The Life Cycle of Seeds: Spring
By Meghan Walla-Murphy
(Part one of a four-part series about the physiology and life cycle of seeds) Part 2
As vernal equinox approaches and spring begins to take hold, hillsides, meadows, grasslands, and even gardens transform. Tender, bright green shoots overtake the brown dormancy of winter. New growth reaches for the sun as the days lengthen and temperatures rise. Winter and spring storms converge over California and drop precious and necessary moisture. And yet while our eye is drawn to the green above ground, our attention should be directed below, toward the seeds responsible for the freshness of spring.
Unfortunately seeds are often taken for granted. They arrive unceremoniously in prepackaged pouches or are laboriously picked off of clothing after a long day’s hike. But really they are a tiny efficient bundle of immense potential energy. The tiny redwood seed (Sequoia sempervirens) provides the beginnings of the tallest tree in the world. Not only that, seeds carry longevity of life by enabling plants to survive over periods of unfavorable growth such as seasonal drought or snow. They even endure years of dormancy to ensure succession over years of poor growing seasons. Seeds are also efficient mechanisms for dispersal, allowing species to spread beyond the physical reaches of the plant itself. Seeds adapted to wind dispersal such as willow, cottonwood, and some asters can travel huge distances before landing in productive soil to germinate.
As with the seasons, the life of a seed is cyclic and follows rhythms that correspond with temperature, light and water. Spring is the season in which once-dormant energy manifests into a living plant, a transition from internally stored energy to external new growth.
Although there are many variations, generalities exist amongst angiosperm (flowering plants) seed cycles. The common garden bean, or a eudicot seed is a good example (Eudicots were traditionally known as dicots but molecular research has changed that). California native plants within the bean/pea family, Fabaceae, that also share this general seed cycle include lupine and milk-vetch.
If split lengthwise along its embryonic axis, the bean seed reveals a seed coat, an epicotyl, a hypocotyl, a radicle, and two fleshy cotyledons. The hard seed coat protects the seed while in dormancy and encases the embryo and its food supply. The epicotyl is located on the embryonic axis below the first pair of miniature leaves and above the cotyledons. The fleshy cotyledons, packed with starch, store food before the seed germinates and form the seed leaves of an embryo. Just below the cotyledons on the embryonic axis is the hypocotyl, which terminates into the radicle. The radicle is the embryonic root of the plant.
The increase of light, warmth and water begins the seed’s cycle of growth. As seeds are extremely dry, they quickly absorb water from the soil, which causes them to swell and burst the seed coat. This rupture ends dormancy and triggers enzymes to digest the food stored in the cotyledons. These nutrients are then transferred to the growing parts of the embryo so that germination can proceed.
First, the radicle emerges from the seed and grows down into the soil, ensuring that the seed and the newly forming plant will be able to secure water and nutrients for sustenance. Next, the shoot tip works up towards the soil surface, forming a hook in the hypocotyl as it pushes above ground. With access to light, the hypocotyl straightens and pulls up the cotyledons and then the epicotyl from under the earth. The epicotyl grows toward the light as the bulky cotyledons open and the first true leaves spread from the shoot. These leaves should be distinguished from the cotyledons or “seed leaves.” As true leaves, they grow and begin the photosynthetic process to produce energy for the plant. The cotyledons then shrivel and fall off the seedling having spent all of their energy on germination.
Other eudicots undergo a similar germination process, but have thin cotyledons. Instead, these seeds rely on an endosperm within the seed coat to provide the necessary nutrients. Monocots, seeds that have only one cotyledon rather than two, have two protective sheaths; a coleoptile, which covers the young shoot and a coleorhiza, which protects the young root. Upon germination the coleoptile functions like a tube through which foliage emerges. The radicle, as with eudicots, grows deeply into the soil. Monocots include grasses, lilies, and other plants with parallel leaf venation.
So as the earth moves around the sun and the northern hemisphere prepares for longer days and shorter nights, we see transference of energy from seeds’ dormant hibernation to the visible manifestation of growth. But this only occurs if conditions support such germination. Seeds activate when the time is right.
In a time for humans when things are changing rapidly, it is all too easy to contract and pull away from those things that nourish and sustain us. But perhaps we should learn from the seeds and find our own ideal conditions, manifest our individual potential energy, and create efficient growth for our California natives.
Meghan Walla-Murphy, with BA in both biology and Japanese history as well as a MFA in creative non-fiction writing, is currently an environmental and education consultant in Northern California. She has spent the last decade delving deeply into environmental studies and natural history, traveling the United States, Africa, Brazil, and China to expand her knowledge base. Meghan is co-author with James Halfpenny’s on the book, “Track Plates for Mammals.” In addition she is a freelance writer for several local Los Angeles papers.