CHAPTER XVI 

 ON VARIETIES IN PLANTS 



The multiplicity and diversity of the varieties of cultivated plants never 

 fail to impress the thoughtful observer. The cereals, fiber plants, legumes, 

 root crops, and tree fruits which comprise most of the important agricul- 

 tural crop plants include some 30 species. It is safe to assume that within 

 this small group of species over 5000 distinct varieties are known at pres- 

 ent. Of rice alone there are thousands of varieties in cultivation. Among 

 flowering plants we find the same diversity. The rose, lily, chrysanthe- 

 mum, violet, carnation, sweet pea, dahlia, gladiolus, tulip, and hyacinth 

 of our gardens and greenhouses represent not more than 200 species, 

 while of roses alone as many as 1000 named varieties are now listed in 

 European catalogues. In general the longer and more widely culti- 

 vated species contain the larger groups of varieties, partly because of 

 the greater opportunity for their discovery and partly because these 

 species have been subjected to conditions most favorable for the pro- 

 duction of varieties. Before attempting to discuss the conditions or 

 operations that lead to the production of new varieties it is necessary to 

 enquire into the natural processes by which varieties have been produced. 



The Origin of Domestic Varieties of Plants. Agriculturists have 

 made use of three general methods in creating new varieties of cultivated 

 plants, viz.: (1) the utilization of mutations or sports; (2) the employ- 

 ment of hybridization and selection methods; and (3) the utilization of 

 clonal diversity. The utilization of mutations should be interpreted 

 to include not only the discovery and multiplication of mutant forms, 

 but also the recombination of mutant characters in new varieties by 

 hybridization. We include selection in the same category with hybrid- 

 ization, because according to the hypothesis which we have championed 

 throughout this text, its effectiveness usually depends upon the existence 

 of germinal diversity such as follows hybridization. In certain cases, 

 of course, selection methods have depended for success upon the utiliza- 

 tion of mutations having minor character effects. The origin of varieties 

 by these three different methods may be illustrated by considering in 

 some detail the horticultural history of certain plants. Since the 

 ancestors of most of our crop plants are now extinct, we may turn for 

 this purpose to some of the more recently domesticated species, the 

 histories of which are known more precisely. 



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