DICOTYLEDONOUS GERMINATION. 855 
root, but it gives off one or more branches of nearly equal size, 
which separately pierce its extremity, and become the rootlets 
(fig. 251, r). Each of these rootlets, at the point where it 
pierces the radicular extremity, 1s surrounded by a cellular 
sheath termed the rovt-sheath or coleorhiza (fig. 251, co). This 
mode of germination is commonly termed endorhizal ; but it is 
not, as already stated, universal in Monocotyledons. 
2. Dicotyledonous Germination.—The seeds of Dicotyledons 
are either albuminous or exalbuminous, and their germination 
in such respects, as a general rule, presents no pecuharity worth 
notice. The two cotyledons either remain within the integu- 
ments of the seed in the form of fleshy lobes, as in the Horse- 
chestnut and Oak, in which case they are said to be hypogeal 
(from two Greek words signifying under the earth); or, as is 
more commonly the case, they burst through the coats, and rise 
out of the ground in the form of green leaves ( jig..18,2¢,.ch in 
which case they are epigeal (from two Greek words signifying 
upon or above the earth). In the course of development the 
cotyledons commonly separate, and the plumule comes out from 
between them (fig. 16, 1). In those cases where they remain 
within the integuments, they sometimes become more or less 
united, so that the embryo resembles that of a Monocotyledon ; 
but a Dicotyledonous embryo may be always distinguished from 
a Monocotyledonous one by its plumule coming out from between 
the bases of the cotyledons, and not passing through a sheath. 
The radicle of a Dicotyledonous embryo (see page 125) is it- 
self prolonged downwards by cell-multiplication just within its 
apex (fig. 248, a), toformthe root. An embryo which germinates 
in this way is termed exorhizal (page 154). 
As a general rule, seeds do not germinate until they are 
separated from their parents; but in some cases, and more 
especially when invested by pulp, as in the Gourds, Melon, 
Cucumber, Papaw, &c., they do so before they are detached. In 
the above plants such a mode of germination is altogether 
exceptional; but in the plants of the order Rhizophoracee, as 
the Mangrove (fig. 255), the seeds commonly germinate in the 
pericarp before being separated from the tree, in which case the 
radicle is protruded through the integuments of the seed and 
pericarp, and becomes suspended in the air, where it elongates. 
CHAPTER 4. 
SPECIAL PHENOMENA IN THE LIFE OF THE PLANT. 
1. DeveLopmMENT oF Heat By Piants.—As the various parts 
of living plants are the seat of active chemical and other changes 
during their development, and in the performance of their 
