Plants and the Seasons: Pt. 2 Spring/Summer

Get Creative in the Garden with Bryn — By on January 1, 2010 12:20 PM

By Bryn Richard

Although the plant world may look dead now, in the middle of winter, it is only asleep. From your garden to the wilder forests and meadows, from the century-old tree to the new seed created last fall, plants are only awaiting the arrival of spring before bursting into life. What triggers these changes and how do they occur?

Seed germination occurs when the right environmental conditions, indicating optimal growing conditions, are present. From an evolutionary perspective, a seed that was not good at determining when to germinate would be unlikely to survive long enough to pass on its genes. “Optimal growing conditions” can vary among plants according to the climate and ecology of the place they are naturally found. It is important to understand this if trying to germinate seeds by hand. Some seeds need darkness to germinate while others need the presence of light. To the former, darkness indicates the presence of a protective layer of soil and the nutrition of decomposed fallen leaves. Plants in the latter category tend to be good at taking advantage of disturbed soils and other openings in the environment. Many plants that grow in fire-prone areas depend on fire to initiate germination. To them, the passage of a fire indicates less competition when they germinate as well as the presence of additional nutrients in the form of ash and charcoal.

For most plants, it is the arrival of spring that initiates germination. Spring is heralded by warming soil temperatures and plentiful water. Soil temperature lags behind air temperature, so if the soil is warming then it means that air temperature has been consistently warming for a while. Seeds depend on soil temperature instead of air temperature because otherwise one of those rare warm February days would trigger germination, only for the seeding to die as normal freezing temperatures return.

Seeds are composed of several different parts, each with their own roll to play. (A nice diagram can be found here.) The seed coat, or outer cover of the seed, protects the seed as it is dormant. It also prevents the entry of water, which at the wrong time of year could be damaging to the seed (ex: by freezing and forming ice crystals that puncture cells.) Therefore the seed coat needs to be damaged, or scarified, before germination can occur. In nature, thin seed coats are broken down by soil fungi and bacteria or are scratched by shifting soil particles. Hard seed coats require stronger methods-either a longer period of time exposed to shifting soil and soil fungi/bacteria, exposure to fire (as mentioned above) or, more commonly, digestion by an animal and contact with stomach acids. Being eaten by an animal also has the added benefit of carrying the seed away from competition with the parent plant, as well as being given a nice fertilizer packet when it is deposited. Gardeners can scarify hard seed coats a variety of ways: with a file or sharp knife, soaking briefly in sulfuric acid (rinse well!), or by shaking in a jar lined with inward-facing sandpaper. Make sure not to damage the seed beyond the seed coat!

As plants at this stage of life are not capable of producing their own food, seeds also contain food-storage structures. This is why seeds are such a nutritious food source for so many animals, including humans. Beans, grains and nuts, to name a few, are all widely valued for the proteins, fats and carbohydrates they bring to our diets. The endosperm is used to nourish the seed during germination, bringing it to the surface. The soft, white pulp in each grain of corn is the endosperm for that seed. The cotyledon, or embryo’s leaves, is also frequently a food storage structure. These are easily seen on bean seeds. They are also why the first set of leaves on a seedling typically looks different than the leaves that follow. Unlike the plant’s true leaves, the cotyledon will shrink as the seedling grows and uses up the stored food. Cotyledons are also one of the distinguishing features used to further classify Angiosperms (flowering, fruit producing, plants) into monocots (Greek: mono, one; cotyledon) and dicots (Greek: di, two; cotyledons). Gymnosperms (plants that produce “naked”, or uncovered, seeds) may have two to twenty-four cotyledons present in a single seed. Although seed packets typically have planting depth instructions written out, a good rule of thumb is to not plant the seed any deeper than its length. A seed planted too deep will run out of food before it is able to produce its own.

The third major part of a seed is the embryo. As expected, this is the part that grows into the seedling. The embryo can be further divided into named parts according to what part of the plant that part will become. The cotyledon is considered part of the embryo, as they are embryonic leaves.

As with any living multi-celled organism, plant growth is due to both cell division and cell enlargement. Cell division occurs in specific areas, called meristems (Greek: meristos, “divided”), leading to organized, controlled growth. Next to the meristem is the area of cell enlargement, where the newly created cells grow to their full size. Plant growth can be divided into two phases. Primary growth occurs in the apical meristem, located at the tip (or apex) of stems and roots. It is responsible for the rapid growth of the seedling reaching for food-producing sunlight, or the roots penetrating deep into the soil for stability, water and nutrients. Secondary growth occurs in the lateral (“side”) meristems and occurs after the stems/roots lengthen, causing their thickening. This thickening provides the structural foundation necessary to support further growth. This website of the University of Texas has good images of slides showing cells in plant tips. Lateral meristem growth can be witnessed in the ring patterns of tree trunks.

A stem’s apical meristem, or apical bud, is responsible for more than just elongating growth. It also initiates the arrangement of leaves and axillary buds (side buds located at the base of leaves.) Axillary buds remain dormant until stimulated by the plant to create branches or to take over for a damaged/destroyed apical bud. By finding the closest axillary bud, you can predict how a plant will grow after a pruning cut (photo, general diagram or rose pruning video).

The growth of both seedlings and mature plants is controlled by a number of hormones, the same as growth is controlled in animals. The best studied growth promoting plant hormone is called auxin, after a Greek word meaning “to increase.” Auxin controls cell elongation. Auxin is responsible for phototropism (Greek: photo, “pertaining to light”; tropos, “to turn”), or the movement of plants toward light (technically, this is “positive phototropism”). Auxin will accumulate on the shaded side of a plant, causing the cells on that side to elongate more and thereby turn the stem back towards the light. For a plant already growing towards light, auxin will be evenly distributed on both sides. Either way, auxin is concentrated at the growing tip, decreasing the farther one moves from the tip.

Auxin also controls geotropism (Greek: ge, “earth”; tropos, “to turn”), or the movement of plants in response to gravity. Positive geotropism is the movement towards gravity (ex: roots). Negative geotropism is the movement away from gravity (ex: stems). Diageotropism (Greek: dia, “across”) is horizontal movement relative to gravity (ex: stolons, or spreading stems near surface level responsible for asexual reproduction). Plagiotropisms (Greek: plagio, “oblique”) is movement at an angle relative to gravity (ex: side roots and stems).

Additionally, auxin inhibits the growth of axillary buds, leading to apical dominance. If the apical bud is pruned, the removal will cause an axillary bud to grow. Axillary buds may also grow when the apical bud moves a significant distance away.

Auxin isn’t the only plant hormone. Gibberellin affects cell enlargement and cell division and controls dormancy, germination and fruit development. Cytokinins stimulate cell division as well and control leaf growth, light response, aging and regulates protein synthesis. Abscisic acid promotes dormancy and flowering and accelerates leaf abscission (leaf drop). Ethylene promotes ripening, aging and flowering. (Apple skins are a good source of ethylene, which is why enclosing unripe fruit in a bag with an apple will cause the fruit to ripen faster.)

So, now that you know something about how plants grow, how about a couple of ways to use this knowledge? Growing your own kitchen sprouts is very easy to do and now you know why sprouts are so nutritious too. You can find seeds through any garden seed source, or possibly at a healthy food store or well-stocked supermarket. The only other supplies you need are a clean glass jar (an old spaghetti sauce jar works well) and a sieve that has holes smaller than the seeds. Soak about a tablespoon (you’ll be surprised at the volume of sprouts from just a tablespoon of seeds) in the jar overnight. Strain and rinse the seeds the next morning, then leave the jar in a warm, low-light location. Rinse again that evening, then twice a day until you have sprouts. Moving the jar into light for a day or two will help green up the new sprouts, if that’s what you want. Most common sprouting seeds only take around 5 days before harvest. If you are sprouting a type of seed that leaves behind a large hull (seed coat), they will typically float and can be removed by agitating the sprouts in a bowl of water then skimming the hulls off the water’s surface. For more detailed instructions on a variety of kitchen sprouts, Johnny’s Selected Seeds has a nice PDF available.

Growing your own microgreens is only slightly more difficult. With microgreens, the seeds are planted in a sterile potting mix in a shallow container. Keep the soil moist, but not soaked (tip: covering the containers with plastic wrap until germination helps retain moisture.) Locate in a well-lit location. Harvest the microgreens by snipping when at least one set of true leaves has developed. Taste periodically to find the best time to harvest. Microgreens typically need 2-3 weeks of growth until harvest ready. Johnny’s Selected Seeds has a PDF overview of microgreen production and common plants used.

For more information on starting seeds for your garden, check out these websites:
http://www.vegetablegardener.com/tag/seedlings
http://www.hortmag.com/ArticleSearch/?category=Propagation
http://www.garden.org/subchannels/care/seeds?q=list

If you are looking for further reading, Botany for Gardeners by Brian Capon is easily readable and full of real-world examples of botanic concepts.

Happy gardening!

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