Inside of a Seed: Two Dicots

“A seed is a living thing that embodies roots, stems, leaves, and fruit in an embryonic state and retains the ability to convert the sun’s energy into a source of food.” — Seedtime by Scott Chaskey

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Few things are more miraculous than seeds. Within them is a living plant in embryonic form. Under the right conditions, these tiny objects expand, pushing out the beginnings of the most minuscule weed to the most humongous tree. Looking at these otherwise unassuming specks, you would hardly guess that they held such potential.

Housed in a seed is the genetic material necessary for growth and reproduction, along with some stored sugars to get the plant started. All of this is enclosed in a protective case. It is a rare moment in a plant’s life – a time when it isn’t rooted in place and can, for a brief period, move around. With the help of agents like wind, water, and animals it can travel anywhere in the world, venturing as far as inches or miles from its parent plant. As long as it finds a suitable place to grow, its voyage is not in vain.

Seeds are the result of sexual reproduction in plants (with rare exceptions, which we will cover in a future post). After pollination, a pollen grain sends three haploid cells into the ovule of a flower. These cells unite with the haploid cells found within. One germ cell from the pollen grain goes to the formation of an embryo, while the other two cells help form endosperm, the food source for the developing embryo. The wall of the ovule becomes the outer layer of the seed, known as the seed coat or testa. The seed matures as the fruit it is nested in ripens. Eventually, the fetal plant within the seed is ready to find a new home.

Seed heads of rubber rabbitbrush (Ericameria nauseosa) – the fuzzy pappus attached to the fruits allows seeds to float in the breeze and travel away from their parent plant.

As with so many things in biology, there is no single type of seed. When it comes to seed anatomy, most seeds consist of the same basic components, but each species of plant has its own unique seed. In fact, a well-trained taxonomist can identify plants simply by observing their seeds. With such a wide variety of seeds, it is difficult to organize them into discrete categories, but we still try. What follows is an introduction to two types of seeds – endospermic and non-endospermic – using two basic examples.

The first thing you should know about these two examples is that both species are dicotyledons (or dicots, for short). This means that when the baby plant emerges, it has two cotyledons, which are also called embryonic leaves because they look like little leaves. All flowering plants have been divided into two groups based on the number of cotyledons they have, the second group being the monocotolydons (or monocots) which have only one cotyledon. This is an old-fashioned way to classify plants, but it is still useful in some instances.

Endospermic Seeds

The seeds of the castor bean plant (Ricinus communis) are endospermic seeds. This means that they retain the endosperm that was formed when two pollen grain cells joined up with the haploid cell in the ovule. The endosperm will help feed the growing embryo as it germinates. The two cotyledons are visible within the seed, but they are thin and broad, leaving plenty of space in the seed for the endosperm. The cotyledons are part of the embryo and are attached to the radicle, which is the embryonic root. The radicle is the first thing to emerge from the seed upon germination. The area between the radicle and the cotyledon is known as the hypocotyl. It becomes the stem of the germinating seedling.

An elaisome is attached to the outside of the seed coat of castor bean seeds. This fleshy, nutrient-rich appendage is particularly attractive to ants. They carry the seeds back to their colony and feed the elaisome to their young. The seeds, however, remain unconsumed. In this way, the ants aid in the seeds’ dispersal.

seeds of castor beans (Ricinus communis)

Non-endospermic Seeds

The seeds of plants in the bean family (Fabaceae) are non-endospermic seeds. This means that as the embryo develops, it uses up the majority of the endosperm within the seed. The food necessary for the seedling to get its start is all stored in its cotyledons. The common pea (Pisum sativum) is a good example of this. The embryo – which consists of the cotyledons, plumula (or plumule), hypocotyl, and radicle – takes up all available space inside of the seed coat. After germination, as the seedling develops, the plumule appears above the cotyledons and is the growing point for the first true leaves and stems.

seeds of the common pea (Pisum sativum)

In future posts, we will look at a few other types of seeds, as well as discuss various other seed-related topics. If you have a story to share about seeds, please do so in the comment section below.

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Poisonous Plants: Castor Bean

A series of posts about poisonous plants should not get too far along without discussing what may be the most poisonous plant in the world – one involved in high and low profile murders and attempted murders, used in suicides and attempted suicides, a cause of numerous accidental deaths and near deaths, developed for use in biological warfare by a number of countries (including the United States), and used in bioterrorism attacks (both historically and presently). Certainly, a plant with a reputation like that is under tight control, right? Not so. Rather, it is widely cultivated and distributed far beyond its native range – grown intentionally and used in the production of a plethora of products. In fact, products derived from this plant may be sitting on a shelf in your house right now.

Ricinus communis, known commonly as castor bean or castor oil plant, is a perennial shrub or small tree in the spurge family (Euphorbiaceae) and the only species in its genus. It is native to eastern Africa and parts of western Asia but has since been spread throughout the world. It has naturalized in tropical and subtropical areas such as Hawaii, southern California, Texas, Florida, and the Atlantic Coast. It is not cold hardy, but is commonly grown as an ornamental annual in cold climates. It is also grown agriculturally in many countries, with India, China, and Mozambique among the top producers.

Silver maple leaf nestled in the center of a castor bean leaf.

Silver maple leaf nestled in the center of a castor bean leaf.

Castor bean has large palmately lobed leaves with margins that are sharply toothed. Leaves are deep green (sometimes tinged with reds or purples) with a red or purple petiole and can reach up to 80 centimeters (more than 30 inches) across. Castor bean can reach a height of 4 meters (more than 12 feet) in a year; in areas where it is a perennial, it can get much taller. Flowers appear in clusters on a large, terminal spike, with male flowers at the bottom and female flowers at the top. All flowers are without petals. Male flowers are yellow-green with cream-colored or yellow stamens. Female flowers have dark red styles and stigmas. The flowers are primarily wind pollinated and occasionally insect pollinated. The fruits are round, spiky capsules that start out green often with a red-purple tinge and mature to a brown color, at which point they dehisce and eject three seeds each. The seeds are large, glossy, bean-like, and black, brown, white, or often a mottled mixture. They have the appearance of an engorged tick. There is a small bump called a caruncle at one end of the seed that attracts ants, recruiting them to aid in seed dispersal.

Female flowers and fruits forming on castor bean.

Female flowers and fruits forming on castor bean.

All parts of the plant are toxic, but the highest concentration of toxic compounds is found in the seeds. The main toxin is ricin, a carbohydrate-binding protein that inhibits protein synthesis. The seeds need to be chewed or crushed in order to release the toxin, so swallowing a seed whole is not likely to result in poisoning. However, if seeds are chewed and consumed, 1-3 of them can kill a child and 2-6 of them can kill an adult. It takes several hours (perhaps several days) before symptoms begin to occur. Symptoms include nausea, vomiting, severe stomach pain, diarrhea, headaches, dizziness, thirst, impaired vision, lethargy, and convulsions, among other things. Symptoms can go on for several days, with death due to kidney failure (or multisystem organ failure) occurring as few as 3 and as many as 12 days later. Death isn’t imminent though, and many people recover after a few days. Taking activated charcoal can help if the ingestion is recent. In any case, consult a doctor or the Poison Control Center for information about treatments.

The seeds of castor bean are occasionally used to make jewelry. This is not recommended. In The North American Guide to Common Poisonous Plants and Mushrooms, the authors warn that “drilling holes in the seeds makes them much more deadly because it exposes the toxin.” Wearing such jewelry can result in skin irritation and worse. The authors go on to say that “more than one parent has allowed their baby to suck on a necklace of castor beans.” I doubt such parents were pleased with the outcome.

castor bean seeds

Castor beans are grown agriculturally for the oil that can be extracted from their seeds. Due to the way its processed, castor oil does not contain ricin. The leftover meal can be fed to animals after it has been detoxified. Castor oil has been used for thousands of years, dating as far back as 5000 BC when Egyptians were using it as a fuel for lamps and a body ointment, among other things. Over the centuries it has had many uses – medicinal, industrial, and otherwise. It makes an excellent lubricant, is used in cosmetics and in the production of biofuel, and has even been used to make ink for typewriters. One of its more popular and conventional uses is as a laxative, and in her book, Wicked Plants, Amy Stewart describes how this trait has been used as a form of torture: “In the 1920’s, Mussolini’s thugs used to round up dissidents and pour castor oil down their throats, inflicting a nasty case of diarrhea on them.”

A couple of years ago, I grew a small stand of castor beans outside my front door. I was impressed by their rapid growth and gigantic leaves. I also enjoyed watching the fruits form. By the end of the summer, they were easily taller than me (> 6 feet). I collected all of the seeds and still have them today. I knew they were poisonous at the time, but after doing the research for this post, I’m a little wary. With a great collection of castor bean seeds comes great responsibility.

The castor beans that once grew outside my front door.

The castor beans that once grew outside my front door.

There is quite a bit of information out there about castor beans and ricin. If you are interested in exploring this topic further, I recommend this free PubMed article, this Wikipedia page about incidents involving ricin, this article in Nature, and this entry in the Global Invasive Species Database. Also check out Chapter 11 (“Death by Umbrella”) in Thor Hanson’s book, The Triumph of Seeds.