12 SEED.—GERMINATION. 
plants and the different parts or organs which they form by 
their combination, it will be necessary for us to give a general 
sketch of the nature and characters of these compound organs, 
and to explain the meaning of the various technical terms which 
are employed for their description. 
We have already stated that a seed contains an embryo, in 
which the essential parts or organs of the future plant are 
present ina rudimentary state. The embryo of the common Pea 
may be taken for the purpose of illustration (jig. 16). Here we 
find a distinct central axis, f, which is sometimes termed the 
tigellum or tigelle, the lower part of which is called the radicle, 
yr; and its upper extremity, which is terminated by two or more 
rudimentary leaves, is termed the plwmule or gemmule, n. 
This axis is united to two fleshy lobes, c, c, whose office is of 
a temporary nature, and to which the name of cotyledons has 
been given. But some seeds only contain one cotyledon in 
Pies Ave 
Fig. 16. Dicotyledonous embryo of the Pea, laid open (magnified). #». The 
radicle. ¢. The axis (tigellum), terminated by the plumule, m. c¢,c. The 
cotyledons. Fig. 17. Bract or carpellary leaf, sc, of a species of Pinus, 
bearing two naked ovules, ov, at its base. mic. The micropyle or foramen. 
their embryo, (fig. 19, c), instead of two as just described in the 
Pea (jig. 16, c, c); and hence we divide the Phanerogams, 
or those plants which are reproduced by seeds, into two great 
classes, called, respectively, Dicotyledones (two cotyledons), and 
Monocotyledones (one cotyledon). The two great divisions of 
plants are, therefore, the Cryptogamia and the Phanerogamia ; 
the former being again subdivided into the Thallophyta and 
Cormophyta ; and the latter into Angiospermia and Gymno- 
spermia, if reference be made to the position of their ovules, 
or into Monocotyledones and Dicotyledones, if we regard the 
number of cotyledons. 
When a seed is placed under favourable circumstances 
(which will be treated of hereafter in speaking of the process 
of germination), its embryo begins at once to develop (jigs. 
18 and 19); the lower part of its axis, t, or radicle, or one 
or more branches from it, growing in a downward direction, 
