Hayes and Garber - Cucurbit Breeding
Hayes and Garber - Cucurbit Breeding Hayes and Garber - Cucurbit Breeding
10 BREEDING CROP PLANTS 1. Variability. It is a matter of common observation that no two individuals are exactly alike. If sufficient individuals are examined the range of variation is found to be quite great. These variations are universally present. 2. A Struggle for Existence. If all the progeny of some of the lower forms grew to maturity and each in turn produced as many progeny, the world would soon be overrun with a single form. There is competition also between different species and genera. 3. Natural Selection. The conclusion would certainly seem reasonable that those forms would survive which possessed characters better adapted to a given environment and therefore gave those particular forms advantage in the struggle for existence. 4. Heredity. Variation produces the material for natural selection to work upon and heredity tends to perpetuate the variations. The mechanism of transmission of characters, the physiological cause of variations, and the inheritance of different categories of variations were unanswered problems. Many criticisms were made of Darwin's work and many were considered by Darwin himself. Nearly all of these have a bearing upon plant breeding. In the improvement of crops, artificial selection takes the place of natural selection. The breeder is constantly faced with the question of the perpetuation of a variation. He also faces the question of whether the useful variation will perpetuate itself in crosses or will be lost. Darwin recognized two sorts of variations, the " fortuitous" or chance variations, i.e., those which are everywhere present and which cause every plant to be slightly different from other plants of the same species. These were considered to be of primary importance in evolution. While he recognized " definite" or discontinuous variations, the so-called mutations, these were not considered of primary importance. The Stability of the Germ Plasm. Weissmann's theories are of much interest. He developed the idea of the continuity of the germ plasm and that external agencies could not modify inheritance without first affecting the germ cells. Plant breeders are not particularly interested in Weissmann's ingenious theories which were outlined to show that the inheritance of acquired characters was an impossibility. Apparently, in order that a new
INTRODUCTION 11 character may be produced there must be a modification of the germ plasm. The real question, then, is what causes germinal change? In considering this question we must keep in mind the importance from the possibility that agencies which are of little standpoint of the plant breeder may be of profound importance in evolution. DeVries' Mutation Theory. The more recent theory of evolution developed by DeVries attacks the question of the sort of variations which furnish the basis for evolution. DeVries gives only slight value to the small continouus variations and advances the hypothesis that large variations are of primary value. He believes in periods of mutation when from some unknown cause a species is producing many new forms, and other periods when stability of the species is the rule. DeVries recognized three sorts of mutations; (1) progressive, when an entirely retrogressive, when an new character appears ; (2) degressive, the appearance of a partially latent or hidden character; and (3) active character becomes latent. The cause, or causes, of these sudden changes was not known. Mutations are frequently not large but small. All sudden heritable changes which cannot be explained by the laws of segregation and recombination are called mutations. The Pure-line Theory. The studies of Johannsen are of particular value from the standpoint of the plant breeder. He worked with self-fertilized crops and found that while the progeny of a single self-fertilized plant varied quite widely, these variations were not inherited. From single commercial varieties he found it possible to isolate numerous lines which in their means differed slightly from each other and which bred true. Johannsen considered a pure line to be the progeny of one or more selffertilizations from a single homozygous ancestor. Selection within such a pure line was of no practical value. Numerous investigations with self-fertilized crops have been made and corroborate the results of Johannsen. Isolated cases of mutations in these pure lines have been reported, and while these are of much scientific interest they occur far too infrequently to be used as a basis for a system of breeding. Johannsen 's pure-line theory has been extended to cover clonal or asexual propagation in both plants and animals. At its proper place evidence will be given to show that in heterozygous organisms which are asexually propagated there sometimes occur
- Page 7 and 8: AGRICULTURAL AND BIOLOGICAL PUBLICA
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- Page 15 and 16: PREFACE Since the early development
- Page 17 and 18: . Early CONTENTS CHAPTER I INTRODUC
- Page 19 and 20: CONTENTS xiii PAGE Color of grain a
- Page 21 and 22: CONTENTS XV PAGE Sorghum 178 Origin
- Page 23: CONTENTS xvn CHAPTER XIX FARMERS' M
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- Page 40 and 41: * CHAPTER II PLANT GENETICS 1 Since
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10 BREEDING CROP PLANTS<br />
1. Variability. It is a matter of common observation that<br />
no two individuals are exactly<br />
alike. If sufficient individuals<br />
are examined the range of variation is found to be quite great.<br />
These variations are universally present.<br />
2. A Struggle for Existence. If all the progeny of some of the<br />
lower forms grew to maturity <strong>and</strong> each in turn produced as<br />
many progeny, the world would soon be overrun with a single<br />
form. There is competition also between different species <strong>and</strong><br />
genera.<br />
3. Natural Selection. The conclusion would certainly seem<br />
reasonable that those forms would survive which possessed<br />
characters better adapted to a given environment <strong>and</strong> therefore<br />
gave those particular forms advantage in the struggle for<br />
existence.<br />
4. Heredity. Variation produces the material for natural<br />
selection to work upon <strong>and</strong> heredity tends to perpetuate the<br />
variations.<br />
The mechanism of transmission of characters, the physiological<br />
cause of variations, <strong>and</strong> the inheritance of different<br />
categories of variations were unanswered problems. Many<br />
criticisms were made of Darwin's work <strong>and</strong> many were considered<br />
by Darwin himself. Nearly all of these have a bearing upon<br />
plant breeding. In the improvement of crops, artificial selection<br />
takes the place of natural selection. The breeder is constantly<br />
faced with the question of the perpetuation of a variation. He<br />
also faces the question of whether the useful variation will perpetuate<br />
itself in crosses or will be lost.<br />
Darwin recognized two sorts of variations, the " fortuitous"<br />
or chance variations, i.e., those which are everywhere present <strong>and</strong><br />
which cause every plant to be slightly different from other plants<br />
of the same species. These were considered to be of primary<br />
importance in evolution. While he recognized " definite" or<br />
discontinuous variations, the so-called mutations, these were not<br />
considered of primary importance.<br />
The Stability of the Germ Plasm. Weissmann's theories are<br />
of much interest. He developed the idea of the continuity of the<br />
germ plasm <strong>and</strong> that external agencies could not modify inheritance<br />
without first affecting the germ cells. Plant breeders are<br />
not particularly interested in Weissmann's ingenious theories<br />
which were outlined to show that the inheritance of acquired<br />
characters was an impossibility. Apparently, in order that a new
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