532 



STRUCTURE AND LIFE HISTORIES 



flowered species, are all illustrations of mutation. Fre- 

 quently the mutative change occurs in a lateral bud, pro- 

 ducing a " bud-sport" (Fig. 400). Such was the origin of 

 the seedless navel orange from the seed-bearing orange. 



FIG. 398. Clover leaves with three to nine leaflets, illustrating a 

 tendency to mutate. The normal clover leaf is a pinnately compound 

 leaf with three leaflets. Plants with leaves having five to nine leaflets 

 constitute a "half-race," i.e. the normal character is active, the anomaly 

 semi-latent. (Photo by the author; specimens from cultures of G. H. 

 Shull.) 



455. The Evening-primrose. In 1886 de Vries began 

 to search for a species that was in a mutating condition, 

 believing that any given species is at some periods in its 

 history more labile or changeable than at other periods. 

 After a long search he found in an abandoned potato field 

 at Hilversum, near Amsterdam, a large number of plants 

 of Lamarck's evening-primrose ((Enothera Lamarckiana) 

 (Fig. 401.) 



"That I really had hit upon a plant in a mutable period 

 became evident from the discovery, which I made a year 



