548 



BREEDING 



BREEDING 



the great importance of such variations in the produc- 

 tion of cultivated varieties and the evolution of species. 

 As is well known to gardeners, these sports or mutations 

 appear suddenly without warning or reason, so far as is 

 known. They cannot be produced, and one must simply 

 wait until they appear and then be prepared to recog- 

 nize and propagate them. Mutations usually repro- 

 duce their characters without much reversion to the 

 parental type except such as is caused by cross-pollina- 

 tion. Mutations of self-fertilized plants thus usually 

 come true to type, while in cross-fertilized plants the 

 mutation must usually be cultivated in an isolated 

 place and carefully selected to weed out the effect of 

 such crossing as has occurred. Many seedsmen examine 

 their trial-grounds regularly for sports or mutations, 

 and many of the best varieties have lesulted from the 

 selection of such sports. Livingston, of Ohio, who dur- 

 ing his life was famous for the number of new varieties 

 of tomatoes which he produced, made it a practice to 

 search regularly the fields of tomatoes, which he grew 

 for seed purposes, for such sports, and almost all of his 

 numerous varieties were pioduced by the discovery of 

 such striking variations. 



A very interesting case of a variety that originated as 

 a. seedling sport or mutation is the now familiar case of 

 the Cupid sweet pea. Until about fifteen years ago the 

 only sweet peas known were the ordinary tall twining 

 sorts which grow to a height of 3 to 6 feet, depend- 

 ing upon the richness of the soil. At this time there was 

 found in California, a small dwarf sweet pea plant only 

 about 6 or 8 inches high. This was growing in a row of 

 the Emily Henderson variety, one ol the ordinary tall 

 sorts from which it evidently had sprung. Seed of this 



640. Stem of pondweed (Potamogeton 

 spiralis), showing narrow submerged 

 leaves and broad floating leaves. (After 

 Britton and Brown.) 



dwarf plant was saved and grown, and it was found to 

 reproduce plants of the same dwarf character. The 

 variety was designated the Cupid, under which name it 

 was introduced to the seed trade and distributed over 

 the world. The Cupid differed from other sweet peas 

 not only in height but in its closely set leaves and general 

 habit of growth. Indeed it is as distinct from other 

 sweet peas as are distinct species of plants in nature. 

 From the original Cupid, there have sprung many 



different sorts, until now there are varieties of Cupids 

 representing almost all variations of color and shape 

 of flower known in the sweet pea family. 



Causes of variation. 



Understanding of the causes of variation is as yet 

 very imperfect. Fluctuations are in general interpreted 

 as the direct physiological action of environment on the 

 plant, or, in other words, environmental reactions. 

 There would seem to be no doubt of the correctness of 

 this view for the cause of ordinary fluctuations, and it 

 may be accepted as the cause of such fluctuating varia- 

 tions as the breeder will commonly meet. Such reactions 

 as the changes in structure and form of the entire air- 

 leaves and finely divided water-leaves of certain butter- 

 cups (Ranunculus) and the floating and submerged 

 leaves of pondweeds or Potamogeton (Fig. 640), and the 

 loss of knees on the bald cypress when cultivated on 

 high land where the soil is well aerated, may be inter- 

 preted merely as extreme environmental reactions. 

 Even these extreme changes are not inherited other 

 than that the ability to react in this way under different 

 environments is inherited. 



To account for mutations is, however, a much more 

 difficult matter and no definite conclusion as to their 

 cause has yet been reached. Lamarck and his followers 

 have strongly maintained the hypothesis that changed 

 environment would stimulate the production of varia- 

 tions that would permanently effect the organism and 

 its progeny in the direction of better adapting them 

 to their environment. Many scientists, even today, 

 believe in the effectiveness of environment in develop- 

 ing adaptive changes. Weisman and his followers, 

 however, appear to have shown that characters acquired 

 through external influences, the so-called acquired 

 characters, do not affect the germ-cells, which are early 

 differentiated in the development of the organism, and 

 are thus not inherited. 



While, in general, it is certain that the ordinary 

 environmental reactions are not inherited, it is known 

 that plants long grown under a certain environment 

 become modified to suit that environment, and that 

 such adaptive changes have in some way so modified 

 the organism that the adaptive changes are rendered 

 heritable. Thus the conclusion follows that in some way 

 environment by its stimulation does occasionally affect 

 the germ-cells and produce changes that are inherited. 

 Plants that have long been cultivated under widely 

 varying conditions almost invariably develop numerous 

 heritable variations that would be classed as muta- 

 tions. The older breeders strongly held to the belief 

 that such conditions as change of food-supply, change of 

 altitude, artificial cultivation, budding, and grafting, 

 indeed the ordinary manipulation of agricultural culti- 

 vation, lead plants to vary in directions of importance 

 to the breeder. Clearly, no problem is of more impor- 

 tance to the breeder than to be able to produce or cause 

 such new characters to appear. 



It is only very recently that the idea has developed that one can 

 go farther than possibly to change the environment. With the 

 publication of MacDougal's researches in 1906, describing mutations 

 that were apparently caused by injecting the capsules of plants with 

 certain solutions, such as zinc sulfate and magnesium chloride, a 

 possible new method of forcing variations was introduced. Mac- 

 Dougal apparently obtained marked variations as a result of his 

 treatment, that were inherited in succeeding generations. 



Tower, by subjecting potato beetles during the formation of the 

 germ-cells to extremely hot and dry or hot and humid conditions 

 with changes of atmospheric pressure, was able to cause the 

 development of marked changes or mutations that were found to 

 transmit their characters true through several generations and which 

 segregated as unit-characters following hybridization. He con- 

 cludes from his experiments "that heritable variations are produced 

 as the direct response to external stimuli." 



Gager has produced similar changes in plants by subjecting the 

 developing ovaries to the action of radium rays, and a number of 

 similar studies by Hertwig and others indicate that radium ema- 

 nations have a very active effect on both plants and animals.^ 



While the evidence favoring the value of such external stimuli 

 as the above in producing new heritable characters is apparently 

 definite and positive, the extent to which the method can be used in 



