56 ELEMENTARY BOTANY. 



the formation of an embryo, or minute plantlet. This gradu- 

 ally encroaches upon the tissue within the ovule, absorbing a 

 part or all of it for its own growth. In some species the 

 embryo at maturity is comparatively small ; in others it occu- 

 pies the entire volume of the mature seed, covered only by 

 the testa or seed-coat. 



16. The embryo consists of a slender stem, or caulicle, hav- 

 ing at one end a bud called the plumule, and at the other a 

 root-tip called the radicle ; it also has one or more seed- 

 leaves called cotyledons. The nourishment which is stored 

 up in the seed to nourish the plantlet during germination, may 

 be largely in the tissue of the seed that surrounds the embrj^o ; 

 or it may be wholly deposited in the cotyledons, which in that 

 case are much thickened or fleshy. The number of cotyledons 

 is constant for the two large groups of plants, as their names 

 indicate, Monocotyls and Dicotyls. As already pointed out, the 

 plants constituting the group called Monocotyls have generally 

 the following associated characters : — (1) One cotyledon, (2) 

 woody strands irregularly scattered through the pith, (3) par- 

 allel-veined leaves, and (4) usually three-parted flowers. The 

 Dicotyls have (1) two cotyledons, (2) woody strands or a 

 woody cylinder encircling the pith, (3) netted-veined leaves, 

 and (4) mostlj'^ five-parted flowers. But few exceptions to the 

 preceding statements occur in our flora. 



All the points enumerated in the last two paragraphs cannot be fully studied 

 without the aid of the compound microscope. If the teacher can exhibit 

 prepared microscopic specimens, the statements of the text can be more sat- 

 isfactorily understood and appreciated. 



