1 8 SEEDS : STRUCTURE AND GERMINATION 



9. The term hyfogean is applied to cotyledons which remain 

 below ground, those coming above being epigean, the relative 

 amount of growth in the hypocotyl and epicotyl determining 

 their position. If the hypocotyl grows vigorously during or 

 after germination the cotyledons are forced above ground ; when 

 only the epicotyl grows the plumule is lifted up above the soil, 

 but the cotyledons remain below where the seed is placed. 



In the broad bean the hypocotyl is very short, and the point 

 where it ends and the root begins is not clearly defined. In a 

 mustard seedling, however, the point of separation between the 

 root and stem is somewhat swollen and readily distinguished 



(Fig. 5)- 



10. All plants whose embryos, like those of the bean and 

 mustard, possess two cotyledons, are known as Dicotyledons ; 

 they form a very large, well-marked class of the flowering or 

 seed-bearing plants. 



T I. The seeds hitherto mentioned contain within their coats 

 nothing but an embryo plant, which depends for the develop- 

 ment of its root and shoot upon the substances stored up in 

 some part of its body, its cotyledons chiefly. This is true even 

 in the case of seeds like those of white mustard, in which the 

 cotyledons of the embryo are comparatively thin. There are, 

 however, a number of plants, such as the ash, mangel and 

 potato, which, although belonging to the Dicotyledons, have 

 seeds in which there are stores of food inside the seed-coat, 

 but free from the embryo and its cotyledons (Fig. 109). Such 

 separate reserve-food, whatever its chemical composition may be, 

 is known as endosperm or 'albumen,' and seeds in which it is 

 present are called endospermous or albuminous seeds. Those 

 like the bean, pea, and vetch, mustard, and turnip, which 

 have no separate reserve-food, are known as exendospermous or 

 exalbuminous seeds. 



Ex. 11.— Take out a seed from the fruit of the ash tree in autumn ; carefully 

 cut thin shavings from the flat side of the seed, starting about the middle of 



