Seed Germination Theory and Practice
by Norman C. Deno

The first edition of Norman Deno's book was written in 1993, and detailed his conclusions on seed germination, and more particularly his findings on the types of inhibitors preventing seeds from germinating, and ways of overcoming these inhibitors, based on experiments with seeds of over 2500 species, in 805 genera, from 145 plant families.

Norman Deno was Professor Emeritus of Chemistry at Penn State University, and during his career in Chemistry published 150 papers, with another twenty papers during his secondary career in horticulture.

The following is a brief synopsis of the second edition of the book. There are also several Supplements, covering experiments on thousands more species.

Chapter 1 - Introduction and Principles

Chapter 2 - Germination, Definition and Description

Chapter 3 - Design of the Experiments

An explanation of the way the experiments were carried out, how the results were recorded and interpreted, with a brief explanation of the main variables which were examined: Temperature, Time, Dry Storage, Light, Oscillating Temperatures, and Gibberellins. The temperatures of 40oF and 70oF were chosen for the experiments as they represent the average winter or summer temperatures. Cycles of three months at these temperatures were used to simulate seasons.

Chapter 4 - Rates of Germination

A detailed account of how the results of the experiments were recorded, and what they mean.

Chapter 5 - Inhibitor Destruction by Dry Storage

Experiments showed that seeds of 50% of temperate zone plants can be collected, put in an envelope and left on a shelf, and will germinate when sown several months later. Professor Deno contends that the dry storage is an essential part of the germination process, and that chemical inhibitors present in the seeds are being destroyed during storage. Many members of the Asteraceae (Daisy), Brassicaceae (Mustard), Campanulaceae (Bellflower) and Poaceae (Grass) Families, including most garden annuals and vegetables, are in this group.

Chapter 6 - Inhibitor Destruction by Moist Conditions

In many species, the germination inhibitors are destroyed by exposure to moisture. There is an induction period before germination begins, and after this period, there is a sudden onset of germination. This pattern has long been recognised but not understood. It is what is happening when seeds are sown in the autumn or winter and germinate in the spring. The induction period has previously been referred to as a period of stratification or 'dormancy', but is in fact the time when the seed is at its most active, destroying chemical germination inhibitors. In some cases, the germination inhibitor is destroyed at 40oF, and germination also occurs at 40oF. This pattern is found in many bulbous species. A related mechanism is when the inhibition system is destroyed at 70oF.

Chapter 7 - Two or More Inhibiting Systems

Many examples were found where there were two or more sets of inhibiting systems, each requiring different conditions for destruction.

Chapter 8 - Seeds Embedded in Fruits

It was found that in many fruits there is an inhibitor in the fruit which prevents the seeds germinating. These seeds need to be washed to remove the chemical inhibitor.

Chapter 9 - Physical Mechanisms for Inhibiting Germination

Seeds of many legumes and others have impervious seedcoats, and these seeds will germinate if a hole is made in the seedcoat. Soaking seeds in hot water will also produce the same effect, because the heat causes the expansion of the seed coat and opens microfissures. Professor Deno says that the belief that freezing and thawing has the same effect is a myth, as true freezing and formation of ice crystals in the seed would be fatal. But oscillation in temperatures opens microfissures.

Chapter 10 - Outdoor Exposure and Oscillating Temperatures

A time honoured procedure has been to sow seeds in pots and place them outdoors in the fall. Germination often takes place in the spring. This procedure provides the low temperatures at which some species germinate and the low temperatures where some inhibitor systems are destroyed. The experiments showed that some species gave good germination when placed outdoors whereas germination was low or none in all other treatments. Further research showed that some seeds require oscillating temperatures to stimulate germination.

Chapter 11 - Photoeffects

The responses to light were complex. The results of experiments showed that seed behaviour fell into seven categories: (1) seeds which germinated only in light at 70oF; (2) seeds which required light and a pre-treatment; (3) seeds where germination was blocked by light; (4) seeds which required light for germination of fresh seed at 70oF but pretreatments removed the photorequirement; (5) seeds that required light for germination at 70oF but also germinated in the dark at 40oF; (6) seeds that germinated in either light at 70oF or in outdoor conditions; (7) seeds whose germination is promoted by light, but some germination occurs in the dark.

Chapter 12 - Exogenous Chemical Effects and the Stimulation of Germination by Gibberellins

Detailed descriptions of methods of using chemicals, particularly Gibberellins, to stimulate germination. Professor Deno contends that gibberellins are natural stimulators of germination in certain species and that the evolution of this requirement is critical to the survival of those species. In the chapter on how the experiments were conducted, details are given of the amount of gibberellic acid found to be effective.

Chapter 13 - Dry Storage and Longevity of Seeds

Ultimately, dry storage is fatal to all seeds. The length of time seeds can be stored and still remain viable depends on temperature and relative humidity. Seeds with impervious seedcoats remain viable for the longest periods. A few species have seeds that retain viability for only a few days, most species remain viable for 1-2 years, some 10-50 years, and some Fabaceae can remain viable in dry storage for 100-150 years.

Chapter 14 - Growing Plants from Seeds

Chapter 15 - Collection of Seeds

Chapter 16 - Plant Nomenclature

Chapter 17 - Endangered Species and Conservation

Chapter 18 - List of Genera Studied Arranged by their Plant Families

Chapter 19 - Rate Theory in More Detail

Chapter 20 - Summary of Data Arranged by Genera

Effectively, a Seed Germination Database for over 2500 species, based on the results of the experiments.

Chapter 21 - The Orchids

Chapter 22 - The Grasses

Chapter 23 - List of Donors of Seeds

Chapter 24 - List of References

Seed Germination Theory and Practice was published privately by Professor Deno. He has now retired (at the age of 89!), so the book and supplements can not now be obtained direct from him. Copies may be available from specialist societies or second-hand bookshops.

The Second Edition of Seed Germination Theory and Practice, published in 1993, is now online, and can be read and downloaded in PDF format from the USDA website. The First Supplement, published in 1996, is here, and the Second Supplement (published in 1998) is here.

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