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.) 
456. 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 
