68 



ORGANOGRAPHY. 



ill a rudimentary state. The embryo of a common pea may 

 be taken for the pm-pose of illustration {fig. 160). Here 

 we find a distinct central axis, t, the lower part of which is 

 called the radicle, r ; and it is terminated at its upper extremity 

 by two or more rudimentary leaves, forming together what is 

 termed the plumule or gemnmle, n. This axis is united to two 

 fleshy lobes, c c, whose office is of a temporary nature, to which 

 the name of cotyledons or seed-leaves has been given. Some 

 seeds only contain one cotyledon in their embryo {fig. 162.. c) ; 

 hence we divide plants which are propagated by them into 

 two great classes, called respectively, Dicotyledons {two coty- 

 ledons), and Monocotyledons {one cotyledon'). 



When a seed is placed under favourable circumstances (which 

 will be treated of hereafter in speaking of the process of ger- 

 mination), the embryo that it contains begins to develope 

 itself 0"(/5. 161 and 162); the lower part of its axis or radicle, 



Fig. 161. 



Fig. 162. 



Fin. 101. Germination of the Haricot or French Bean, a Dicotyledonous 

 Plant, r. The rootH springing from the lower end of the axis. 1. t. Die 



axis. c.c. The cotyledons. <l. The leaves /Vy. 1<!2. Oermmntiou ..t 



I^Iuizc, u Monocotyledonous Plant, t. The oxid giving oft roots from its 

 lower extremity, c. The cotyledon, g. The kavcB and young stalk. 



or one or more branches from it, growing in a downward direc- 

 tion, while the up])er i)art elongates a]iwards, carrying the 

 ]tluniulc with it, while at the same time the cotyledonary 

 jxjrtion becomes developed and forms the first leafy organs. 

 We have thus jn-oduced a central axis developing in two op- 

 posite directions, the lower part is called the descending a.xis or 



