PREFACE. 
XI 
into equally well marked groups. Some of them grow by the addition of 
new woody matter to the outside of their stem beneath their bar k ; these 
are EXOGENS ; others grow by the addition of new woody matter to 
the inside of their stem near the centre : those are ENDOGENS. But 
Exogens have two or more cotyledons to their embryo, and hence are 
called DICOTYLEDONS, while Endogens have only one cotyledon, 
and are, therefore, MONOCOTYLEDONS. Exogens have the 
young external wood connected with the centre by medullary pro- 
cesses ; Endogens having no occasion for such a provision are destitute 
of it. In Exogens the leaves have their veins disposed in a netted man- 
ner ; in Endogens the veins run parallel with each other. The number 
of parts in the flower of an Exogen is usually five, or its multiples ; in 
an Endogen it is as usually three, or its multiples. In germination the 
young root of Exogens is a mere extension of the radicle ; but of En- 
dogens it is protruded from within the radicle ; hence the former have 
been named EXORHIZjD, and the latter ENDORHIZJE. In 
this case then, as in the last, we have two groups differing entirely from 
each other in their germination, the structure of their stem and leaves, 
their mode of growth, the arrangement of the parts of the flower, and 
in the organization of their embryo. It is impossible, therefore, not to 
recognize such groups also as natural. 
To this separation of the vegetable kingdom into Exogens, Endogens, 
and Acrogens, or by whatever synonymous names those groups may be 
known, many Botanists confine themselves. But there are two others, 
of subordinate importance perhaps, but nevertheless characterized by 
circumstances of a similar nature, and, therefore, I think to be esteemed 
of equal dignity with them. In true Exogens and Endogens the ferti- 
lizing principle of the pollen is communicated to the young seeds through 
the medium of a stigma which terminates a case or pericarp in which 
they are enclosed. But in some plants othervdse Exogens, the fertiliz- 
ing influence of the pollen is applied immediately to the seeds, without 
the intervention of any pericarpial apparatus, and they bear the same 
relation to other Exogens as frogs and similar reptiles to other animals. 
These plants, therefore, are separated as a distinct class under the name 
of GYMNOSPERMS. Like the other groups of the same grade, 
these are also found to possess peculiarities of a subordinate nature. 
For instance, they have in many cases more cotyledons than two, whence 
they have been called POLYCOTYLEDONS , their radicle usually 
