Botany for Gardeners - The Basics of Seeds
Two, or One?
In my previous article, "The Basics of Fruits", I mentioned that, botanically, the fruit of a plant is the structure that contains the seeds. These compact units of sexual reproduction are found in two types among flowering seed plants and one type among non-flowering seed plants. I have previously introduced you to the terms angiosperm, referring to the flowering seed-bearing plants, and gymnosperm, referring to the non-flowering seed-bearing plants. These terms describe a fundamental difference between these two groups of seed plants. This difference is whether the seeds are surrounded by an ovary wall (angiosperms) or whether the seeds are borne naked (gymnosperms).
Next, within the angiosperms are found two different groups, distinguished in their seeds by the number of seed leaves, or cotyledons, each produces. Those that have seeds with two cotyledons are known as dicotyledons, or dicots for short. Common dicots are many familiar trees, such as ash, willow, poplar, sycamore, mango and avocado, and plants such as rhubarb, tomato and sunflowers. The other group has seeds with one cotyledon, and these are known as monocotyledons, or monocots for short. These are most often seen as plants and not trees, although in the tropics a large family of trees, the palm family, belongs to the monocots. Other familiar monocots are onion, corn, bamboo, grasses, bananas, lilies, and cannas.
How about the gymnosperms? These have cotyledons that range in number from as few as 2 to as many as 24, arranged in a whorl at the top of the hypocotyl, or embryonic stem (see picture at left).
Above, or Below?
In looking at the actual structure and germination of seeds, we see other differences. The dicot seed, as diagrammed in the thumbnail picture above, right, shows a number of parts, but all seeds have in common three basic components: the embryo, the endosperm and the seed coat. In dicots, the endosperm is often relatively rudimentary while most of the stored nutrition for the soon-to-develop embryo is found in the cotyledons. In monocots, the endosperm plays a much more crucial role in the nutrition of the embryo, while the single cotyledon acts as a sort of interface between the embryo and the endosperm, facilitating the utilization of nutrients from the endosperm. Also of note is that in dicots, the cotyledons emerge from the seed coat, above the soil and are known as the "seed leaves", while in monocots the single cotyledon remains underground or within the seed while the new shoot of the sprouted seed emerges into the light.
While the essential seed structure is straightforward and relatively simple, nature provides many variations and added complexities to help in seed dispersal and in long-term survival. Some seeds are equipped with hooks or spines on the seed coat, enabling them to hitch a ride on a passing animal. Others have structures that allow them to take to the air and be dispersed in that manner (see Dandelion seed image at right). The orchid seeds are unique in that they are not only dust-like, but consist of just an embryo enclosed in a sac made from just a single layer of cells. This means that conditions for germination must be immediately available and favorable or else the seeds will not survive. In many cases, the fruit surrounding the seed serves as an attractive meal for animals, enabling dispersal when the seeds pass through the digestive tract and are excreted in a remote location.
Chilled or Warm?
Many of the temperate species produce seeds requiring a period of cold, or stratification, before they will germinate. By contrast, seeds from the tropics will require warm temperatures for germination. Seeds in both cases may have thick seed coats, requiring a process of degradation to allow the entry of moisture and the start of germination. When we as gardeners work with such seeds, we need to nick, or scarify, them in order to allow them to absorb water. Many leguminous seeds, such as beans, do better when they are scarified. The thickened seed coat enables seeds to travel on the ocean for great distances and still arrive at their new destination in good enough shape for germination. Thick seed coats can also serve as flotation devices, as in the case of the coconut, providing a similar opportunity for dispersal. For other seeds, passage through the digestive tract of an animal provides just enough degradation of the seed coat to allow germination to begin after exit from the animal.
Some seeds stay alive, or viable for months or even years, enabling the seed to wait for favorable conditions to arise. Other seeds must have favorable conditions upon ripening or else they will perish quickly. Scientists have worked with various methods to prolong seed viability so as to help preserve valuable species and varieties. One of the most promising methods is deep cold, or cryogenic, storage. Even some tropical seeds and pollen can be preserved long term by this method if properly prepared beforehand.
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