SEEDS by J A J RICHARDSON

 

SEEDS

 

by J A J Richardson 2009

 

Seeds sit at the roots of cultivation and propagation. At the base of every plant, and the beginning of every clone, sits a seed. In Nature mechanisms of seed dispersal, seed dormancy, and seed development are a habitual affair, governed by the seasons and individual micro-climates. As Horticulturalists the scenario is not always so simple. Cultivated seeds tend to come in paper packets, the origin and treatment of which often remains unknown by the grower. These seeds are after-all procreated by hand. A basic understanding about seed physiology and morphology therefore sits at the foundations of gardening success. In this report we shall take a brief look at mechanisms of of Seed Dispersal, Seed Dormancy, and Seed Development.

 

Seed Mechanisms

 

Many of the mechanisms behind the sowing of seed are often taken for granted by Horticulturalists. It is normally accepted that seeds are seeds (much and much the same); that one seed-line can/should be treated as another seed-line, under the same controlled conditions, to the same result. That failure in the garden is the result of “poor breeding” , a “bad packet” and/or an “infertile seed-line”. Here we hope to highlight the fact that individual seed-lines, and in some cases individual seed, each seek slightly different methods of treatment in order to trigger the best patterns of growth.

 

Seeds are unique. Upon sowing, each seed, no matter how closely related to its siblings, becomes an individual in its own right. Let us not forget that from seed, seedlings grow into adult plants (each of which has slight if not major differences from the next plant). During the earliest stages of seed development and germination even the smallest alteration in micro-climate can manipulate the physiology of a seeds potential. In this way, under artificial conditions, individual seed (and collectives) alter their metabolism in accordance with their climatic surroundings (the condition beneath which they are kept and sown). To understand these processes we shall consider seed mechanisms against the order that they occur in Nature.

 

Seed Dispersal

 

All annual plants set-to-seed from their flowers once a year. As the nights get longer, these pollinated (seeded) flowers ripen. At this point Seed Dormancy begins. The swollen sepals, which are at this stage seed laden, begin to split and the seed are aired. Some seed are cast from their shell-pods onto the ground, whilst some seeds remain hidden far deep within the flowers themselves.

 

In Nature, whilst related to plant size, the number of seed per plant is often relative to subspecies and micro-climate. During winter and early spring, thawing ice and snow aid the dispersal of highland species over distances, whereas under under moderate climates, rainfall aids the dispersal of lowland species over local landscapes. This reflects a need for competition in certain subspecies and/or an inherent mechanism of seed dispersal in others. Those seeds that are not disturbed by beast, bird or human return to the earth from whence their origins began.

 

As an inherited mechanism of seed dispersal, animals play an extremely functional role in the dispersal of seed for several reasons. Firstly via digestion animals transport seed into new locations. Whole seed are passed into pre-fertilised piles of dung/manure. This gives annual seeds a nice home to over-winter in (a place where seed are not likely to get eaten by birds). In this way a population of seed can easily find their way uphill or across rivers. This also provides the seeds with the ideal conditions for seed dormancy. Secondly, large herbivore churn-up the ground surrounding the plants upon which they browse. This creates a perfectly tilled environment upon which new seed can compete with established weed species. Thirdly, seed-feeding birds hold no-bounds and may carry seeds hundreds of miles on the wing. Finally, humans transport seed for both food and cultivation.

 

In wild populations, micro-climate plays the most important role in seed dispersal by governing the rate of soil morphology itself (the way a soil site moves). This dictates the location of seed populations across landscapes and encourages the uptake of natural selection. Seed that descend from higher ground will naturally be washed into lower ground. Some seed float in water while others tend to sink (ever wonder why this is?). Seeds that readily float are washed the furthest away from the source of the parent plant by heavy rainfall/snow. Likewise strong winds aid seed seed dispersal by literally blowing seed from calyx and stem. Floods, storms, landslides, earthquakes, de-forestation, periods of drought and/or urban development may all contribute towards the process of seed dispersal across any population, within any environment. There are quite literally thousands if not millions of variables involved.

 

Where/when wild populations are subjected to human intervention and/or unnatural methods of procreation, the natural ebb and flow of Nature and natural seed dispersal, is dramatically damaged. Those warning signs that read - “Please do NOT pick the wild flowers” are there for good reason. And must be perpetuated if wild populations are to remain.

 

 

Seed Dormancy

 

Seed dormancy is a naturally occurring 'survival factor' found within many species of plant. Under 'natural condition' the dormancy of many annuals is then quite normal. The 'over-wintering' of seed is then a naturally occurring phenomenon. That is, when under dormancy (even where perfect germination conditions exist) a seed will NOT germinate!

 

Under 'artificial conditions' cultivated seeds tend to display lower levels of dormancy. The seed of cultivated plants, especially those used for food, have an unnaturally shallow dormancy period. The 'survival factor' has quite literally been bred out of popular cultivar in order to ensure a homogeneous crop/harvest. There are three different types of seed dormancy. Exogenous Dormancy (external, Endogenous Dormancy (internal), and Combined Dormancy (external and internal):

 

Exogenous Dormancy is governed by the external covering of the seed itself (the seed coat). With many annual plants the seed coat is relatively thick. The seed is slowly permeable to water via a fissure line in the shell. This mechanical resistance inhibits dormancy. A period of warmth, followed by a period of cold usually encourages germination.

 

Endogenous Dormancy involves the internal development of the seed embryo (both morphological and physiological). Whilst internal seed morphology is aided by 'warm stratification', both small and immature embryos are always found in combination with another type of dormancy (mostly physiological).

 

Physiological endogenous dormancy is the most common type of dormancy found in Nature. An inhibitor inside the embryo and/or an incomplete chain of chemical reactions imposes dormancy. Most annual species experience a relatively 'shallow dormancy' period. Shallow dormancy can be overcome by 'after-ripening (drying seeds). A period of 'cold stratification' is then required in order for physiological dormancy to be complete. Growers use cold-rooms and refrigerators for this reason.

 

Combined Dormancy is displayed when several types of dormancy (often internal and external) are displayed in the same seed. This may involve exogenous dormancy (a seed coat), coupled alongside endogenous morphological resistance (embryo development), alongside physiological dormancy (a shallow inhibitor), all within the same seed structure. The underdeveloped embryos MUST first develop into their full size during a period of 'warm stratification' before a period of 'cold stratification' can be effective.

 

The best way to test levels of seed dormancy against levels of seed viability is to play with seeds. Try keeping collectives of seeds under different conditions (dry, wet, hot, cold...). Then sow each seed lot into separate trays of soil. Place each tray into a contrasting environment (dry, wet, hot, cold...) make observational notes and record the results.

 

 

Seed Development    

 

Seed development is a funny old thing... one that never ceases to amaze. The smaller the seed, the more amazing the transformation from genetic time capsule into seedling. Let's face it, this is where the ingrained magic of growing plants begins. What people in white-coats like to call “plant science” is in fact just Mother Nature doing her own thing.

 

Now many people may call on the traditionalists, but seeds like things best in soil. There is just no escaping this. With 'paper towel' and 'glass of water' methods of germination people can look, touch and prod the seed as it germinates (and I'd like to think this is one of the best ways to learn the habits of seeds). Seed development is however an extremely delicate process. For this reason seed development is often perhaps best left untouched by hand.

 

The germination and viability of seed is foremost governed by patterns of seed dormancy. Only once the natural cycle of embryo development is complete will the germination of seed successfully begin. Fresh seed are usually far less viable than those that have been subjected to dry conditions (warm stratification) followed by cool conditions (cold stratification) prior to sowing. Only following a period of dormancy can true seed germination and accurate measurements of seed viability begin. Before which seed tend to grow into weaker seedlings.

 

 

After thoughts...

 

Always remember that mechanisms of seed dispersal, seed dormancy, and seed development hold a direct influence over the seeds that grow and the seeds that do not grow in the garden. Seed may have travelled a long way from the hands of the Breeder to the hands of the Gardener. Any number of climatic and/or environmental factors may then have already inhibited and/or aided the initial stages of seed dormancy and seed development. It all depends upon the condition(s) beneath which the seed have previously been kept.