INTRODUCTION. 15 
happen to consult a book which makes special mention of the genus 
in these several classes, he will be altogether at fault. Suppose that some 
more knowing Botanist tells him that his plant is a Polygonum, he will 
again turn to his book, wondering how he could have overlooked it: 
but he will find the genus in the class Octandria, in spite of the different 
number of stamens in the specimen before him, and he will then learn 
that it is placed in the genus Polygonum on account of its strong gene- 
ral resemblance to other Polygonums, although differing from them in 
characters which are ordinarily considered as sufficient to establish 
classes and orders. Again, the greater part of the species of the genus 
Rhamnus (buckthorn) possesses both stamens and pistils iff the same 
flower; but the species most generally known in this country on account 
of its purgative properties is Dicecious, the staminiferous flowers being 
on one plant, and the pistiliferous on another. The student who meets 
with it therefore would seek for it in the class Dicecia, where he would 
be disappointed as before ; since, as in most species of flowers are com- 
plete, it is placed in the class and order to which the number of its sta- 
mens and pistils would refer it. 
10. Such exceptional cases occur much more frequently than is com- 
monly supposed. It has been proved that in fourteen divisions of the 
Linnean system, there are no less than forty three exceptions,—one quar- 
ter of the whole; and that out of two hundred and seventy four genera ~ 
of the Northern United States, belonging to eighteen Linnzan sections, 
there are seventy eight exceptions,—rather more than a quarter. These 
facts are important, both as preparing the student to meet with such diffi- 
culties, even in the study of the Linnzan system, which is generally con- 
sidered so easy of application; and also as showing the imperfection of 
the system itself, which is of no importance whatever beyond the tem- 
porary purpose of facilitating the early studies of the Botanical student. 
In well arranged descriptions of American plants (such as Wood’s Class 
book of Botany, which may be strongly recommended for this purpose) 
the most perplexing of these cases are noticed, in such a manner as to 
prevent the loss of time and labor, in vain attempts at discovering gen- 
era in wrong classes or species in wrong genera. 
PRINCIPLES OF THE LINNZZAN SYSTEM. 
11. In consequence of the advantages of the Linnean system for a 
beginner, it is desirable to give an outline of the principles upon 
which its divisions are founded, which may serve as an introduction to 
the regular systematic treatises upon the subject. The Phanerogamia 
or flowering plants are distributed under twenty-three classes, all of 
which are characterised either by the xwmber, or particular arrange- 
ment of the stamens. In the first twelve of these, number alone is re- 
a . 
