INTRODUCTION. vii 
In addition to these main ranks, subordinate ones are sometimes employed, when further 
division is desirable: thus a Class may be separated into Subclasses, as the Class Angio- 
spermae into the Subclasses Monocotyledones and Dicotyledones; Families may be separated 
into Tribes, as in the treatment of Gramineae in the following pages; Genera are often separ- 
ated into Subgenera; and in the case of Species, where certain individuals are found to ex- 
hibit features of structure or aspect differing more or less constantly from the majority of 
the kind, these are set off as Varieties or Subspecies. Often the varieties or subspecies are 
subsequently found to be distinct species. 
The variability of some species, caused by the operation of the forces collectively known 
as the factors of organic evolution, is so great, and the variation being often toward a re- 
lated species, that it is sometimes very difficult to determine whether certain individuals be- 
long to one or to the other, or to a variety of one or the other. This leads to different opin- 
ions. The actual fact, whether of the same species or not, may usually be ascertained by the 
close examination of a large number of specimens, or by growing the forms in question side 
by side, when, if they are the same, their rapid approximation will be manifest; though if 
their natural habitats are in different soils, this latter experiment may not bea satisfactory test. 
The grouping of Species into Genera, and of Genera into Families, though based upon 
natural characters and relationships, is not governed by any definite rule that can be drawn 
from nature for determining just what characters shall be sufficient to constitute a Genus or 
a Family. These divisions are, therefore, necessarily more or less arbitrary and depend upon 
the judgment of scientific experts, in which natural characters and affinities, as the most im- 
portant and fundamental factors, do not necessarily exclude considerations of scientific con- 
venience. The practice among the most approved authors has accordingly been various. 
Some have made the number of genera and families as few as possible. This results in as- 
sociating under one name species or genera that present marked differences among 
themselves. The present tendency of expert opinion is to separate more freely into 
convenient natural groups, as genera and families, according to similarity of structure, 
habit, form or appearance. While this somewhat increases the number of these divisions, it 
has the distinct advantage of decreasing the size of the groups, and thus materially facilitates 
their study. This view has been taken in this work, following in most instances, but not in 
all, the arrangement adopted by Engler and Prantl in their recent great work, ‘‘ Natiir- 
liche Planzenfamilien,’’ * not yet quite completed, in which all known genera are described. 
Systematic Arrangement. 
The Nineteenth Century closes with the almost unanimous scientific judgment that the 
order of nature is an order of evolution and development from the more simple to the more 
complex. In no department of Natural Science is this progressive development more marked 
or more demonstrable than in the vegetable life of the globe. Systematic Arrangement 
should logically follow the natural order; and by this method also, as now generally recog- 
nized, the best results of study and arrangement are obtained. ‘The sequence of Families 
adopted 50 or 75 years ago has become incongruous with our present knowledge; and it has 
for some time past been gradually superseded by truer scientific arrangements in the later 
works of European authors. f 
The more simple forms are, in general, distinguished from the more complex, (1) by fewer 
organs or parts; (2) by the less perfect adaptation of the organs to the purposes they sub- 
serve; (3) by the relative degree of development of the more important organs; (4) by the 
lesser degree of differentiation of the plant-body or of its organs; (5) by considerations of 
antiquity, as indicated by the geological record; (6) by a consideration of the phenomena of 
embryogeny. Thus, the Pteridophyta, which do not produce seeds and which appeared on 
the earth in Silurian time, are simpler than the Spermatophyta; the Gymnospermae in which 
the ovules are borne on the face of a scale, and which are known from the Devonian period 
onward, are simpler than the Angiospermae, whose ovules are borne in a closed cavity, and 
which are unknown before the Jurassic. 
In the Angiospermae the simpler types are those whose floral structure is nearest the 
* Berlin, 15 volumes, 1890-1896. 
} Engler und Prantl, ‘‘ Nattirliche Pflanzenfamilien;’’ Warming, ‘“‘Systematic Botany, 1895,” 
Vines, ‘‘Student’s Handbook of Botany, 1895;’’ Richter, ‘‘ Plantae Europeae, 1890;’’ Thomé, “ Flora 
von Deutschland, OEsterreich und der Schweiz, 1886-1889;'’ Potonié, ‘‘Illustrirte Flora von Nord- 
und Mittel-Deutschland, 188 Schlechtendahl, Langethal und Schenck, ‘“‘ Flora von Deutschland,”’ 
fifth edition by Hallier, 1880-1885. 
