56 



THE CANADIAN HORTICULTURIST 



March, 1910 



The Principles of Plant Breeding' 



Prol. Wm. Lochhead, Macdonald College 



H\HRIDIZATION involves a know- 

 ledge of the parts of the flow- 

 er and of thoirp.irlicui.'ir functions. 

 It is based on the f.ict of the soxiiality 

 of plants. When ripe pollen from the 

 stamens of a flower belonging to one 

 variety or species is placed on the mature 

 stigmas of a flower belonging to anoth- 

 er variety, the pollen grains send down 

 slender tubes through the styles into the 

 ovary, where tiiey enter the ovules and 

 come into contact with the egg-cells. A 

 male germ cell then passes oi't of tho 

 tube and fuses with the nucleus of the 

 egg-cell. This process is known as 

 fertilization of the egg-cell by the male 

 germ cell. The fertilized egg-cell soon 

 divides into many cells and becomes an 

 embryo. The plant that develops from 

 this embryo is a hybrid, and the process 

 of formation is called hybridisation. 



The principles of hybridization of 

 plants were unknown before the eigh- 

 teenth century. The development of our 

 knowledge of hybridization is largely due 

 to Kolreuter (1760), Knight (18 — ) and 

 Darwin. Later additions to our know- 

 ledge were contributed by Gaertner, 

 Naudin, Focke, Vilmorin, Mendel and 

 others. To Darwin we owe the phrase : 

 "Nature abhors perpetual self-fertiliza- 

 tion," which does not hold true in the 

 case of many vigorous plants such as 

 tobacco, wheat and barley. Dr. East 

 says Darwin's phrase should probably 

 be changed to read: "Nature resists any 

 sudden change in long established condi- 

 tions." 



It is well known that many plants have 

 special adaptations in their flowers, 

 whereby self-fertilization is prevented ; 

 that the highly colored flowers are usu- 

 ally cross-fertilized by insects ; that the 

 more inconspicuous . flowers are cross- 

 pollinated by wind, etc., etc. Darwin 

 proved by numerous experiments that 

 the products of crosses were usually more 

 vigorous than the parents of the hybrid. 

 It has also been observed that "in gen- 

 eral the closer the botanical relations of 

 two plants, the more easily they will 

 cross. Crosses between varieties are 

 generally very easy to make ; those be- 

 tween Linnaen species have been made 

 in quite a number of instances, while 

 crosses between genera and families are 

 rare." Moreover, it has been observed 

 that hybrids arising from parents not 

 closely related are usually much more 

 likely to be sterile than are those from 

 parents nearly related. 



While a host of facts regarding hy- 



•In last June issue of The Canadian Horticul- 

 tiiriBt, the improvement of plants by selection 

 was disc^usssd. The theory of mutations was 

 dealt with in the September number. The ar- 

 ticle on hybridization that appears on this page 

 will be followed by further information on the 

 same subject. 



bridization had been accumulated, no 

 general principle had been established 

 until Mendel published a report in 1865. 

 The experiments embodied in the report 

 were made between 1855 and 1865, and 

 were published in the transactions of an 

 ob.scure society in Hrunn, Austria. 

 This publication lay unnoticed until 1900. 

 If one turns to the works on plant 

 breeding published before 1900 he will 

 realize how vague at that time were our 

 notions of the laws regading hybrids. 

 No person seemed to be able to predict 

 with any degree of certainty the result 

 of crossing varieties of plants. In fact, 

 contradictory results are often reported 

 by different plant breeders. "The facts 

 were wonderful enough, but they showed 

 no signs of falling into an orderly ar- 

 rangement." Mendel's results were 

 formulated in two laws : 



1. The Law of Dominance, which 

 may be expressed thus: "If two contrast- 

 ed characters which have previously bred 

 true are crossed, one only, the dominant 

 character, appears in the hybrid." 

 (East) ; and 



2. The Law of Inheritance, which 

 may be stated as follows: "In succeed- 

 ing generations, self-fertilized plants 

 grown from .seeds of the cross reproduce 

 both characters in the proportion of three 

 of the dominant character to one of the 

 recessive character. Furthermore, the 

 recessive character continues ever to 

 breed true, while those plants bearing 

 the dominant character are one-third 

 pure dominants, which ever after breed 

 true to the dominant character, and two- 

 thirds hybrid dominants which contain 

 the recessive character in a hidden con- 

 dition." (Ea.st). 



Mendel's experiments in cross-breed- 

 ing were made with the common garden 

 peas, which are capable of self-fertiliza- 

 tion, and which have numerous varietal 

 forms, distinguished by the color and 

 shape of the seed, the color of the flow- 

 ers, the color of the pods, the lens^th of 

 the stems, and the arrangement of the 

 Powers on the stem. He determined the 

 heredity first of all, of each .set of char- 

 aiters; i.e., yellow and green seeds, 

 round and angular seeds, smooth and 

 wrinkled seeds, and so forth. He found, 

 for example (a) that when yellow and 

 green .seeded varieties were cros.sed he 

 obtained only yellow-seeded hybrids. 

 (Generation F.i) the yellow being dn- 

 niinant to the green which is recessive. 



(b) \\'hen, however, the hybrid plants 

 were self-fertilized, the .seeds obtained 

 in this second generation (F2) were com- 

 posed of both yellow and green forms, — 

 in the proportion of three yellow to one 

 green. 



c) When the plants arising from green 



seeds of the second generation were self- 

 fertilized, only nlants wi'h (.'r^'cn (F3) 

 seeds were obtained. 



(d) When the yellows of the second 

 generation were self-fertilized, some 

 gave rise to plants with yellow seeds 

 only, while others gave rise to plants 

 with yellow and green seeds in the pro- 

 portion of three to one, as in the second 

 generation (F2.) 



In like manner Mendel crossed peas, 

 each possessing one of a set of charac- 

 ters and obtained similar results. He 

 found "round .seeds dominant over 

 wrinkled, colored .seed coats over white 

 seed coats, tallncss over dwarfness," 

 etc. The similarity of the results led 

 "Mendel to the conception of pairs of 

 unit-characters of which either can be 

 carried to any gamete, or sex cell, to the 

 exclusion of the other." De Vries 

 adopts this idea of an organism being 

 composed of a bundle of unit-characters 

 in his theory of mutations, and con- 

 siders a mutation to differ from the par- 

 ent plant in the addition of a unit-char- 

 acter, not previously possessed by the 

 parent. Such is the idea of a discontin- 

 uous variation. 



Mendel carried on experiments where 

 peas possesing two or more pairs of con- 

 trasting characters were cros.sed, and 

 found that the separate pairs were trans- 

 mitted entirely independently of one an- 

 other. "When, for example, ;i tall yel- 

 low-seeded pea was crossed with a 

 dwarf green-seeded one, the Fi plants 

 all exhil)ited the dominant character of 

 each pair, and were tall yellows. In the 

 next generation appear, as usual, tails 

 and dwarfs in the ratio of 3. i , and aI.so 

 yellows and greens in the same ratio. 

 If we suppose that there are 16 plants, 

 it is clear that 12 of these will be tall, 

 and that the other 4 will be dwarf. Now, 

 of every 4 tails, 3 will be yellows and 

 the other green. Out of our 12 tails, 

 therefore, 9 will be yellows and 3 will 

 be green. Similarly, of the 4 dwarfs, 

 3 will be ve'low and one will be green. 

 Consequently, the F2 generation arising 

 from the cross will consist of 9 yellow 

 tails, 3 green tails, 3 yellow dwarfs, and 

 one dwarf green. In other words, there 

 will be for every 16 plants a class of 

 each showing the dominant character of 

 one pair and the recessive of the other ; 

 and one plant with both recessive char- 

 acters. - Mendel established by experi- 

 ment that these were the proportions 

 that actually occurred, a result which 

 has been amply confirmed since his time 

 for other plants as well as for animals. 

 .\nd the principle may be extended inde- 

 finitely for any number of pairs of char- 

 acters." (Punnett). 



More care must be taken in .spraying 

 fapanese plum trees than with most 

 other kinds of fruit. They are more 

 easily injured. 



