In the vast and diverse plant kingdom, we encounter a fascinating division between seed-producing and non-seed producing plants. The latter includes lichens (thallophyta), mosses (bryophyta), and ferns (pteridophytes). On the flip side, we find two prominent groups among the seed producers: gymnosperms (conifers) and angiosperms (flowering plants).
Gymnosperms: The Ancient Conifers
Gymnosperms, which have been around for approximately 200 million years, encompass cone-bearing plants like conifers and yew trees. These hardy, ancient giants are known for their unenclosed seeds, a distinctive feature that sets them apart from their angiosperm counterparts.
Angiosperms: The Flowering Plants
Angiosperms, the more recent arrivals on the botanical scene, encompass all flowering plants. This includes a wide array of greenery, from grasses and herbs to the fruits and vegetables we cultivate in our gardens. These plants produce seeds enclosed within fruits and flowers, creating a dazzling spectacle of colors and forms.
Angiosperms: Monocots and Dicots
Angiosperms can be further categorized into two main groups: monocots and dicots.
Monocots: Monocots encompass plants like grasses and bulbs. They are especially relevant for food growers because they include staple crops like wheat, oats, sweetcorn, asparagus, onions, and leeks.
Dicots: Nearly all other plants in your garden fall into the dicot group. This division is important because it affects the way these plants grow and develop.
The most distinguishing feature between monocots and dicots lies in the number of cotyledons, which are the seed’s embryonic leaves. Monocots have one cotyledon, while dicots have two.
Seed Anatomy: Unlocking the Potential for Growth
At the heart of seed germination lies the intricate structure of a seed. It comprises an embryonic root, shoot, and leaves, all connected to a food store within the seed. Cotyledons, the “seed leaves,” serve as both the food source and the mechanism for nutrient transfer from the seed’s endosperm to the growing shoot. In some seeds, such as peas and broad beans, cotyledons serve as both the food source and the transfer mechanism for growth.
The amount of energy stored within a seed, and consequently the growth it can support, depends on the seed’s size. This is why it’s crucial to plant seeds at the correct depth. If a small seed is sown too deep, it might not have enough stored energy to grow the shoot above the soil level. Sowing depth plays a significant role in the germination of very small seeds, like carrots, and careful attention to this detail can ensure a successful outcome.
Factors Affecting Seed Germination: Moisture, Oxygen, and Temperature
Seed germination hinges on several factors, including moisture, oxygen, and temperature.
Moisture and Oxygen: Seeds have a moisture content ranging from 4% to 12% when they come from a seed pack. Planting them in dampened compost increases their moisture content to between 25% and 50%. This extra moisture initiates enzymes in the seed that digest the stored food, providing energy for the embryo’s growth. However, efficient digestion requires oxygen, and excessively wet compost can hinder the process.
Moisture and Dormancy: Some seeds, particularly those from woody species, have natural dormancy mechanisms that moisture alone can’t break. This isn’t a concern for most vegetable growers, but it does apply to woody herbs like rosemary, which can be challenging to germinate. To overcome dormancy, a process called ‘Cold Stratification’ is used, involving refrigerating seeds for about 12 weeks in damp conditions.
Temperature: While most seeds don’t require light to germinate, temperature plays a crucial role. A minimum temperature of approximately 5°C is required for germination, with an optimum range of 25 to 30°C. For cool climate crops, like those grown in Ireland and the UK, 18°C is recommended, while warm climate crops should be at 20 to 25°C. This temperature-light relationship can lead to issues like leggy seedlings when the balance isn’t maintained.
While most vegetable seeds don’t rely on light to germinate, some, like lettuce, celery, and celeriac, require light to trigger their germination. These seeds should either be lightly covered with fine compost or left uncovered. This light-sensitive adaptation in lettuce seeds can be traced back to their ancestors, which colonized disturbed ground, taking advantage of sunlight when trees fell in forests, disrupting the forest floor. This adaptation is a double-edged sword, as it applies to many garden weeds as well.
Photoreceptors and Timekeeping
The mechanism by which plants measure day length and determine the timing of events like leaf shedding and flowering is called photoperiodism. Photoreceptors, specifically phytochromes, play a crucial role in this process. These photoreceptors switch between Pr (phytochrome red) and Pfr (phytochrome far red) states in response to red and far red light.
In full sunlight, red light is abundant, whereas in the shade, it’s filtered out, leaving primarily far red wavelengths. Therefore, when seeds and trees are exposed to strong red light, they perceive it as sunshine, while far red light signifies shade. This ability to perceive light conditions helps seeds germinate in the most favorable conditions, utilizing the sun’s energy for growth.
How Trees Measure Time
Trees use this phytochrome-switching mechanism to measure day length (hours of darkness), which helps them decide when to shed their leaves. During daylight, Pr switches to Pfr. In darkness, Pfr slowly reverts to Pr at a fixed rate. Longer nights lead to more Pfr at sunrise, signaling the approach of winter and prompting leaf fall.
This same phytochrome system controls flowering, bud setting, and vegetative growth in most plants, including many of the vegetables we grow. For example, certain vegetables like spinach and Asian salads bolt or go to seed in summer but remain steady in spring or autumn. This is because they are “long-day” plants programmed to flower in summer. Adjusting your planting times or choosing bolt-resistant varieties can help you work with these natural patterns.
The fascinating relationship between light, phytochromes, and hormonal action in plants highlights the intricate, precise mechanisms governing their growth and development, making the world of botany endlessly intriguing.
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