Water uptake and movement to the xylem · ⁨水的吸收与向木质部的移动⁩

Water from soil to xylem

• Water gets in at the root hair cells 根毛细胞.
• It then crosses the root to reach the xylem.
• A clever waterproof barrier lets the plant control what gets through.

Into the root hair

• A root hair cell has a lower water potential than the soil water around it.
• So water enters by osmosis 渗透, down the water-potential gradient.
• Root hairs give a huge surface area for this uptake.

Two pathways across the root

• Apoplast 质外体 pathway: water moves through the cell walls and spaces between cells — without entering the cytoplasm. This is fast.
• Symplast 共质体 pathway: water moves through the cytoplasm, passing cell to cell through the plasmodesmata 胞间连丝.

The apoplast (through walls) and symplast (through cytoplasm) routes; the Casparian strip 凯氏带 blocks the apoplast.

The endodermis 内皮层 and Casparian strip

• At a ring of cells called the endodermis, the apoplast route is blocked by the Casparian strip — a waterproof band of suberin 木栓质.
• This forces all the water through the cell membranes, so the plant can control what enters the xylem.

The wood of a tree trunk is xylem; each ring is one year of growth.

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