The different cell types reflect different functions of the cells. The upper epidermis and lower epidermis secrete a layer of wax called the cuticle that prevents water loss. The lower epidermis has pores in it that are surrounded by two guard cells. Together the guard cells and pores are called stomata and functions as an inlet for carbon dioxide needed in photosynthesis and outlet for the escape of water vapor and oxygen. The photosynthetic cells are arranged in two ways in dicots. The upper photosynthetic tissue is arranged like stakes of a fence and is appropriately called the palisade mesophyll. The lower photosynthetic cells are loosely arranged with numerous intercellular air spaces between the cells; hence these cells are called the spongy mesophyll. In this cross section there is also a vein that delivers water to the mesophyll cells and removes food to other parts of the plant. The structure of the leaf is ideal for photosynthesis. However, the structure also fosters the loss of water vapor from the plant by a process called transpiration. To see how transpiration functions as the driving force that pulls water through the plant, observe the following animation.

Transpiration

In summary, evaporation of water from the mesophyll cells establishes a high concentration of water vapor in the intercellular air spaces. The low concentration of water vapor in the air means that the water vapor diffuses from the intercellular air spaces into the air. Evaporation continues pulling water ultimately from the xylem of the veins within the leaf.

Movement in the xylem is different from that of the cells of the leaf. Xylem cells are dead cells. They have thick cell walls on the sides but lack end walls. Each cell is like a piece of pipe. Xylem cells are aligned end-on-end to form a pipe or tube. There are multiple xylem "tubes" forming a continuous pathway for water movement from the root to the leaves. Let’s look at a cross sectional cut in the stem to see the arrangement of xylem within.

Dicot Stem Tissues