SEEDS

Kinds of Seeds

Many structures commonly referred to as "seeds" are actually dry fruits. 

Sunflower seeds are sold commercially while still enclosed within the hard wall of the fruit, which must be split open to reach the seed.

Different groups of plants have other modifications, the so-called stone fruits (such as the peach) have a hardened fruit layer ( the endocarp) fused to and surrounding the actual seed. 

Nuts are the one-seeded, hard shelled fruit, of some plants, with an indehiscent seed, such as an acorn or hazelnut.

Seed Reproduction

Seeds are produced in several related groups of plants, and their manner of production distinguishes the angiosperms ("enclosed seeds") from the gymnosperms ("naked seeds").

Angiosperm seeds are produced in a hard or fleshy structure called a fruit that encloses the seeds, hence the name. (Some fruits have layers of both hard and fleshy material).

In gymnosperms, no special structure develops to enclose the seeds, which begin their development "naked" on the bracts of cones. However, the seeds do become covered by the cone scales as they develop in some species of conifer.

Seed production in natural plant populations vary widely from year-to-year in response to weather variables, insects and diseases, and internal cycles within the plants themselves. Over a 20-year period, for example, forests composed of loblolly pine and shortleaf pine produced from 0 to nearly 5 million sound pine seeds per hectare. Over this period, there were six bumper seeds, five poor seeds crops, and nine good seed crops, when evaluated in regard to producing adequate seedlings for natural forest reproduction.

Structure of a Seed

The outer covering of a seed is called the seed coat. Seed coats help protect the embryo from injury and also from drying out. Seed coats can be thin and soft as in beans or thick and hard as in locust or coconut seeds. Endosperm, a temporary food supply, is packed around the embryo in the form of special leaves called cotyledons or seed leaves. These generally are the first parts visible when the seed germinates.

Plants are classified based upon the number of seed leaves (cotyledons) in the seed. Plants such as grasses can be monocots, containing one cotyledon. Dicots are plants that have two cotyledons.

Seeds are the dispersal and propagation units of the Spermatophyta (seed plants): Gymnosperms (conifers and related clades) and Angiosperms (flowering plants). A comparison of these to major groups is presented on the "Seed evolution" webpage. Several seed-related issues differ between gymnosperms (750 species) and angiosperms (250000 species). If not otherwise stated, the information given in this website refers to typical Angiosperm seeds.

Seeds are mature, fertilized ovules. Ovules are structures of seed plants containing the female gametophyte with the egg cell, all being surrounded by the nucellus and 1-2 integuments. In angiosperms the double fertilization results in formation of the diploid embryo and the triploid endosperm.

Embryo: Young sporophyt, dipoid (2n), result of fertilization. The mature embryo consists of cotyledons (seed leaves), hypocotyl (stem-like embryonic axis below the cotyledons),radicle (embryonic root).

Seed and embryo types were defined by Martin (1946). These and the resulting evolutionary trends are found on the "seed evolution" webpage.

Endosperm: Food storage tissue, triploid (3n), result of double fertilization, 2/3 of the genome is of maternal origin.

Testa (seed coat): Outer protective layer of the seed, developed from the integuments of the ovule, diploid maternal tissue.

Fruits are mature, ripened ovaries containing seeds. The pericarp ("fruit coat") is diploid maternal tissue.

Perisperm: Diploid maternal food storage tissue originates from the nucellus. Only in some species, e.g. Beta vulgaris, Piper nigrum, Coffea arabica, many Caryophyllales.

Endospermic seeds: The endosperm is present in the mature seed and serves as food storage organ. Testa and endosperm are the two covering layers of the embryo. The amount of endosperm in mature seeds is highly species-dependent and varies from an abundant endosperm layer (Nicotiana tabaccum) to a single layer (Arabidopsis thaliana).

Non-endospermic seeds: The cotyledons serve as sole food storage organs as in the case of pea (Pisum sativum). During embryo development the cotyledons absorb the food reserves from the endosperm. The endosperm is almost degraded in the mature seed and the embryo is enclosed by the testa.

Seed Germination

Seeds remain dormant or inactive until conditions are right for germination. All seeds need water, oxygen, and proper temperature in order to germinate. Some seeds require proper light also. Some germinate better in full light while others require darkness to germinate.

When a seed is exposed to the proper conditions, water and oxygen are taken in through the seed coat. The embryo's cells start to enlarge. Then the seed coat breaks open and a root or radicle emerges first, followed by the shoot or plumulethat contains the leaves and stem.

Many things can cause poor germination. Overwatering causes the plant to not have enough oxygen. Planting seeds too deeply causes them to use all of their stored energy before reaching the soil surface. Dry conditions mean the plant doesn't have enough moisture to start the germination process and keep it going.

Some seed coats are so hard that water and oxygen cannot get through until the coat breaks down. Soaking or scratching the seeds will help break down the seed coat. Morning glories and locust seeds are examples. Other seeds need to be exposed to proper temperatures. Apple seeds will not germinate unless they are held at cold temperatures for a period of time.

General Functions of a Seed

Seeds serve several functions for the plants that produce them. Key among these functions are nourishment of the embryo, dispersal to a new location, and dormancy during unfavorable conditions. Seeds fundamentally are a means of reproduction and most seeds are the product of sexual reproduction which produces a remixing of genetic material and phenotype variability that natural selection acts on.

Seeds serve several functions for the plants that produce them. Key among these functions are nourishment of the embryo, dispersal to a new location, and dormancy during unfavorable conditions. Seeds fundamentally are a means of reproduction and most seeds are the product of sexual reproduction which produces a remixing of genetic material and phenotype variability that natural selection acts on.

Embryo nourishment Seeds protect and nourish the embryo or young plant. Seeds usually give a seedling a faster start

than a sporeling from a spore, because of the larger food reserves in the seed and the multicellularity of the enclosed embryo.

Seed dispersal Unlike animals, plants are limited in their ability to seek out favorable conditions for life and growth.

As a result, plants have evolved many ways to disperse their offspring by dispersing their seeds (see also vegetative reproduction). A seed must somehow "arrive" at a location and be there at a time favorable for germination and growth. When the fruits open and release their seeds in a regular way, it is called dehiscent, which is often distinctive for related groups of plants, these fruits include; Capsules, follicles, legumes, silicles and siliques. When fruits do not open and release their seeds in a regular fashion they are called indehiscent, which include the fruits achenes, caryopsis, nuts, samaras, and utricles.

Seed dispersal is seen most obviously in fruits; however many seeds aid in their own dispersal. Some kinds of seeds are dispersed while still inside a fruit or cone, which later opens or disintegrates to release the seeds. Other seeds are expelled or released from the fruit prior to dispersal. For example, milkweeds produce a fruit type, known as a follicle, that splits open along one side to release the seeds. Iris capsules split into three "valves" to release their seeds.

Seed dormancy Seed dormancy has two main functions: the first is synchronizing germination

with the optimal conditions for survival of the resulting seedling; the second is spreading germination of a batch of seeds over time so that a catastrophe after germination (e.g. late frosts, drought, herbivory) does not result in the death of all offspring of a plant (bet-hedging).Seed dormancy is defined as a seed failing to germinate under environmental conditions optimal for germination, normally when the environment is at a suitable temperature with proper soil moisture. This true dormancy or innate dormancy is therefore caused by conditions within the seed that prevent germination. Thus dormancy is a state of the seed, not of the environment. Induced dormancy, enforced dormancy or seed quiescence occurs when a seed fails to germinate because the external environmental conditions are inappropriate for germination, mostly in response to conditions being too dark or light, too cold or hot, or too dry.

Seed dormancy is not the same as seed persistence in the soil or on the plant, though even in scientific publications dormancy and persistence are often confused or used as synonyms.

Often seed dormancy is divided into four major categories: exogenous; endogenous; combinational; and secondary. A more recent system distinguishes five classes of dormancy:morphological, physiological, morphophysiological, physical and combinational dormancy.

Seed hibernation 

is different from seed dormancy. While seed dormancy can be defined as "a seed not germinating when conditions, e.g. light, water/nutrient availability or the presence of activating substances like smoke, are favourable", hibernation is the ability of a seed to remain in hibernation when there is a lack of things essential to its development (water, sunlight, nutrients, etc.) or in harsh conditions (extreme cold, extreme heat, hard ground, etc.). There is no specific time limit in which a seed may hibernate; Seeds found in the Canadian arctic and thought to be 10000 years old and still viable were finally shown to be approximately 10 years old by radio carbon dating (Zazula et al., 2009 New Phytologist). The oldest seeds that have been found to be viable are of the Date Palm (Phoenix dactylifera): several seeds found in the Israeli fortress of Masada were radiocarbon dated to be 2000 years old. One seed was still able to germinate (Sallon et al., 2008 Science). When conditions are right for the particular type of seed, they can come out of hibernation and grow.

Seed Micro-organisms

Plant microorganisms are essential to a healthy garden. In previous entries I talked quite a bit about bacteria and fungi and the proper balance needed between the aerobic and anerobic microorganisms in your hydroponic growing medium.

The beneficial bacteria and fungi in your plant root systems perform 1 of 3 possible functions: they free up soil nutrients for plant use by the breakdown of organic matter, they can enter a symbiotic relationship in the root systems meaning that the microorganism as well as the plant benefit from one another and, lastly, they can help suppress plant disease.

Symbiotic microorganisms include Rhizobia, a nitrogen-fixing bacteria, that attach symbiotically to plant roots and convert nitrogen in the air to proteins that plants use and mycorrhizal fungi that help conduct water and nutrients through plant roots.

Bacteria such as Bacillus Cereus, Circulans and Paeniticillus Polymxa help suppress plant pathogens or disease. You can see that such microorganisms are very important in growing healthy plants so be aware they are there and needed, and do not get rid of them needlessly out of some preconceived notion that 'the only good bacteria is dead bacteria'.