PLANT BKEEDING AND EVOLUTION 



199 



I 



The Uncle Sam type appeared among these variations and has 

 proved to be constant when grown from seed. In tobacco, there- 

 fore, as in many other plants, two kinds of variations frequently 

 occur which are of very different value and importance in the 

 improvement of these plants by breeding. The inconstant, or 

 fluctuating, variations are not of great value for selection in 

 the making of an improved 

 race, while the stable varia- 

 tions called mutations are 

 of the greatest interest and 

 importance to breeders of 

 new kinds of plants and 

 animals. 



THE MUTATION THEORY 



Certain varieties of plants, 

 like the Uncle Sam Sumatra 

 tobacco, have long been 

 known to have arisen sud- 

 denly as a result of varia- 

 tions affecting one or more 

 parts of the plant body. 

 The plants manifesting these 

 new variations have usually 

 been called sports in agri- 

 culture and horticulture, but are now called mutants. The 

 variations which distinguish them from the parent species are 

 called mutations. Among the best known of such sports, or 

 mutants, are the following: the moss rose and the nectarine, 

 which are bud sports from a cultivated rose and from the 

 peach ; the various cut-leaved varieties of the willow, maple, 

 and birch ; many white-flowered varieties springing from plants 

 with colored flowers ; and probably many varieties of vegetables, 

 forage plants, cereals, and fruits whose history, when traced 

 back, indicates a sudden origin from wild species. The muta- 

 tion theory for the origin of new varieties by sudden constant 



FIG. 109. (Enothera lamarckiana 



This is the mother plant of mutants dis- 

 covered and produced by De Vries. From 

 Babcock and Clausen's " Genetics in Rela- 

 tion to Agriculture." After De Vries 



