208 ELEMENTS OF BOTANY 
genus, the Rose genus, the Clover genus, the Golden-rod 
genus, the Oak genus. The number of species in a genus 
varies widely, —the Kentucky Coffee-tree genus con- 
tains only one species, while the Golden-rod genus com- 
prises more than forty species in the northeastern United 
States alone. 
257. Hybrids. — If the pollen of a plant of one species 
is placed on the stigma of a plant of the same genus but a 
different species, no fertilization will usually occur. Ina 
large number of cases, however, the pistil will be ferti- 
lized and good seed be produced. This process is called 
hybridization, and any plant grown from such seed is a 
hybrid? Many hybrid oaks have been found to occur in 
a state of nature, and hybrid forms of grapes, orchids, 
and other cultivated plants are produced by horticulturists 
at will. 
258. Varieties. — Oftentimes it is desirable to describe 
and give names to subdivisions of species. All the culti- 
vated kinds of apple are reckoned as belonging to one 
species, but it is convenient to designate such varieties as 
the Baldwin, the Bellflower, the Rambo, the Gravenstein, 
the Northern Spy, and so on. 
259. Family.— Genera which resemble each other some- 
what closely are classed together in one family. The 
particular genera mentioned in Sect. 255, together with a 
large number of others, combine to make up the Buttercup 
family. In determining the classification of plants most 
points of structure are important, but the characteristics 
of the flower and fruit outrank others because they are 
more constant, since they vary less rapidly than the char- 
acteristics of roots, stems, and leaves do under changed 
1 See L. H. Bailey on Evolution of Plants, in Science, March 20, 1903. 
