How do nutrients made in the leaves reach the entire plant? How is water absorbed by the roots carried to the leaves? Let us study the plant’s transport system.
Basic Knowledge
Inside stems are bundles of tubes called vascular bundles (vascular bundle), which consist of two types of tubes:
・Xylem (xylem): Carries water and inorganic nutrients from the roots → leaves. On the inner side of the vascular bundle.
・Phloem (phloem): Carries photosynthetic products (sugars) from the leaves → throughout the plant. On the outer side of the vascular bundle.
In dicot stems, vascular bundles are arranged in a ring, and cambium is located between the xylem and phloem (allowing the stem to grow thicker).
In monocot stems, vascular bundles are scattered, and there is no cambium.
Roots absorb water and inorganic nutrients (such as nitrogen, phosphorus, and potassium) from the soil. Numerous thin projections called root hairs (root hair) are found near the tips of roots. They increase the surface area and improve absorption efficiency. Roots do not have chloroplasts (they do not carry out photosynthesis), and nutrients are sent from the leaves through the phloem.
Vascular bundle (vascular bundle)（A pathway formed by xylem and phloem bundled together）
Xylem (xylem)（Carries water and inorganic nutrients from roots to leaves. Inner side of a vascular bundle）
Phloem (phloem)（Carries photosynthetic products from leaves throughout the plant. Outer side of a vascular bundle）
Cambium (cambium)（Between xylem and phloem. Makes stems thicker through cell division. Only in dicots）
Root hair (root hair)（A thin projection on the root surface. Increases surface area）
Inorganic nutrients（Nitrogen, phosphorus, potassium, and others. Called the three major nutrients）
Further Study (Background and Significance)
Xylem consists of connected dead cells, forming hollow tubes. This allows water to move smoothly from roots to leaves without obstacles. Phloem, on the other hand, consists of living cells and actively transports sugar throughout the plant.
The cambium, found only in dicots, divides once a year and makes the stem thicker, forming annual rings. Counting annual rings reveals a tree’s age, and their width can be used to estimate the climate at that time (dendrochronology).
The plant’s three major nutrients are nitrogen (N), phosphorus (P), and potassium (K). Nitrogen is important for leaves, phosphorus for flowers and fruit, and potassium for root growth. These are also components of chemical fertilizers.
The total length of the root hairs of a single corn plant can sometimes reach several hundred km, showing how efficiently plants absorb water and nutrients. If roots are damaged when a plant is transplanted into soil, the root hairs break, absorption capacity drops sharply, and the leaves may wilt.
- Xylem = water and inorganic nutrients (roots → leaves), inner side of the vascular bundle
- Phloem = organic nutrients (leaves → entire plant), outer side of the vascular bundle
- Dicots = ring-shaped vascular bundles + cambium
- Monocots = scattered vascular bundles + no cambium
- Cambium makes stems thicker and forms annual rings
- Root hairs improve absorption efficiency
- Three major plant nutrients: N, P, and K
Points to Note (Easily Confused)
① Do not reverse xylem (water, inner side) and phloem (nutrients, outer side). Remember: “water = xylem.” ② Only dicots have cambium. Monocots (rice) do not grow thicker. ③ Roots absorb water and inorganic nutrients. Organic nutrients are sent from the leaves through the phloem. Roots do not carry out photosynthesis. ④ A root hair is a projection of a root epidermal cell, not an independent organ.
Practice
- What is the tube that transports sugar made in the leaves called?
- Which have cambium, dicots or monocots?
- Briefly explain the significance of root hairs.