FAMILIAR SEEDS, STRUCTURE AND GERMINATION 7 
cotyledon are called MonocoTyLEpDons, those whose 
seeds contain two cotyledons are called DicoTYLEDONS. 
We shall see that along with this difference go many 
others, and hence the distinction between these two 
kinds of seeds is of fundamental importance. 
The single cotyledon is a shield-like organ and is 
therefore called the ScurELLUuM. When the seed 
germinates, the cotyledon remains below the ground 
absorbing the food material from the endosperm. 
Hence the cotyledon is hypogeal. Between the scu- 
tellum and the endosperm is a layer of cells called 
the EPITHELIAL layer which secretes a ferment which 
changes the starch into sugar during the germination. 
The plumule and radicle are both covered with sheaths. 
The radicle only grows for a short time, the main root 
system being developed from the base of the stem by 
lateral branches. Hence the roots are clustered to- 
gether in a bundle at the base of the stem and form a 
fibrous root. These roots are called ADVENTITIOUS roots, 
since they are not produced in the normal order hy 
tranching of the tap root. 
5. The Mustard Seed.—This is a much smaller seed 
(fig. 4) than the two we have already described. The 
testa is yellow and mucilaginous, the cotyledons are 
thin and leaf-like and are folded on themselves one 
inside the other and enclose the radicle. 
During germination these cotyledons come out of the 
seed-coat, unfold themselves, turn green, and act as the 
first foliage leaves. Hence the cotyledons are epigeal. 
The seed is exalbuminous. 
6. The Sunflower Seed.—The so-called “ seed”’ is, as 
in the case of the mealie, really a fruit, the hard black 
