(1) In the figure, there are ① seed coat, ② endosperm, ③ cotyledon, ④ embryo, ⑤ hypocotyl and ⑤ radicle. Corn seeds are composed of ① seed coat, embryo and ② endosperm, and the seed coat has protective function. The embryo includes embryo, hypocotyl, radicle and a cotyledon, and the endosperm is rich in nutrients.

(2) Cotyledon ③, embryo ④, hypocotyl ⑤ and radicle in the figure are collectively called embryos, which are developed from fertilized eggs.

(3) Corn seeds will germinate under suitable conditions. At the initial stage of germination, the radicle grows fastest. First, it breaks through the seed coat and grows to the ground, and then it develops into a root. Then, the hypocotyl elongates, and the embryo breaks through the seed coat and grows into a stem and a leaf. Through this process, the embryo of the seed develops into a seedling.

So the answer is: