208 ESSENTIALS OF BOTANY 
genus. The number of species in a genus varies widely, — 
the Kentucky Coffee-tree genus contains only one species, 
while the Golden-rod genus comprises 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. In: 
a large number of cases, however, ovules will be fertilized 
and good seed be produced. This process is called hybrid- 
zation, and any plant grown from such seed is a hybrid.1 
Many hybrid oaks and violets have been found to occur 
in a state of nature, and hybrid forms of wheat, orchids, 
grapes, and other cultivated plants are produced by plant 
breeders at will (Chapter XXIx). 
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 (in the case of seed- 
plants) the characteristics of the flower and fruit outrank 
others because they are more constant, since they vary 
less rapidly than the characteristics of roots, stems, and 
leaves do under changed conditions of soil, climate, or other 
surrounding circumstances. Mere size or habit of growth 
1 See L. H. Bailey’s Plant Breeding, The Macmillan Company, New York. 
