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Figure 13.2 (Structure of Vascular Tissue)
As you can see, the vascular bundles in dicots are much larger and more consistently
arranged. Monocot species, on the other hand, spread the xylem and phloem of the vascular
tissue around throughout the stem. These two methods reflect the structure of the plants
themselves. Monocots tend to be plants like grasses, which have veins and leaves which run
in parallel. In dicots, such as many flowering trees and fruiting plants, the leaves and veins in
the leaves branch off in various patterns. This organization favors a vascular tissue which is
more organized, and can branch as the plant grows.
In woody dicots, the vascular tissue is even more organized, with a vascular cambium layer
producing xylem on the inside and phloem on the outside. These layers are produced
seasonally, which give woody plants their characteristic “rings”. By adding to the vascular
tissue every season, these plants can handle an increase in growth and become very large.
Some monocots such as palms have adopted a secondary growth technique while
maintaining a scattered arrangement of vascular tissue.
Functions of Vascular Tissue
Vascular tissue functions mainly in maintaining the water balance and sugar balance of a
plant. Not only does the plant’s cell need water to complete basic biological functions, they
also need the minerals and nutrients found in the soil to complete their work. Most plants
have small pores in the leaves called stoma , which allow water to evaporate and gases to
exchange. To get more water and nutrients into the cells of leaves, these small pores open.
As the water evaporates, the forces of adhesion and cohesion pull the water up the tubes of the
xylem. As water is absorbed through the roots, this also creates a pressure from the bottom to
force the water upward. The tubes of the xylem are narrow to support this action, but there are
many of them bundled together. The xylem portion of the vascular tissue can be seen below,
on the left.
As the water moves up and into the leaves, some of it is needed to dissolve the sugars created
by photosynthesis and carry them back down the plant. Remember that photosynthesis
creates glucose, which the plant will use as energy. The plant combines glucose molecules to
create sucrose, a temporary storage sugar. The root cells, and other cells in the stems and
leaves, do not create their own glucose and rely on the plant to provide them energy. The
phloem cells work to transport this created energy all throughout the plant from source cells ,