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Meristems generate cells that differentiate or specialize quickly and become permanent tissue.
These cells adopt specific functions and lose their ability to divide further. They are classified into
three types: dermal, vascular, and ground tissue.
Dermal tissue
covers and protects the plant, and
vascular tissue
transports water, minerals, and sugars to different parts of the plant.
Ground tissue
serves as a site for photosynthesis, provides a supporting matrix for the vascular tissue, and helps to
store water and sugars.
Secondary tissues can be simple (made up of similar cell types) or complex (composed of
different cell types). Dermal tissue, for example, is a simple tissue that covers the plant's outer surface
and regulates gas exchange. Vascular tissue is a complex tissue
composed of two specialized
conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to
different parts of the plant and includes three different cell types: vessel elements and tracheids (both
of which conduct water), and xylem parenchyma. Phloem tissue is made
up of four different cell
types that transport organic compounds from the site of photosynthesis to other parts of the plant:
sieve cells (which conduct photosynthesis), companion cells, phloem parenchyma, and phloem fibers.
Phloem conducting cells, unlike xylem conducting cells, are alive at maturity. The xylem and phloem
always lie adjacent to each other (Figure 90). In stems, the xylem and the phloem form a structure
called a
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