Transport of Water in Stem

Following plants have a system of vessels for transport of water in the stem. These vessels are called xylem.

Xylem:

Xylem tissue consists of visual elements and tracheids. Vessel elements from long tubes while tracheid are long cells with overlapping ends.

Xylem Vessel:

A Xylem vessel is a long, hollow, tubular structure from root to leaf; it is made up of many dead cells arranged vertically. The walls of these vessels become strong by the deposition of lignin.

Water and Mineral Transportation:

When a leaf transpires (loses water), the water concentration of its mesophyll cells drops. This drop causes water to move by osmosis from the xylem of leaf into mesophyll cells.When one water molecule move up in the xylem of the leaf it creates a pulling force that continues all the way to root, this pulling force created by the transpiration of water is called transpiration pull.
Following are the reasons for the creation of transpiration pull.

Water is held in a tube (xylem) that has a small diameter.
Water molecules adhere to the walls of the xylem of xylem tube (adhesion).
Water molecules cohere to each other (cohesion).

These attractions make an overall tension among water molecules. This tension forms columns of water. The columns of water move from root to shoot and the water content of the soil enters in these columns.

Transportation of Food in Plants:

Phloem:

The tissue that transports nutrients like amino acids and sugar from leaves to various parts of plants is called phloem. Phloem is a complex, food conducting tissue in vascular plants.In dicots they are located outside the xylem.

Structure:

Phloem is composed of four elements:
Sieve tubes are elongated living cells with performed end walls, which do not possess nucleus and are main conducting cells.
Companion cells which are closely associated with sieve tubes and control the sieve tubes.
Phloem parenchyma which is mainly concerned with storage of organic matter, tannins etc.
Phloem fibres are elongated cells with lignified walls.

Functions:

They transport food and other metabolic products from leaves to various parts of the plant.This transport is called translocation of food.