of fiber break off and add to the microplastic load in our water supply. 

Cleaning up the mess that is already there, and proper disposal of future plastic waste represent a huge challenge.  There is a glimmer of hope with the discovery of bacteria that can digest plastic. They potentially could help with disposal and the cleanup effort, but so far have not been used on a practical scale. 

Silkworms feed on mulberry leaves and excrete silk
filaments to make a cocoon. This natural process is
analogous to making viscose, but without the harsh
chemicals!
Photo cc Fastily at English Wikipedia

To slow the accumulation of plastics, and maybe even stop it, however, we can turn to natural biodegradable alternatives. Animal products, like wool, leather, etc., can take up some of the slack, but plants can provide  an even more massive source of organic building blocks to replace fossil fuels. The unique plant compound  cellulose, which strengthens cell walls, fibers, and wood, can effectively replace fossil fuel feedstocks in almost any application. Plants have always long provided us  with natural fibers from cotton, linen, and silk (the latter with a little help from a  caterpillar!). If we can shift back more to these natural fabrics, we can make a big difference.

Other cellulose-based products that have been with us for over a century are plastic-like celluloid, cellophane, and rayon.  Celluloid was formerly used to make billiard balls and other solid objects, as well as movie film. Its use was discontinued due to its unfortunate tendency to explode or catch fire upon impact. Cellophane and  rayon, continue to be in use, but have their own associated hazards.     They are made by treating cellulose with harsh chemicals to make a mush that can be extruded through tiny pores to slender soft fibers, or through thin slits to make cellophane sheets. Fabric fibers made in this way are called viscose. The handling and disposal of the chemicals used in this process are bio-unfriendly, if not done carefully. Before purchasing such products, including those puzzlingly soft bamboo socks, one should check whether the manufacturer is environmentally responsible.  

Modern bioplastics have been devised to avoid these problems. One cleaner process converts the plant cellulose into lycocells, similar to viscose, but using less chemicals. Tencel is a product made in this way. There are now many commercial ventures starting to manufacture environmentally-friendly, plant-based bioplastics including styrofoam-like materials, other packaging materials, and even biodegradable disposable diapers. If these catch on, we can significantly shift away from plastics. 

Bamboos are the poster child for fast-growing 
sources of cellulose. New stems are continuously
produced from underground rhizomes, and can
grow up to 3 feet per day.

The cellulose for these processes can be extracted from a variety of sources,  Fast-growing trees, like Eucalyptus, Casuarina, Populus, and Paulownia, along with bamboo and sugar cane stalks, can be grown like other agricultural crops, as a source of cellulose. 

Eucalyptus harvest in Cameroun. Eucalyptus  is
native to Australia, but has been widely planted in
warm parts of the world as a source of wood and cellulose.
It unfortunately often becomes invasive, displacing native 
vegetation. Photo by Jean-Louis Heckly, cc Wikipedia
commons/

Cellulose, of course comes in premade forms like wood and bamboo. Wood can be carved into almost any shape: spoons, spatulas, bowls cups, toys, toothbrushes, etc., for which cheaper plastics are often used today. In traditional and modern technology, bamboo culms can be cut and used directly for housing, flooring, scaffolding, water pipes, containers, paper, and hundreds of other structural materials. 

The drawback to wider use of cultivated bamboo and other cellulose sources is that it requires more land to be converted into agriculture, diminishing the biodiversity of natural plant communities, and creating disease and pest-prone, fertilizer and water consuming monocultures. 

With so many people on the planet, there is no technology that is without environmental cost, but overall, cultivating biodegradable cellulose sources, if done responsible, is far better than continuing to load our oceans and drinking water with plastics. 

The plastic problem seems overwhelming, and much of the solution will have to come from government legislation and reform of manufacturing processes, but individuals can take small steps that will make a difference.  Can you commit to one or more of the following?

1. Use less clothing made of synthetic fabrics and go back to cotton and other natural plant based fabrics. Use an iron if wrinkles offend you!

2. Support emerging bio-friendly technology, including environmentally-friendly diapers, whenever available to replace plastics. 

3. Trade in your plastic toothbrush for one made of wood and natural fibers.

4. Use glass, stainless steel, wood, or bamboo-based kitchen utensils, bowls, storage containers, etc. instead of plastic ones. 

5. Filter your own water and use glass or stainless steel containers for carrying it around. 

Cactus? Look again!

Aloe erinacea superficially resembles a 
cactus, but closer examination reveals 
that the plant consists of closely-spaced,
spirally-arranged succulent leaves with
spines along the edges. In cacti, leaves 
are done away with altogether, or adapted
as spines.

 The picture at the right is a member of the genus Aloe. We all know the most common member of this genus, Aloe vera,  grown as a garden ornamental, as a source of skin ointment, or for its edible leaves. The species pictured, Aloe erinacea, with its compact, rounded overall shape and prominent spines, superficially resembles a small barrel cactus. Aloes and cacti are both succulent plants adapted to survive arid conditions by storing water in their tissues. In cacti, it is the stems that are modified to store water, while in aloes, it is their leaves. A careful look at this plant reveals that it does in fact consist of a compact series of spirally arranged leaves.  

When unrelated organisms come to resemble each other, it is an example of convergent evolution - similarity due to common adaptation, but from very different ancestors.

I've spoken of this a number of times on this site, as succulents in general are a spectacular example of convergent evolution and the all-important process of adaptation. I've been retired for a number of years, but if were to go back into the biology classroom again today, I would walk up to the front and write ADAPTATION on the board (or powerpoint screen!). I would then proceed to show how everything we see in organisms is a result of this process, which unites genetics, ecology, evolution, and systematics.

Cacti and aloes have very different evolutionary histories leading to their convergence. In this post, I want to emphasize the historical dimension of adaptation. The current functional features of a plant have histories of gradual change, sometimes adding or improving functionality, sometimes changing the function altogether. Leaves themselves underwent extensive series of adaptations in early plants just to become the flat, efficient light-gathering antennae that we know today, and similar series of adaptations happened multiple times in different groups of plants. the very leafiness of leaves itself is convergent in true mosses, clubmosses, ferns, different groups of seed plants.

 In various lineages of plants, leaves were further modified into the parts of gymnosperm cones, the parts of the flower and other reproductive structures. Leaves have also been modified through adaptation into grasping tendrils, sticky insect-catching traps, and other specialized structures. In cacti, leaves disappeared, or were converted into spines, as the stems adapted simultaneously for photosynthesis and water-storage. In aloes, leaves retained their photosynthetic function, while adding water storage.

Both cacti and aloes thus came from "normal" plants adapted for less arid conditions. Among the nearest non-succulent relatives of cacti are carnations, and of aloes they are daylilies, asparagus, and amaryllis. Cacti are eudicots, which typically have prominent stem systems and relatively small leaves. Aloes are monocots, which typically have condensed, inconspicuous, and mostly underground stems, but prominent, elongate leaves. It was "easier" for cacti to adapt their already exposed stems for water storage, but for aloes it was easier to add that function to their leaves, than to redesign their underground stems.  So in adapting to new conditions, organisms modify what they already have, in the simplest way ("along the lines of least resistance").

So in considering the process of adaptation, we must keep in mind that organisms adapt to new or changing conditions by modifying pre-existing structures. Developing new organs from scratch happens rarely, if ever. In plants, stems and leaves have been the most plastic of organs, forming a wide variety of distinctive adaptive organs.

For some zoological examples, take the wings of birds. These highly specialized flight organs evolved from the front legs of their non-flying ancestors, radically changing their function.  It happened separately in flying pterosaurs and in bats, using arm and hand bones differently. It did not end there with the birds, for wings went through another transformation in penguins, from flying organs to swimming organs. In snakes,  legs disappeared altogether or remained as a set of tiny useless bones buried within their muscles, what we refer to as vestigial organs. The same thing happened in whales, where the front legs were modified into flippers, while the hind legs were reduced to buried vestigial structures. So one possible endpoint for a history of adaptations is to disappear!

Studying the history of adaptations in particular animals, or particular organs, is one of the most fascinating areas of biology, helping us understand the strange bedfellows resulting in modern classification (carnations and cacti, asparagus and aloes!), as well as the process of adaptation and the ecology of organisms. Proposed evolutionary scenarios must always include a plausible evolutionary (i.e. adaptive) history of how they came to be. 

The aloes and some closely related genera, incidentally provide another opportunity for a theme and variations expedition like I did with the Amaryllis family a little while ago. These leafy succulents are native primarily to Africa, and here are some photos from my collection:

Close-up of an unidentified Aloe
showing the emergence of new
leaves in the center.

Aloe (or Kumara) plicatilis is
unusual in having its leaves
arranged in a single plane.

Aloe (or Gonialoe) variegata

Aloe pictifolia

Aloe dichotoma is a rare example of a monocot that becomes a tree 
through an unusual type of secondary growth.

The difference between blackberries and mulberries and why it matters

Blackberries grow on prickly vines or brambles, 
and are members of the Rose Family (Rosaceae).

 As I was picking mulberries from a tree in my back yard the other day, I was reminded of the similarity between blackberries and mulberries. They are strikingly similar in appearance. 

Like most dark fruits, they are both rich in nutrients and protective phytochemicals. For the consumer, the primary differences are the somewhat milder, less sweet flavor, and the annoying little green stems of of mulberries. Depending on the climate, one or the other may be easier to grow, and fresh blackberries are generally more widely available in stores. Dried mulberries, however, are becoming increasingly popular and more available. Beyond all that, it's a matter of taste.

Mulberries grow on trees, and are 
members of the Mulberry Family
(Moraceae).

In terms of the teaching of botany and evolution, however, the similarities and differences between the two berries tell a powerful story. Though they function the same way in natural fruit dispersal, they are not related at all. Blackberries are members of the the very fruitful family, Rosaceae, which includes raspberries, strawberries, plums, cherries, peaches, apricots, apples, pears, rose hips and many more.  Mulberries, on the other hand are members of the Moraceae, which includes figs, breadfruit, and rubber trees. 

The structures of the two fruits are quite different. Blackberries are aggregate fruits, which means that the cluster of drupelets derive from a cone of separate carpels belonging to a single flower. Mulberries on the other hand are technically infructescences, or compound fruits, similar to pineapples. Each drupelet forms from its own tiny flower. So there are fundamental developmental differences that lead to the similar looking fruits.

This might seem like a geeky bit of botanical trivia that would quickly make dinner guests fall asleep, but in the classroom, however, it illustrates some of the most central phenomena of evolution: adaptation, adaptive radiation, and convergent evolution.

These fruits, first of all, are adapted for dispersal by animals, primarily birds, though I have had to keep an eye out for hungry black bears as well while picking berries along roadsides in Washington State. Both go through green and red phases before turning black at ripening. This primes the animals for the coming feast. The berries are sweet, juicy, and flavorful. The animals gobble down the fruits, and the digestive process strips away the juicy tissues, leaving the tiny seeds to pass through the alimentary canal. The animals tend to move about after feeding, leaving seeds in their feces. (See also "What is an Adaptation?)

The different kinds of fruit to be found in the Rose Family are an example of adaptive radiation - the evolution  of a variety of descendant species from a common ancestor. As the descendants of the common ancestor  began spreading into new habitats and new geographic areas, they adapted to local conditions, including local fruit dispersers. 

As other families went through their own adaptive radiations, some descendants encountered the same opportunity for dispersal, and developed similar physical characteristics, but with tell-tale differences in underlying structure. This is convergent evolution - the development of very similar adaptations from unrelated ancestors. The ancestors of the Rose Family happened to have flowers with multiple separate carpels, and so easily evolved into aggregate fruits, while the ancestors of the Mulberry Family had tiny flowers with just one carpel in each, so a similar fruit was most easily developed by grouping the fruits of many flowers together.  I have posted earlier about evolution of cactus-like members in  unrelated families, along with numerous examples of convergent evolution in animals. (See "Of cacti and humans – are certain life forms inevitable?"

Why do coconut palms lean?

 

Coconut palms commonly grow along tropical coastlines
in a zone of salt-tolerant vegetation, but not directly in
saltwater. Coconuts may fall onto the beach and be carried
away by high tides, but not usually directly into the water.

 Coconut palms have a distinctive, arching growth form, which is somewhat unusual among palms. Most solitary, tree-like palms grow straight upward rather rigidly. The reason for the coconut palm's graceful arch has led to much speculation online, some of it rather goofy, such as that they lean out over the shoreline in order to drop their coconuts into the water for dispersal. Slightly more plausible is that they lean toward the light, or that they are bent by the coastal breezes. 

While these factors may contribute somewhat to the ultimate shape of the mature palms, I'd like to point to a more fundamental factor: the phase of development that all palms go through after germination called establishment growth.  This is something peculiar to tree-like monocots, which have neither a taproot system nor layered secondary growth. In dicotyledonous trees, stem thickness increases gradually throughout the plant, and the root system branches to keep up with it. (See The Root of the Root Problem)

While coconut palms may appear to all lean toward
the ocean (to the left in this picture), they in fact may lean
inland as well, at least at the beginning. Only a few
at the far upper left of this photo are actually leaning
toward the ocean. Note that the bases of the stems emerge from
the ground at a distinct angle. This is the result of the
early phase of horizontal establishment  growth. 

Most monocots keep their main stems underground as rhizomes, corms, or bulbs, and produce adventitious roots. Leafy shoots and/or flower stalks typically arise directly from these underground stems, and die back after their reproductive cycle. Becoming trees, as in palms, screwpines (Pandanus) or traveler's "palms"  (Ravenala), was an evolutionary afterthought, for which new ways to develop trunk thickness and a sufficient root base had to be invented. (See also The Invention and Reinvention of Trees.)

Monocot trees do this by developing their full stem thickness, along with a mass of permanent adventitious roots, at, below, or close to the ground before beginning their vertical growth. The trunk base can widen only by extending more roots into the soil. This is what we call establishment growth. 

The underground stem of a cabbage
palmetto during establishment growth 
is shaped roughly like a saxophone, with 
the mouthpiece representing the seed,
and the opening of the bell
representing the ever-widening shoot
apex. You have to imagine roots
sprouting along the body of the
saxophone, and leaves emerging from the
open end of the bell. 
Drawing from Drawforkids.com.

There are several ways to do this. In cabbage palms (Sabal spp.), for example, the shoot apex first grows downward into the soil, sending up its juvenile leaves  and sprouting adventitious roots as it goes. The stem tip gradually widens and then turns upward. The overall shape of the stem at this stage resembles a saxophone. By the time the shoot apex (stem tip) reaches the soil line, it is as wide as it is going to get, and begins forming a an upright trunk. This takes some 25 years for a Sabal palm.

The production of s series of aerial stilt
roots allows this palm to increase the
thickness of its stem while growing upward.

Other palms, as well as screw pines, begin growing upward immediately out of the seed, as very slender stems that widen as they grow upwards and produce adventitious roots that remain for the life of the plant in the form of stilt roots. 

The horizontal establishment growth of the coconut
palm stem will proceed to the right in this example.
Photo by Vencel, CC attribution 3.0. 

It appears that the coconut palm follows a third model by establishing its basal thickness along with  a mass of adventitious roots, through a period of condensed horizontal growth, with the lower side of the trunk remaining in contact with the soil. Once it achieves full thickness, the trunk gradually curves upward to achieve a more-or-less upright growth, though it often continues to lean. Since a coconut seedling sprouts out of one end of the coconut, the direction of the horizontal growth phase and the eventual upward curve, will depend on which way the coconut is facing when it sprouts - not so much for any functional reason. 

This is my hypothesis anyway. Those of you who have grown coconut palms from seed can perhaps verify or correct it. 

The major breakthroughs of plant evolution

 As plant life evolved, several major breakthroughs allowed them to greatly expand their footprint across the globe. These breakthroughs were major macroevolutionary shifts brought about by a series of small microevolutionary adaptations. The essential characteristics of Plants are each associated with one or more of these major breakthroughs. Such events are described in more detail in Plant Life: a Brief History, I present here a brief synopsis of those major events:

The earliest known fossil cyanobacteria
formed layered colonies that slowly
built pillar-like formations called
 stromatolites, like these from
present-day Australia. Photo by Paul
Harrison, CC BY-SA 3.0

1. Origin of photosynthesis - this central plant process not only marked the beginning of plant life, but also opened up a vast new energy supply to all life on earth and providing the oxygen supply that allowed for complex food webs and distinctive ecosystems. Though seemingly a long, complex process, different parts of  photosynthesis  evolved separately in more ancient bacteria and were brought together through horizontal gene transfer.  Carbon-fixation or the Calvin Cycle, had its roots in earlier chemoautotrophic organisms, where it was driven, not by sunlight, but by energy captured from sulfur and other compounds bubbling up from undersea volcanic vents. The ability to capture sunlight evolved among other bacteria, likely producing only ATP as its product. When coupled with the carbon-fixation process ,simple forms of photosynthesis came into being. The first organisms capable of modern photosynthesis, which releases oxygen as a byproduct, were the Cyanobacteria, which are still abundant today. Solid evidence of their existence goes back nearly 3 billion years, but they may have been present even earlier. (The first plants) (Cyanobacteria - the super heroes of evolution)

Chlamydomonas, a single-
celled alga CC by-SA 2.0

3. Origin of eukaryotic algae -  Primitive animal-like cells, already equipped with mitochondria, captured cyanobacteria through endosymbiosis, which were "domesticated" to become chloroplasts. (Plants and animals and kleptoplasts - oh my!) This occurred a number of times, resulting in multiple unrelated organisms called algae,  which at first floated as part of the phytoplankton of the seas. Sexual reproduction via cells specialized as sperm and egg evolved in these early algae, along with mitosis and meiosis.

Freshwater charophytes
are related to land plants

4. Origin of multicellular plants - With cells remaining attached to one another, and usually also anchored to rocks and other substrates, multicellular algae were able to branch into extensive light-gathering antenna systems, resulting in the various kinds of seaweeds and freshwater plants like charophytes.

Mosses were among the
earliest land plants, and
continue to thrive in moist
habitats. Modern Sphagnum
mosses pictured here form
vast peat bogs, particularly 
in boreal regions.

5. Invasion of the land - Green algae adapted already to freshwater habitats, colonized the land, becoming the ancestors of both bryophytes (mosses, liverworts, and hornworts) and tracheophytes  (vascular plants like ferns, gymnosperms, angiosperms). Early land plants survived by developing water-retaining outer layers and internal systems for storing and transporting water. While such plants remained close to bodies of water at first, they created the vegetation that supported the first animal life to leave the water. The hydrostatic, or turgor, pressure within terrestrial plant cells maintains cell and tissue rigidity and drives cell expansion. It also drives the transport of food-laden fluid in the phloem tissue water and, in combination with evaporation and transpiration, helps drive the movement of water from the roots to the leafy plant tops, even in trees 100 meters tall. (How does water get to the top of a redwood tree?) Turgor pressure is also the basis of plant movements, such as the closing of of leaf traps in the Venus fly trap.  (How plants do everything without moving a muscle?)

Ferns produce wind-dispersed spores
that sprout into gametophytes.

6. Invention of wind-dispersed spores.  In the earliest land plants, and still in modern bryophytes and seedless vascular plants like ferns, sexual reproduction was essentially unchanged from what it was in aquatic algae. Sperm cells had to swim to eggs through water-filled channels and films in the soil. Since the distance sperm cells could travel was very limited, early plants produced dormant, wind-dispersed spores through meiosis from diploid sporophyte plants that developed from fertilized eggs. Spores could carry genetic information between populations, thus promoting genetic diversity and greater adaptability.  Spores germinated into haploid gametophyte plants that produced another round of sperm and egg.(The truth about sex in plants) 

Ovules contain the stages of
reproduction, from spore,
to gametophyte, to embryo
surrounded by food (seed). 
In this cycad, ovules are
borne on modified leaf-like
structures.

6. Evolution of the seed - The seed, called in its early development an ovule, is both a chamber for internal sexual reproduction and a vehicle for the dispersal of the embryo once it matures. Eggs are produced by highly reduced gametophyte plants within the ovules, while sperm cells are produced by even smaller gametophyte plants within specialized spores called pollen grains, which are brought to the ovule by wind or insects.  This liberates plants from the need for water-transmission of sperm to egg, enabling them to live and reproduce in drier habitats, the same way as internal fertilization and laying of desiccation-resistant eggs allowed reptiles to spread through dry terrestrial habitats. The earliest seed plants, took the form of seed ferns, in which pollen-producing sporangia (pollen sacs) and ovules were borne directly on large, fern-like leaves. In more advanced gymnosperms, pollen and ovule forming leaves became distinct from the vegetative leaves and took different forms, most often scale-like structures grouped into strobili (cones if rigid, catkins if soft and flexible). Among living gymnosperms, only some cycads have ovule-bearing structures that still resemble leaves. 

Magnolia flowers have numerous distinct
carpels (uppermost), numerous stamens, all
subtended by a number of tepals (petals/sepals)

7. Origin of the flower -  flowers evolved as a means of manipulating insects and other animals for transfer of pollen from one plant to another (pollination). The seemingly endless  diversity of flowers is reflected in an equal diversity of  insects, birds, and other animals adapted to recognize and feed in  flowers with specific combinations of shape, color, fragrance, and nectar production. Flowering plants, or angiosperms, evolved from ancient seed ferns in parallel with the various groups of modern gymnosperms. Their pollen-bearing structures (stamens) and ovule-bearing structures (carpels), are surrounded by leaf-like petals and sepals, and arranged in a distinctive order in each flower. Carpels mature as fruits that aid in the dispersal of seeds. (What's so primitive about Amborella?)

Grasses dominate extensive areas with
alternating wet and dry seasons. Photo by V. S.
Dustin CC BY SA-3.0

8. Re-evolution of the herbaceous habit -  While ferns and other non-seed-bearing plants were herbaceous, early seed plants and all gymnosperms are woody, as required for the slow development and maturation of their seeds. Angiosperms further shortened the reproductive cycle, so as to make quick-growing, winter-dormant herbs possible again. The most significant group of angiosperm herbs are the monocots, with grasses being the most widespread and ecologically significant herbs on the planet. (How the grass leaf got its stripes) (The grasses that would be trees) Grasses support vast food webs on seasonally dry savannas, and their seeds provide the major source of sustenance for humans around the globe. 

9. Evolution of varied secondary plant compounds - All through the evolution of plants, which are both nutritious and immobile, animals evolved to feed upon them. While some plants, like many algae and grasses, could multiply fast enough to overcome such predation, many other plants have evolved  deterrents, including hard fibers and various forms of spines, thorns, etc., but most importantly toxic or repellant chemicals. Plant chemicals that deter animal herbivores have become numerous and diverse as different species of animals developed immunity to some but not all. By nature, secondary plant compounds are physiologically active and while poisonous in some circumstances, often have valuable medicinal effects. As such, they have been vital to the survival of the human species. (Medicinal plants in our own backyard)

A perfect storm of weeds

 A weed is sometimes defined as a plant out of place - or more often an overwhelming mass of plants popping up where we don't want them. It's a definition based on our futile attempts to to remake a landscape into something a human vision of tidiness. To be fair weeds are often exotic plants - invasive species from another continent freed from their usual constraints of competitors and predators. And so, weeds are also bad for our natural ecosystems, not just to our landscaping vanity.

Weeds are mostly plants that are really good at spreading into disturbed habitats. They multiply rapidly, often asexually, and fill vacant ground or the exposed sides of forests. They are a vital part of succession, preserving soil, nutrients and moisture. And so such plants are good for their native ecosystems. Our native grape vines and blackberries, however, can also become a nuisance at the edge of woods, sometimes creeping into yards, and so give the landscaper an ethical dilemma.

A native species of morning glory begins to reclaim an abandoned logging road in Papua New Guinea.

Up north, fastidious weed-haters spend hours in the 

Syngonium (Nepthytis) podophyllum is valued as
a house plant, but it can escape into native woods.
Here it clambers into a conservation area near a
housing development in Florida.

spring and summer pulling up dandelions one-by-one from their lawns, only to have them repopulate the next season from the one that got away, its seeded parachutes having been blown across the yard by a visiting grandchild.

While you in the north can relax during the winter, we in Florida, continue to battle with the "Vines from Hell" that never take a rest. Our nastiest weeds are climbers and creepers that can smother a bush within months, or just as easily march through beds and across lawns. They are vines that not only grow upward, but also on the ground, sprouting roots as they go, and this is where they are most troublesome. A simple vine can be severed at the base and pulled from the trees, but removing the rooted bits of one of these creepers from the soil is a nightmare. We chop them up, pull them up, dig them up, but if we leave one tiny fragment, it will come to life again like the splinters of the broomsticks in the Sorcerer's Apprentice. All it takes is a single node, with a single tiny bud.

Missed bits of Syngonium resprout in an area that
was recently cleared of it.

So in our Florida yards some of our worst weed nightmares are Nepthytis (Syngonium podophyllum), flame vine Pyrostegia venusta), air potato vine (Dioscorea bulbifera), and skunkvine (or stinkvine), (Pedaria foetida). Skunkvine comes from tropical Asia and air potato from tropical  Asia and Africa. Flame vine and Nepthytis come from tropical America. 

The air potato is a member of the true Yam Family (Dioscoreaceae), while sweet potatoes, which are sometimes called yams, are in the Morning Glory Family (Convulvulaceae). Real potatoes are in the Tomato Family (Solanaceae).  Air potatoes can evidently now be controlled by a beetle from Nepal, and so is not seen in Florida quite as much.

There are a couple more that don't climb trees, but are even more adept at creeping horizontally through beds and lawns. The Boston fern (Nephrolepis exaltata), a popular house plant, will escape into moist woods and form dense colonies in Florida. It is a native of tropical regions around the world, and will actually freeze to death if it attempts to escape anywhere near Boston! Marsh pennywort (Hydrocotyle vulgaris), native to Europe and North Africa, can choke out lawn grass, particularly in moist areas near ponds.

One of the most rampant vining weeds in central Florida is Skunkvine, here twining its way through a Ligustrum hedge.

Flame vine is an attractive ornamental vine, 
but it can smother trees and also spread across
the ground, rooting as it goes.

 

Boston fern seems to be an innocuous house or bedding
plant, but can form thickets in moist woodlands when it
escapes. It extends horizontally through the soil by
slender runners that sprout new plants every few feet.

T

The flowers of pennywort are in umbels, demonstrating its 
relationship to members of the Carrot Family (Apiaceae or 
Araliaceae). Seed production is not important for local
spreading, but is likely responsible for long-distance dispersal.

Air potato vine is a member of the true Yam
Family (Dioscoreaceae). It clambers into
trees.

Air potato vines produce small tubers on their
stems, which fall to the ground and start new vines.
Caution - they are not edible. Photo by Karen Brown;
University of Florida; posted on USDA National
Invasive Species Information Center.