212 
ORGANOGRAPHY. 
BOOK I. 
It is distinguished into three parts; viz. the radicle (Plate 
VI. fig. 2. Z>, &c.) (rhizoma ov rostellum) ; cotyledons (fig. 2. a, 
&c.), QXid plumule (or gemmule) (fig. 2. c.) ; from which is also 
by some distinguished the caulicule or neck (collet, scapus, sea- 
pellus, or tigelle). Mirbel admits but two principal parts; 
viz. the cotyledons, and what he calls the blast^me, which com- 
prises radicle, plumule, and caulicule. 
Upon certain remarkable 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. Dicotyle- 
dons; 2. Monocotyledons; and, 3. Acotyledons. In order to 
understand 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, fleshy 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 cauli- 
cule will extend upwards ; the radicle will become elongated 
downwards, forming a little root ; the cotyledons will elevate 
themselves above the earth and unfold ; and the plumule will 
lengthen upwards, and give 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 ; 
and, 4. in their inequality. A cohesion of the cotyledons takes 
place in those embryos, which Gaertner called pseudomonocoty- 
ledonous^ and Richard macrocephalous. In the horse-chestnut, 
