As spring approaches, visions of bountiful gardens, greenhouses, and windowsills inspire classroom growers to plant seeds indoors. By learning a bit about what makes seeds tick, you can better focus students' seed observations and investigations, and enrich their understanding of what these little treasures need to spring to life.
The question of whether seeds are alive or not perennially challenges classroom scientists. In fact, these seemingly lifeless objects do lack "vital signs" and so, are considered dormant, but capable of coming to life. When conditions are right to support growth, seeds waken from their dormancy and burst forth -- germinate -- in a period of intense activity.
What might be the advantages of remaining hard and dry, unassuming, and "asleep" for long periods of time? In a dry, inactive state, seeds can survive adverse conditions such as freezing temperatures, drought, or fungus attacks, which the adult plant could not. Imagine what would happen if seeds didn't have this ability and, for instance, germinated in the fall right before a cold winter.
The tough outer layer, or seed coat, protects the seed and the young plant or embryo inside. The embryo consists of a preliminary root, shoot, and one or two seed leaves called cotyledons, which store food for the embryo. Some seeds also have endosperm tissue that contains food reserves to nourish the young plant until it can make its own food using light energy.
Just like humans, seeds have needs that must be met if they are to thrive and grow. Armed with the genetic information needed to make a new plant, seeds wait to break dormancy until they have an ample supply of water, optimum temperatures, and a well-aerated soil or other spot in which to dig in.
The first step in a seed's awakening is absorbing water. This activates enzymes that, in turn, make the stored food available to the embryo. As water is taken in, often doubling the original seed volume, the coat splits, making oxygen in the soil available to the tiny plant. The energy that drives the seedling's cells to quickly divide and grow comes from the food stored in the cotyledons and endosperm. During the process of respiration, energy from this stored food is "burned" in the presence of oxygen. While humans breathe in oxygen for the same process, it diffuses into plants from their surroundings, including the soil. In a heavy or saturated soil, there's too little oxygen available to support this crucial step and the seed may rot.
The tip of the root, which emerges first from the seed, anchors the plant and enables it to absorb water and nutrients. Next, the young shoot begins to grow, relying in the early stages on food supplies from the cotyledons and endosperm. When the seedling's first real leaves come through the soil, the plant finally shifts to making its own food through photosynthesis. The greater the stored food supply (i.e., large seeds), the deeper a seed can be planted and survive until the plant begins producing its own food.
Although relative warmth is required for germination and growth, the ideal range of temperatures varies with different seeds. Not surprisingly, seeds of many plants native to warmer climates (e.g., tomatoes and peppers) require warmer temperatures to germinate than those native to cooler climates (e.g., lettuce).
Although germinating seeds are not dependent on sunlight to produce energy, in some cases light can trigger or prevent germination. Often, small-seeded species require light. Consider the possible advantages of this adaptation. Imagine what would happen if small seeds were buried deeply in the dark soil. Without the food reserves to reach the surface, seeds would sprout and die. Requiring light ensures that buried seeds will remain dormant and able to survive to sprout until conditions are right for germination and growth.
Now that you are armed with an understanding of how seeds tick and come to life, consider the implications and opportunities for classrooms gardeners and scientists. Invite your students to "think like seeds" as they try to nurture them in the classroom. Consider having them pore over seed packets and observe sprouting seeds up close, then generate seed-starting questions they can tackle with classroom investigations. Some examples follow.
Why are seeds planted at different depths? What would happen if we planted the same type of seed at different depths? Does the size of the seed correlate with ideal planting depth?
What type of soil mix is best for starting seeds? Based on what we know about seed needs, why might some types of soil be better for seed sprouting?
How do different types of seeds germinate? (Have students observe and describe how seeds spring to life. When they see roots emerging first, have them theorize why that might benefit the young plant.)
How could we determine the best germination temperatures, light conditions, or spacing for different types of seeds?
Why do roots grow down and shoots grow up? Can we influence this response?
Although most garden seeds germinate fairly easily and reliably, native species often have additional requirements. Seeds in some areas, for instance, have chemicals in their coats that inhibit germination until they are washed away by heavy rains. Some seed coats require "scarring" before they will germinate while others require a period of cool, moist temperatures that simulate winter. Many seeds that are encased in an alluring fruit must pass through an animal's digestive system before being deposited away from the parent plant. Some even require heating by fire before they'll sprout! All of these adaptations help the seeds survive in different types of environments, increasing the chances that they'll germinate when conditions are right for growth. If your class is raising native plants for butterfly gardens, wildlife habitats, prairies, and so on, consider researching and experimenting with different methods of breaking seed dormancy and triggering germination.