Tag Archives: Fruit

A Journey Through Plant Evolution at the Lincoln Park Conservatory

I recently spent some time at the beautiful Lincoln Park Conservatory. It is very hot and humid in the greenhouse, which may not have been the most obvious choice on a day that was already 88º outside. Nevertheless, I thought it would be a good way to talk about the evolution of plants. My specialty in biology is animals, but I have always loved the story of plant evolution. Like all other major groups of life on Earth, plant life began in the water. Early plants were very reliant on water, but plants became less and less dependent on water as they evolved. In the water, life is easy. Dehydration is not a problem. Nutrients can be absorbed directly out of the water into the cells. The water will carry sperm for reproduction and disperse the offspring. Life in the water is good, but there was a lot of space to grow on land.

The Lincoln Park Conservatory is a great place to go. A cooler day is better.

The Lincoln Park Conservatory is a great place to go. A cooler day is better.

Liverworts

One of the first types of plants to live on land were the liverworts. They are very small, with leaves that lie almost flat on the ground. Liverworts have no ability to draw water up out of the ground, as later plants are able to do. As a result, they cannot grow very tall and must be damp all of the time. They cannot survive or reproduce without being wet.

Liverworts are one of the first plants to live on land.

My favorite room contains the primitive plants: ferns, moss, liverworts, and cycads.

My favorite room contains the primitive plants: ferns, moss, liverworts, and cycads.

Moss

Like liverworts, moss also cannot draw water up into their bodies. Are able to grow a little bit taller than the liverworts because they grow in dense mats that can trap water between individual plants. This allows them to grow up to about four inches tall.

Mosses do not dry out as easily as liverworts, but they do rely on water for reproduction. The male sperm must swim through the water to find a female plant.

Like an idiot, I forgot to take a picture of moss. This one form wikipedia.org will have to do.

Club Moss

Club moss are sometimes called “ground pines” because they can resemble pine trees, but they are neither pines nor moss. Modern club moss usually only grow to be a few inches tall, but during the Carboniferous period, when they were the dominant land plant, they grew as tall as modern trees.

Club moss are one of the first type of plants to have vascular tissue, which lets them draw water from the ground up into their bodies. This adaptation is of unparalleled importance for plants on land. For this reason, club moss were one of the first types of plants to be able to grow more than a couple of inches tall. Without vascular tissue, a plant more than an inch or two has no way of getting water to the upper part of the plant. Plants don’t need very much to live, but access to sufficient sunlight is one of their main requirements. When all of the plants around you are only two inches tall, a plant that can grow to be several feet tall or taller has an enormous advantage when it comes to getting sunlight. You can grow taller than your neighbors and spread out to get all the sun you want.

Club moss (not moss). Image from bio.sunyorange.edu

Coal is made of fossilized plants from the carboniferous period. The majority of coal is made up of ferns and club moss. Sometimes coal preserves the structures of the plants it was made from and we can use the coal fossils to learn about ancient plants.

A thin section of coal clearly shows the features of the stem of an ancient plant (in cross-section). Image from http://www.ucmp.berkeley.edu

Ferns

Evolutionarily speaking, ferns are slightly newer than the club moss. Like the club moss, ferns are have vascular tissue (so does everything else from here on).

Compared to most other plants, ferns grow sideways. The stem lies horizontally underground, and the fronds grow up out of the ground from it. When you see a cluster of fronds sticking up together, they are usually from the same plant.

Fern frond

Fern frond

Structures on the underside of the fronds, called “sori,” contain spores. These capsules break open, releasing the spores, and new ferns grow where the spores land.

Sori are clearly visible on the underside of fern fronds. These contain spores.

Sori are clearly visible on the underside of fern fronds. These contain spores.

Cycads

Cycads look superficially like palms, pineapples or yuccas, but is not closely related to any of them. They were one of the dominant types of plants during the mesozoic era — the age of the dinosaurs.

Cycads were among the first plants to use pollen in reproduction. Pollen is produced by the male structures on plants, and is responsible for carrying sperm to the ovule in the female structures on other plants. Unlike the earlier plants, which require water for the sperm to swim through, pollen is carried by the wind. This is great for plants that live away from water and want to be able to reproduce with individuals that are far away. The problem is that it is fairly inefficient. Plants whose pollen is carried by the wind need to produce vast quantities of the stuff in order for some of it to get to other plants. I grew up in New Hampshire, where there a lot of white pine trees (which are not cycads, but also reproduce with wind-borne pollen). I got up many a morning to find my car completely covered in yellow pollen. All of that pollen that didn’t end up on the right part of the female plants is wasted energy.

Cycads were also among the first plants to have seeds, instead of spores like the older plants. Spores are fine, but they cannot travel over long distances or lie dormant for a more opportune time to sprout. A seed contains the plant embryo as well as nutrients to keep it alive for months or years. If a spore happens to land on the back of a bird on its way to the other side of the country, the embryo inside may not survive the trip because its mother didn’t pack it lunch. An embryo inside a seed will survive the same journey because it is surrounded in an oil-rich substance called “endosperm.” When we eat nuts, it is the endosperm that we are after.

Cycads can superficially resemble pineapples, yuccas or palms, but they are not part of the same group.

Cycads can superficially resemble pineapples, yuccas or palms, but they are not part of the same group.

Flowering Plants

Flowering plants became the dominant plants of the world during the late mesozoic, and today account for the majority of plant species.There are over a quarter million living species of flowering plants, compared to only about 12,000 species of fern, and fewer than 10,000 species of liverwort.

The flowering plants have two evolutionary advancements that allowed them to be so successful: flowers and fruit. These allow plants to solve two big problems in the area of reproduction.

Flowers represent an exchange of goods and services between plants and animals. Big, colorful, aromatic flowers are nature’s equivalent of an “eat here” sign. Flowers produce sugar-rich nectar that animals like ants, butterflies, birds, and bats like to eat. While these “pollinators” are eating the nectar, they get covered in pollen. When they go to the next flower, they drop some pollen off and pick up some more. This results in animals carrying pollen directly from one plant to the next, with very little waste. Remember all that energy that earlier plants wasted trying to pollinate my car? Flowers allow plants to use their energy more effectively. The energy this saves over relying on wind pollination is part of why flowering plants are so successful evolutionarily.

Flowers attract certain animals, which carry pollen between flowers, helping the plants reproduce.

Flowers attract certain animals, which carry pollen between flowers, helping the plants reproduce.

Spreading seeds is another problem for plants. If seeds just fall off the parent plant and onto the ground, some will roll away or get kicked away. The others will sprout right next to their parent and compete for the same nutrients and light. This will reduce the success of both the parent and the offspring. Some (but not all) flowering plants produce fruit to solve this problem.

Fruit is a sugar-rich substance that is easy to get eat, which surrounds the seed, which contains an oil-rich substance that is protected by a hard shell. In human terms, the plant “wants” you to eat the fruit, but does not “want” you to eat the seed. If you eat the seed, you are eating the tree’s offspring. If you (or another animal) eat the fruit, there is a good chance that you will swallow the seeds by accident. The hard shell protects it from being broken in your mouth or digested in your stomach. After eating the fruit, you (or whatever animal) will walk or fly away and eventually deposit the seeds far away in a nutrient-rich pile of fertilizer.

Flowering plants are better at life on land than any other plants. They can draw water and nutrients out of the soil through their roots and vascular tissue, and they are very good at reproducing and spreading their offspring without the aid of water. This is why they are beating all of the other plants.

Cladogram showing the evolutionary relationships of the plant groups and major evolutionary advancements.

Cladogram showing the evolutionary relationships of the plant groups and major evolutionary advancements.

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Drug-Resistant Diseases

Drug-resistant pathogens are in the news more and more these days. The World Health Organization recently released a report about the declining effectiveness of antibiotics. Many believe that the age of antibiotics is coming to an end. This is very troubling, given that antibiotics are unquestionably one of the greatest advancements in the fight against disease.

Infectious diseases are caused by a variety of organisms: viruses (HIV, chicken pox, dengue fever, ebola, influenza, rotavirus), bacteria (tuberculosis, meningitis, gonorrhea, tetanus, whooping cough), worms (schistosomiasis, ascariasis, whipworm, pinworm, elephantiasis, river blindness), protists (malaria, sleeping sickness, giardia, leishmaniasis, chagas disease, cryptosporidium), and fungi (athlete’s foot, thrush, valley fever, aspergillosis).

Organisms that cause disease (such as those listed above) are not fundamentally different from the organisms that do not cause disease. Parasites are subject to evolution by natural selection just like anything else.

A toxin may broadly be defined as a chemical that negatively affects the physiology of an organism when the two come in contact. Drugs that we use to fight parasitic infections within a person’s body are toxins. Ideally, the toxicity of the drug is much greater to the parasite than it is to the host.

There are many naturally occurring toxins in the environment. Ethanol, along with many other alcohols, is a very powerful toxin to most living things. It is because of this toxicity that we use various alcohols to sterilize various surfaces (including our hands, in the case of hand sanitizers) and sometimes instruments.

Humans have a fairly high resistance to ethanol, but not to other alcohols. Most people can drink the equivalent of a couple of ounces of pure ethanol and be fine. The same quantity of methanol or propanol (which are chemically very similar to ethanol) would lead to pretty severe physiological consequences, including death. The ability of humans to metabolize ethanol is attributed to our evolutionary history of eating fruit. Fruit contains a lot of sugar, which is a good source of energy. Ancient people who ate fruit whenever it was available had more energy available in their bodies for building muscles, repairing damage, reproduction, hunting, and whatever else they needed to do. If fruit sits on the tree (or the ground) too long, yeast lands on it and starts eating the sugar for itself. When yeast eats sugar, ethanol is produced as a waste product. If a person eats that fruit, they will also be eating some ethanol. A person who is able to withstand a lot of ethanol is able to eat a lot of fruit and gain the energetic benefits of doing so. Over time, the average human became more and more able to cope with consuming toxic ethanol. Modern humans can safely consume quantities of ethanol that would be fatal to many other organisms.

Methanol, ethanol and propanol are all very similar molecules. Ethanol is the only one we can consume.

Methanol, ethanol and propanol are all very similar molecules. Ethanol is the only one we can consume.

So what does any of this have to do with diseases? Just as humans evolved to cope with the toxicity of ethanol, pathogens can evolve to deal with the toxicity of the drugs we use to fight them.

The most toxic substance to an organism is going to be one that is “evolutionarily novel” — that is, the substance is one that the species has not encountered over its evolutionary history. Why? Because a species that frequently encounters a particular toxin will evolve physiological mechanisms to resist the negative effects of the toxin. For example, if you want to poison a human with an alcohol, you’d use methanol, not ethanol, because ethanol is more evolutionarily familiar to us.

When we use the same drug to fight the same pathogen over many years, the drug becomes evolutionarily familiar to the pathogen, and the pathogen can evolve to cope with it. For decades, physicians used the same antibiotics to treat bacterial infections, including those caused by Staphylococcus aureus. After they used the antibiotic methicillin for a long enough time, the methicillin became a normal part of the bacteria’s environment and the bacteria evolved to cope with it, eventually becoming Methicillin-Resistant Staphylococcus aureus, or MRSA.

The rate of evolution for any given species is inversely-related to the generation time of the species — species with long generation times evolve very slowly, and species with short generation times evolve very quickly.

Organisms with a shorter generation time evolve faster

Organisms with a shorter generation time evolve faster

This is because evolution is a process that happens in between generations. If new generations occur more closely together, more evolution can happen in a shorter time. The human generation is about 20-30 years. A generation for bacteria can be as short as 20 minutes — that’s 72 generations per day, over 26,000 per year, and over a half million within the span of a single human generation. For perspective, a half million human generations is about 10 million years. 10 million years ago, gorillas, chimpanzees and humans had not yet evolved into separate species. What does this mean for antibiotic resistance? It means that bacteria can evolve very quickly to be immune to a given antibiotic.

This is obviously bad news for us. Not all pathogens evolve as quickly as bacteria, but they are all pretty fast. Recent history is full of examples of drugs that worked well until the disease evolved immunity towards it.

Malaria, tuberculosis, HIV, Staphylococcus aureus, Escherichia coli, gonorrhea and many others all have strains that have evolved immunity or resistance to the drugs we use to treat or cure them. We are going to need a new way of fighting these diseases.

Have a topic you want me to cover? Let me know in the comments or on twitter @CGEppig