Pathway Of Water From The Soil To The Leaf

Water flows from the soil into the plant and out through the leaves in a continuous process that is controlled by the water demand of the environment surrounding the tree canopy (Figure W1.1). Water is absorbed through the root hairs, nonwoody roots, and to some extent by the woody root system. Water moves into the root through spaces in the cell wall and pores between cells, called the cortex, until it reaches the endodermis. The endodermis contains a suberized layer of cells,

Endodermis

FIGURE W1.1. Movement of water from the soil to the leaf (Source: Modified from Jones, 1992.)

Endodermis

FIGURE W1.1. Movement of water from the soil to the leaf (Source: Modified from Jones, 1992.)

the Casparian strip, that blocks water movement unless the water moves through the cell membrane into the cell. Once water is in the cell, it can move through the Casparian strip through plasmodesmata into the stele, and in the stele, water moves back across a cell membrane into the xylem. The xylem is composed of vessel elements and tracheids that are functional only when their cells have matured and are dead. After the xylem cells are produced, the protoplasts are absorbed by adjacent cells. Before a cell dies, it builds a secondary cell wall that adds strength and prevents it from collapsing when tension develops in the xylem. Tracheids are longer and more narrow than vessel elements. Both cell types have pits in the sides or ends that allow water to flow from one cell to the next. In this manner, a continuous column of water is supported within the xylem from the stele, up the trunk of the tree, out the branches, and to the leaves.

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