What is a stem of a plant? 

Except for the root, leaf, flower, and fruit, the part that remains is called the stem

How is the stem formed of a plant? 

See, the plant is formed from one or the other seed, and on top of this seed there are two coverings which are called seed cover or seed coat. 

What happens inside this seed, is an embryo is found and in this embryo,  

there are two structures of cells, one of which is called Plumule and the other is called Radicle

See, when the seed is put in the soil, after getting the mineral elements and water present in the soil, 

the plumule and radicle present in the embryo start developing, out of which the plumule later forms the stem and the radicle forms the root. 

Now you know that the stem is formed from the plumule present in the embryo.