MANUAL OF THE GRASSES OF THE UNITED STATES 11 



are represented by fossils. Knowledge of the ancestry of the kinds 

 of plants now on the globe is necessarily very incomplete. Hence, 

 ideas of the relations of groups to each other are largely inferences 

 based upon morphological resemblances. Those individuals which 

 are so much alike as to appear to be of one kind, with, presumably, a 

 common ancestor in recent geological times, are regarded as belonging 

 to the same species. The species is the unit of classification. For 

 convenience, species are grouped into genera and genera into families. 

 For example, the white oak, red oak, black oak, and other lands or 

 species of oak belong to the oak genus (Quercus), all the species of 

 which have one character in common — the fruit is an acorn. The oak 

 genus, the beech genus, the chestnut genus, and a few allied genera 

 are grouped together as a family. 



The grass family (Gramineae or Poaceae) is one of the largest in 

 number of genera and species, and, among flowering plants, is prob- 

 ably the largest in the number of individuals and is one of the most 

 widely distributed. Some genera, such as the bluegrasses (Poa), 

 the bromegrasses (Bromus), and the immense genus Panicum, contain 

 numerous often closely allied species. Some genera contain but a 

 few species or even but one. 



When an attempt is made to classify a group of related variable 

 species the question always arises whether there are several closely 

 related but distinct species or a few distinct species, each of which 

 shows great variation. It is but natural that botanists should differ 

 in their conclusions. This explains in part the different classifica- 

 tions of the same group given by botanists of different periods or even 

 of the same period. A satisfactory classification depends upon the 

 study of abundant material both in the field and in the herbarium. 

 By observation in the field one learns the range of variability of a 

 species, while in the herbarium one can compare plants from different 

 localities, interpreting the dried specimens in the light of field experi- 

 ence. 



In the classification of variable species it is found convenient some- 

 times to separate variants as varieties. A variety comprises those 

 individuals of a species that show a definite tendency to vary in a 

 certain direction, but which are connected with the species by rather 

 numerous intergrades. Sometimes a variety is founded on a single 

 variation which is distinct but trivial, for example, pubescent speci- 

 mens of a glabrous species. A variation supported by a distinct 

 geographical range or even by a distinct habitat is given greater 

 weight than is a variation found in a few individuals growing among 

 plants of the typical form. 



The study of a vast amount of material in field and herbarium 

 during some 30 years has resulted in the recognition of relatively few 

 varieties, the intergrades proving to be more numerous than fairly 

 clear-cut variants. Well-marked varieties are given a separate 

 paragraph in the text, but are not usually given in the keys. Less 

 well-marked varieties are given in the paragraph with the species. 

 Many additional forms are indicated in a descriptive statement 

 without being formally recognized as species or varieties. For 

 example, under Digitaria gracillima appears, "A tall plant with 

 * * * has been called D. bakeri (Nash) Fernald"; and under 

 Eriochloa michauxii, "a form with * * * has been described as 

 E. mollis var. longifolia Vasey." 



