by Miller & Levine
[complete Table of Contents]
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Roots, Stems, and Leaves
23-1: Specialized Tissues in Plants
The three principal
organs in seed plants are roots, stems, and leaves.
Plants consist of three tissue systems: dermal tissue, vascular tissue,
and ground tissue.
Meristematic tissue is the only plant
tissue that produces new cells by mitosis.
Vascular tissue contains several different
cell types. Xylem consists of tracheids and vessel elements, and phloem
consists of sieve tube elements and companion cells.
The two main types of roots are taproots,
found mainly in dicots, and fibrous roots, found mainly in monocots.
A mature root has an outside layer of
epidermal cells and a central cylinder of vascular tissue separated by
a large area of ground tissue called the cortex.
Roots anchor a plant in the ground and
absorb water and dissolved nutrients from the soil.
Stems have three important functions:
they produce leaves, branches, and flowers; they hold leaves up in the
sunlight; and they transport various substances between roots and leaves.
In monocots, vascular bundles are scattered
throughout the stem. In dicots and most gymnosperms, vascular bundles
are arranged in a cylinder.
In all seed plants, primary growth of
stems is produced by cell divisions in the apical meristem.
In conifers and dicots, secondary growth
takes place in lateral meristematic tissues called the vascular cambium
and cork cambium.
The structure of a leaf is optimized
for absorbing light and carrying out photosynthesis.
Plants keep their stomata open just enough
to allow photosynthesis to take place, but not so much that they lose
an excessive amount of water.
23-5: Transport in Plants
Root pressure, capillary action, and
transpiration work together to move water through the xylem tissue of
even the largest plant.
When nutrients are pumped into or removed
from the phloem system, the change in concentration causes a movement
of water in that same direction. As a result, phloem is able to move nutrients
in either direction to meet the nutritional needs of the plant.