The Mystery of Seed
This was the goal of the leaf and the root.
For this did the blossom burn its hour.
This little grain is the ultimate fruit.
This is the awesome vessel of power.
For this is the source of the root and the bud….
World unto world unto world remolded.
This is the seed, compact of God,
Wherein all mystery is enfolded.
-George Starbuck Galbraith,
The New York Times, May 6, 1960
There is a beauty and wonder in seeds, an enigmatic captivation, a fascination full of hope and promise for the future. As I sit here, futilely trying to resist the urge to place “just one more” order for seeds online, having already just come home from Agway with a few packets tucked into my bag of seed potatoes, I am inspired to consider for the umpteenth time, what is the irresistible allure of seeds. What makes me scour page upon page of seed catalog after seed catalog, 30 or more in a season – what fuels my desire to collect, grow and possess seeds. Continually searching for new seedspeople, I travel across the country just to gather with fellow seed enthusiasts. My Instagram feed is riddled with seed savers, seed breeders, seed historians and seed librarians, my bookshelves safeguard a collection spanning 1000s of years of human connection to seed and the many uses of plants to nourish, cloth, protect, and enrich our lives.
Take a moment and ponder with me, what is a seed? Why would you read this article about seeds? Where does your imagination take you when you gaze down at that handfull of seeds just before sowing them gently in the warm spring soil? Where did they come from? How many lives intersected with the past of that seed and the decisions made that collectively and ultimately created that seed? That very seed there in your hand! What will happen to it once it leaves your hand?
Through botanical and ethnobotanical texts, college and post-graduate degrees, conferences, symposia, and endless discussions with fellow seed addicts, I have shared this fascination and sought answers to these questions for decades. Since I was 11 and opened my very first Seeds Blüm catalog, excitedly shared with me by my mother, not only seed, but its stunning diversity enchanted me. I was immediately taken in by the concept that each seed carries a history with it and that the potential within a seed can be handed down through generations like any other precious family heirloom.
I was most taken by seed diversity and the ties of that diversity to our individual cultural identities. My mother was a horticulturist in central Maine during the 1970’s and ‘80’s and was taken by the ‘back to the land’ movement so much so that she had us experimenting with and practicing organic agriculture and growing variety trials in our back yard for local seed companies and the old Farmstead Magazine out of Freedom Maine. We grew tomatillos, three different kinds, and daikon radishes in 1982. Not just the one spinach variety you bought seed for at the hardware store, but 6 others grew in tidy rows in dad’s proud raised beds, nourished by compost and protected by shredded leaf mulch! None of my friends at school knew what I was talking about when I explained what we had for dinner the night before! We bought seeds from Seeds Blüm, Johnny’s, Vessey’s and I’m sure a myriad of others, and grew them alongside the varieties Farmstead asked us to grow, just to experiment on our own – to grow the history and diversity within our growing seed collection.
Seed diversity… some seeds wear their diversity on their coats brilliantly patterned with a myriad of colors while others hide their light inside, waiting for just the right conditions to sprout and reveal the potential held within — potential to respond, acclimate and survive to the fickle environment in which they may sprout.
Fascinated, captivated, and enchanted by seeds, my lifelong journey with seeds and plants has lead me to the deep belief that seeds are our agricultural history and the fundamental unit of our food system. They carry and define our cultural identities as our ancestors evolved alongside the foods they domesticated and cultivated. Every seed holds the potential to continue that journey of co-evolution with humans. With a deeper understanding of how seed is created, we can all continue to engage in that co-evolutionary dance that has been man’s greatest privilege since the onset of civilization — a dance that has driven the very rise of civilization!
What is a seed?
Plants reproduce themselves in many ways including asexually via cloning themselves. In flowering plants, however, biodiversity is increased much faster through sexual reproduction and the complex mixing of genetic material that is contained in the resultant seed. To begin to understand how this process may play out on our farms, let’s start with a simple question: what is a seed? It is symbol of hope, poetic muse, primary component of human food worldwide, extreme survivalist, and protector and progenitor of its species. Botanically, a seed is a complex structure containing a dormant embryo that is capable of enduring extended periods of components of a seed are: the embryo; food storage; and a protective coat called the ‘testa’.
The embryo is the tiny dormant plant, complete with a nascent root, the ‘radicle’, and shoot, called the ‘plumule’. Connecting these is the ‘hypocotyl’, an embryonic stem that will elongate as the seed germinates and pushes the first leaves out of the ground after the radicle takes hold in the soil.
Food is necessary for the embryonic plant as it awakens and resumes physiological function within the seed. To germinate, the seed coat splits and the radicle is the first structure to grow out of the seed, into the dark soil. Once the radicle takes hold and anchors the seed, the hypocotyl elongates and the plumule emerges from the soil. All of this happens before the tiny plant is able to photosynthesize. The seed’s stored food is all the energy the seed has to get its very first leaves into the sun. Food is stored either as endosperm surrounding the embryo, or within the cotyledons or ‘seed leaves’. The type of food energy stored varies between seed type. Corn, and many other small grains, for instance, primarily stores starch, while oilseeds such as flax, sunflower, or canola predominantly store oil and fat. Legumes are high in protein.
The seed coat protects the embryo and food source from environmental elements. It is often leathery and capable of expanding when the seed is ready to germinate. Many seeds can persist for years in the soil without dying because of the testa’s ability to protect its precious contents so completely. It would be wrong to say one of these structures is more important than another. If one of them fails, the seed will likely die or suffer severely reduced vigor. Collectively, they are a biomechanical masterpiece.
How is a seed formed?
The genesis of these miraculous structures is a much more complex topic. Yet taken piecemeal, it is an elegant and logical process that unravels like any other of life’s great mysteries. It begins with a seemingly simple flower and requires an understanding of flower form, function, and biology – i.e. botany!
Botany of seed formation: Flower development
The single purpose of a flower is to produce seed. It is the setting for the development of male and female sex organs and where fertilization of an egg by sperm occurs. Evolution has favored an enormous degree of variation in the strategies plants use to form flowers, fruits and seeds. Let’s explore the common features shared amongst flowers before we dive into the deeper waters of diverse mating systems.
Fundamentally, a flower is a modified shoot, i.e. stem, of a plant. It starts out as any other bud with the potential to become either another branch or a flower. The genes that control the development of a branch and flower are the same. To become a flower, a plant turns these genes on and off at different times and rates than if it were directing the development of a new branch. The structure that develops and holds flowers, is called an ‘inflorescence’ instead of branch. There are many types of inflorescences. Flowers can be arranged in many ways on the inflorescence stem that is called a ‘peduncle’. This includes a simple solitary flower at the end of a single peduncle and multiple flowers arranged on panicles including racemes, spikes, umbels, and capitula. The arrangement of flowers on an inflorescence affects, in part, how those flowers can or can not cross pollinate and mate.
The peduncle terminates in a swollen tip called a receptacle and the flower sits atop that. The flower itself begins formation hidden inside a bud and is comprised of four separate whorls of modified leaves: sepals, petals, stamen, and pistil, each with an important function. The sepals are the most leaf-like in both form and color. They are the green layer you see on the outside of a flower bud and their function is to protect the bud as the young flower parts form within. The petals form just inside the sepals and their main function is to attract pollinators. Petals are often highly pigmented with both the rainbow of colors the human eye can see and ultraviolet colors that only pollinators can see. Petals also often contain nectaries, small glands that produce nectar that serves as a reward for visiting pollinators. Nectaries can be produced on other parts of the flower and plant, but are most often associated with the petals.
The last two whorls in a complete flower are the separate male and female organs. The stamen, produced just inside the petals, is comprised of a filament and an anther and is the male sex organ of a plant. The filament contains vascular tissues and carries nutrients to the anther, inside which thousands of pollen grains are formed. Pollen is a tiny, autonomous structure capable of living outside of the parent plant and is composed of three cells, two are sperm and one, the ‘tube cell, is a cell that will ultimately form a pollen tube.
Nestled and protected within all of these layers is the fourth and final whorl that is the female portion of the plant, the pistil. The pistil consists of a stigma, style, and an ovary (also often called a carpel). A pistil can contain one or several carpels fused together. The stigma is the outer most portion of the pistil and is the site upon which a pollen grain lands. When the flower is mature, the surface of the stigma is said to be receptive and is sticky which makes picking up pollen easier. The stigma reacts biochemically to pollen grains to determine if the pollen grain is compatible to mate with the flower or not. A potential barrier to fertilization is that under extreme heat and dry conditions the stigma of a mature flower may dry up making pollen recognition impossible. The style is specialized tissue that connects the stigma and ovary through which pollen tubes will grow. It is also thought to help the stigma determine pollen grain compatibility.
Located at the base of the pistil is the ovary, or gynoecium (the “female house”), containing several layers of maternal tissue. Deeply held and protected within the ovary are the ovules, complex structures that contain more layers of mother tissue and, ultimately, an embryo sac. The embryo sac contains the female sex cells, namely an egg and the “polar nuclei”. An ovary can contain one to several hundred ovules. It is noteworthy to mention that the multiple layers of protective tissue surrounding the ovules are an evolutionary trend in the development of complex flowering land plants. This development ensures not only the long-term survival of the species, but its ability to disperse itself across diverse habitats and ecosystems.
Flowers can have all of these whorls (the sepals, petals, stamen, and pistil), or just a few of them. Flowers possessing all of these parts are said to be “complete”. Flowers missing one or more major parts are “incomplete”. Flowers can also be classified according to their sex. Some flowers are bisexual and contain both stamen and pistils. Other flowers are male and do not contain a pistil. While still other flowers are female, and do not contain a stamen. Finally, some flowers are asexual and lack reproductive organs altogether. Hybridization of ornamental flowers to have more petals, often involves breeding to change the gene expression that directs the development of each whorl so that where a wildflower would have created stamens and pistils, it’s hybridized sister will produce more whorls of petals. Look closely the next time you see a daffodil or Echinacea with multiple layers of petals and see if you can find any stamen or pistils! Flowers that are bisexual are called “perfect flowers”, while flowers lacking one of the sex organs are “imperfect”. We will discuss the implications of flower sexuality later on when we examine plant mating systems.
As the inflorescence grows, the flower bud develops protected within the sepals. In most species, when the flower is almost mature, the sepals will open and the petals will unfurl and grow. Inside the stamens and pistils, if present, the pollen and ovules will mature. When everything is fully developed the stigma will become receptive to compatible pollen grains The egg and polar nuclei inside the ovule will be ready for fertilization by the sperm carried within the pollen. It is time for the pollen to take its dangerous and sometimes long journey to reach the stigma of the waiting flower.
Botany of Seed Formation: Pollination
Plants have adapted diverse strategies to facilitate that union. Many involve the exploitation of animal accomplices by providing tasty and nutritious rewards such as nectar, or intoxicating scents to lure animals to the flower. Pollen that relies on being carried by animal counterparts is often large, sticky, and has appendages that help the pollen stick to the animal. Other plants rely only on the wind to carry pollen. This pollen is often smooth, tiny, and extremely light. The pollen of many wind-pollinated plants can travel up to 5 miles on a strong breeze. The delivery of pollen to the stigma is called pollination and is susceptible to many environmental factors that can thwart its completion.
Pollen is released from the anthers when the anthers dry and crack open, a process called dehiscence. In extreme heat and dry conditions, the anthers can dry up themselves or dehisce too early, exposing unformed pollen to unfavorable weather. Healthy “viable” pollen can still be damaged by extreme heat and dry conditions and die once it leaves the anther. Other barriers to successful pollination include cold and wet conditions that some pollinators, such as honeybees, refuse to work in. Rain can wash pollen from the anthers onto the ground. Wind pollinated species are affected by those breathless, windless, hot summer days when there isn’t even a light gust to pick up and carry pollen. Pollen is often considered the weakest link in the fertilization and “fruit set” of a crop.
Botany of Seed Formation: Fertilization
Now that the pollen grain has reached the female, the stigma must recognize it as compatible. This is a biochemical process and once successful, the pollen grain begins to hydrate and germinate taking up moisture from the stigma. During hot, dry weather, the stigma can dry up, stopping the process altogether. Germination of the pollen grain entails one of the three cells growing into a pollen tube that grows down through the style all the way to one ovule within the ovary. As the tube grows, the remaining two cells of the pollen grain, the sperm, begin to travel down the tube, ultimately being delivered directly to one ovule. The growth of the pollen tube is another potential barrier to successful fertilization. It is a temperature dependent process with the optimal temperatures for its growth being the same as the optimal growing temperatures for that particular plant. For instance, many squashes thrive in 80º-90ºF heat, and so do their pollen tubes. Pollen tube growth will slow or cease altogether if temperatures get too cold. This is common on early or late summer evenings when temperatures can drop 20ºF or more as the sun is setting.
Pollen tubes also have a very definite life span of just a few hours, ranging from 18-36 hours. If their growth slows or stops before it reaches an ovule, there is very little chance it will resume when temperatures increase the following day. Therefore, pollination that occurs late in the afternoon or evening has much less potential for resulting in fertilization than pollination that occurs earlier in the day.
Botany of Seed Production: Seed and Fruit Formation
Once the pollen tube reaches an ovule, the sperm enter the embryo sac. One sperm fertilizes the egg creating a zygote that further develops into an embryonic plant. The other sperm fertilizes the two polar nuclei creating the endosperm tissue that will become the stored food within the seed. This is called “double fertilization” and once complete, the rest of the ovule outside of the embryo sac will develop into the seed coat. Thus, a fully fertilized ovule will become one seed. Every ovule inside the ovary needs to be fertilized by a separate pollen grain in order to develop into a seed.
During the later stages of seed development, chemical and physical mechanisms take hold to put the embryo and seed into a state of dormancy. Only a very certain set of environmental circumstances can break that dormancy, allowing the seed to finally germinate. Dormancy is a strategy used by a plant to survive extreme environmental conditions that might otherwise kill the parent plant — such as winter in temperate regions, years of drought and high heat in deserts, or flooding monsoon seasons in the tropics.
As each of the ovules is fertilized, they send signals to the surrounding ovarian tissue to begin to change and develop into a fruit. While we often call these ‘vegetables’, botanically, any structure ripening from the fertilized ovary of a flower and bearing seeds is named a “fruit”.
Ovules are connected to the ovary wall through a placenta-like structure called the “funiculus” and they ripen together after fertilization of the ovules. The health of the mother plant is crucial to ensuring the healthiest, most robust seeds. Providing ample growing time and optimal growing conditions suitable to specific plant groups is critical. Heat-loving plants need heat. Plants that prefer cool temperatures, such as spinach, may grow and flower in the summer, but will produce the biggest and best quality seed in consistent temperatures under 80ºF. Proper spacing is important as seed producing plants need more room than those grown for vegetables. Staking heavy seed stalks or additional trellising may be required. Diseases and pest damage affect the amount of energy a plant puts into ripening the ovules and fruit. Good soil health will boost your plants’ immune systems. Certain diseases can also be transmitted to the seed through various parts of the flower. Seeds infected this way can carry diseases with them and potentially infect the new fields where they are planted. If you plan to share your seeds, you should be careful to keep this in consideration.
Fruit maturation serves many purposes for the seed. During development, fruit tissue photosynthesizes and produces both food and protective anti-oxidants to nourish and protect the seeds. The concept of maturity in fruits can be confusing. As a consumer of fruit, we often consider market ripeness as when it tastes the best and has the most pleasing texture in our mouths. But physiologically, fruits are considered mature when the seeds inside are ripe and ready for dispersal. Often, market and physiological maturity are very different things! For example, when we eat “green beans”, the pods (which are the fruit) and seeds are green and soft and are nowhere near mature. When physiologically mature, bean pods are brown, dry and very brittle and the seeds within, leathery and tough.
Fruit types and seed dispersal:
Mature fruit types vary greatly and the final version is entirely tied to the species’ strategy for seed dispersal. Each fruit type executes a different mode of seed dispersal. There are two major categories of fruit, and of course numerous sub-categories within each that you could spend years memorizing. The major types are “fleshy” and “dry”.
Many fleshy fruits generally mature to have sweet and often colorfully attractive fruits. They have evolved to be eaten as a mode for dispersing seed. Their seeds have chemically resistant coats that protect the seed while going through the digestive tract of an animal. After an animal eats the fruit, they may travel miles before depositing it somewhere, thus spreading the seeds far and wide. Other fleshy fruits aid their seeds in germinating by rotting. Many tomatoes are a good example and we often see the seeds happily sprouting from the edges of compost piles after the fleshy fruit has rotted away. The gel outside a fresh tomato seed contains chemicals that keep the seed dormant, rotting digests this gel away and allows the seed to awaken when the soil warms.
Other fruits are dry at maturity and can be divided into “dehiscent” and “indehiscent” sub-types. Dehiscent fruits, such as beans and brassicas, are designed to split open when mature so that seeds can fall out, similar to pollen release through anther dehiscence described above. Sometimes they split suddenly so that seeds are actually ejected several feet away from the pods, lupines are a good example of projectile dehiscent seed dispersal! Indehiscent fruits are fruits we often mistake as seeds such as dill, fennel, beet, spinach, lettuce, and carrot seeds because the fruit wall itself is reduced to a hard dry coating that tightly surrounds the seed itself. As they mature, the entire inflorescence dries and they remain attached for quite a while. In the northeast, you might even see some hanging on through the winter! These fruits can be dispersed by the wind where parts of the fruit wall form tiny parachutes or wings. Other indehiscent fruits simply fall from dried inflorescence when shaken by the wind or a passing animal. Other indehiscent fruits have hooks and barbs on their seed coats and easily latch on to the fur of a passing animal and are thusly carried far away from the parent plant.
In the northeast, dry fruits can be particularly susceptible to diseases because they ripen in the late summer when we have more rain and humidity and cooler nights. To harvest healthy seeds takes foresight and planning. If you are growing a large crop, consider growing it under a hoop house. If growing a field crop, watch the weather carefully. If extended rain and gloom are predicted and the seeds are almost ripe, harvest the longest stalks possible of the entire plant (including roots if you can keep the soil away from contaminating the seed heads), and bring into a dry protected place such as a greenhouse or barn. Keep rodents away and the seed will continue to ripen as the plants die down.
If planning to grow and harvest seed on your farm, homestead, or back yard, it is important not only to understand the botanical concepts already explained, but to understand the various mating systems plants have evolved. Mating systems are strategies plants use to exchange and recombine genes as they reproduce themselves. Differing greatly from animals that are mobile and can move around freely to find a suitable mate, plants have evolved a number of methods to carry out sexual reproduction.
First, it is important to know who can mate with whom. Knowing the scientific name of your plants, including the cultivar, is a very important part of that. The scientific name is the genus and species and should always be written in italics on your seed packets. For example, the common bean’s scientific name is Phaseolus vulgaris. Knowing there are thousands of cultivars of beans, we need to further designate the species name to include the cultivar name. The scientific name, then, for the bean called ‘Black Knight’ is: ‘Phaseolus vulgaris ‘Black Knight’.
We have all been taught that the definition of a species is a group of organisms that can intermate with each other, and that includes cultivars within that species. While that simple definition of species holds true in plants, plants also throw a curve ball into that arena. Many plants can intermate with plants of another, very closely related, species. Let’s look at peppers. The most common species is Capsicum annuum. There are two other capsicum species that can freely intermate with C. annuum, those are C. frutescens and C. chinense. When two species mate, it is called an “interspecific cross”. A noteworthy comment about interspecific crosses, since it is a very common question, is that the three common species of the Cucurbit genus do not readily cross with each other. That is, Cucurbita maxima, Cucurbita moschata, and Cucurbita pepo, can be grown side by side with very little chance of an accidental, interspecific cross happening. If it does, the resulting fruit is almost always sterile.
The simplest mating system found in plants is self-pollination. Known as “selfing”, “selfers” need nothing more than their own eggs and sperm to carry out sexual reproduction. The main characteristic that facilitates selfing is a botanically perfect, i.e. bisexual, closed flower where the anthers and stigma are in very close proximity to each other. In some cases, the anthers encircle the stigma and in others, the anthers are fused together in a cone surrounding and enclosing the stigma. We mentioned earlier that most flowers open just before the male and female portions mature. In selfers, male and female organs mature before the sepals and petals open. Pollination and fertilization happen inside the flower before it opens, and sometimes the flower never opens at all. Selfing is quite common and is seen in most small grains including barley, oats, sorghum, rice and wheat, and other crops such as lettuce, peas, and beans and to a slightly lesser extent in eggplant and tomatoes. These plants are highly inbred but suffer no ill consequences from that.
The vast majority of plants, however, reproduce via “cross-pollination” and are called “crossers” or “out crossers”. Cross-pollinated plants suffer greatly when inbred, a condition known as “inbreeding depression”, exhibiting loss of vigor and often times early death. These plants must intermate and share pollen with a large population of its species and cultivar. Cross-pollinated plants rely on some external mode of pollinaton, whether insect or animal mediated or wind, and can have either perfect bisexual flowers or separate, imperfect male and female flowers. If flowers are perfect, the plant employs a few methods to prevent self-pollination. One method blocks pollen recognition by the stigma. Another, blocks pollen tubes from delivering sperm to the ovule. A third method involves the anthers and stigmas of the same flowers maturing at different times so that when the stigma is receptive, the pollen from that flower is not viable and vice versa.
If the cross-pollinating plant has evolved to produce separate male and female flowers, there are also variations on that theme! In some plants, such as most cucurbits and corn, the male and female flowers are found on the same plant but in separate locations making a proximal barrier to self-pollination. Often, the male flowers form several days before the female flowers which further blocks the plant’s ability to pollinate itself which adds an additional, temporal barrier to self-pollination. These plants are called “monoecious” meaning “one house” that includes both male and female flowers.
Another condition is that of the male and female flowers born on separate plants entirely. Spinach is a common example and the male and female plants even look different. Pollen often needs to be carried quite a distance from the male plant to the female. These types of plants are called “dioecious”, meaning two houses, one for male flowers, and one for female. Dioecious plants require the greatest degree of cross-pollination to maintain healthy seed production.
Isolation to prevent unintended cross-pollination:
The line between selfers and crossers is a scale along a continuum rather than two distinct buckets. Plants’ abilities to self or cross vary. Let’s take peppers again as an example. The flowers are self-fertile, however, peppers are known to out-cross with other pepper cultivars up to 60% of the time! Tomatoes also are thought to be self-pollinating, but outcross with other tomato cultivars up to 40% of the time. As we discussed the closed morphology of self-pollinating flowers previously, tomatoes, peppers, and eggplant possess flowers where the stigma is enclosed by an anther cone and are often self-pollinated before the flower petals fully open. However, if you look closely, and you really should if you want to save seeds from these plants, you might notice that the stigma of some of the tomato flowers actually protrudes out from the rim of that anther cone and is exposed to the air and any activity by pollinating insects.
To maintain the genetic and varietal integrity of the plants you wish to save seed from, a grower needs to prevent out-crossing with other cultivars of the same species. At the same time, the grower must also ensure they are not promoting inbreeding depression and needs to allow a large enough population of plants to intermate with each other. Respectively, that means the grower must isolate groups of plants from each other and plant enough plants, the more prone to inbreeding depression a plant is, the most plants need to be in the mating population.
Isolation can be accomplished in several ways. One way is to physically separate the groups of plants from each other by a specified distance, see table. Physical barriers, such as tree lines and wooded areas, also aid in distance isolations. Isolation of insect pollinated crops can also be accomplished simply by building a cage around the group of plants that you want to intermate and covering it thoroughly with fine insect netting. Yet another method of isolation is temporal, i.e. time the planting of your crops so that cultivars of the same species are not blooming and setting fruit at the same time.
Seed production is one of the most fascinating life processes you can witness and steward on your farm. Learning, observing, and becoming fully in tune to the whole life cycle, from seed to seed, of the crops you grow is one of the most meaningful and satisfying journeys you will ever take. Once you master saving high quality seed with strong cultivar integrity, you can begin the journey to adapt certain cultivars to your farming systems and even breed your own new varieties… but that is another subject altogether!
Hannah Traggis is Senior Horticulturist at the Massachusetts Horticultural Society and is available for questions about this article at firstname.lastname@example.org