250 
ORGANOGRAPHY. 
BOOK I. 
It is distinguished into three parts; viz., the radicle (Plate 
VI. fig. 2.^, &c.) [rhizoma or rostellum) ; cotyledons (fig. 2. a, 
&c.) ; and plumule (or gemmule) (fig. 2. c.) ; from which is also 
by some distinguished the cauliculus or neck (scapus, scapellus^ 
or tigelle), Mirbel admits but two principal parts; viz., the 
cotyledons, and what he calls the blastema, which comprises 
radicle, plumule, and cauliculus. 
Upon certain differences in the structure of the embryo, 
modern botanists have divided the whole vegetable kingdom 
into three great portions, which form the basis of what is 
called the natural system. These are, 1 . Dicotyledons ; 
2. Monocotyledons ; and, 3. Acotyledons. In order to under- 
stand exactly the true nature of the embryo in each of these, 
it will be requisite first to describe it fully as it exists in 
dicotyledons, and then to explain its organisation in the two 
others. 
If a common Dicotyledonous embryo (Plate VI. fig. 2.), 
that of the Apple for example, be examined, it will be found 
to be an obovate, white, fieshy body, tapering and solid at 
the lower end, and compressed and deeply divided into two 
equal opposite portions at the upper end ; the lower tapering 
end is the radicle, and the upper divided end consists of two 
cotyledons. Within the base of the cotyledons is just visible a 
minute point, which is the plumide. The imaginary line of 
division between the radicle and the cotyledons is the cauli- 
cule. If the embryo be placed in circumstances favourable 
for germination, the following phenomena occur : the caulicule 
will extend so as to separate the cotyledons from the radicle 
by an interval, the extent of which varies in different plants ; 
the radicle will become elongated downwards, forming a little 
root; the cotyledons will either elevate themselves above the 
earth and unfold, or, remaining under ground, will part with 
their amylaceous matter and shrivel up ; and the plumule will 
lengthen upwards, giving birth to a stem and leaves. Such 
is the normal or proper appearance of a dicotyledonous 
embryo. 
The exceptions to it chiefly consist, 1. in the cohesion of the 
cotyledons in a single mass, instead of their unfolding; 2. in 
an increase of their number ; 3. in their occasional absence ; 
