Biol. 360 Botany Lab

Week 3

Roots, Stems, and Leaves 

A.     Roots

At the tip of a root, the root apical meristem, the site of growth, is surrounded by the root cap.  The root cap is composed of several layers of cells that protect the meristem as the root grows through the soil.  Up the root from the meristem is the zone of elongation.  Cells in this zone are actively growing.  It is this growth that forces the root through the soil.  In the zone of maturation, growth has stopped.  It is in this zone that root hairs are found.  Root hairs are not hairs, but cytoplasmic extensions of the epidermis that specialize in water/mineral uptake.  Further up the root, the cells are older and the root eventually becomes suberized (covered with a layer of wax called suberin) and  is unable to take up water.  Uptake occurs primarily through root hairs in the zone of maturation.

The interior structure of the root contains tissue for storage as well as vascular tissue.  The epidermis is the outer layer and the interior is the cortex.  The cortex is often specialized for starch storage.  The vascular tissue is surrounded by the endodermis.  The endodermis and the Casparian strip, a waxy material that binds the endodermal tissue together, act as a filter, preventing unwanted material from moving into the plant.  Interior to the endodermis is the pericycle.  The pericycle is the tissue layer from which lateral roots arise.  At the center, the vascular tissue can be arranged in numerous patters.  In dicots, the center (stele) is small; in monocots, it occupies the majority of the root volume. 

Root systems have two general morphologies, tap and fibrous.  Taproots have a main, deep primary root while fibrous roots are shallow in the soil.  The primary and secondary roots in fibrous root systems are often difficult to tell apart.

Examine the prepared slides of roots.  Make a good sketch (from what you see) of a root cross section and longitudinal section for monocots and dicots.  Label everything you want to learn!  But make sure the picture you draw is big enough to allow for enough detail. 

B.     Stems

In most plants, as roots leave the soil, differentiation into stem tissue occurs.  The outer most tissue layer in stems is also the epidermis.  Within a vascular bundle, phloem is on the exterior side while xylem is in the interior.  In monocots, vascular bundles are scattered throughout the stem.

Gymnosperms and dicots that live for more than one year exhibit secondary growth. Secondary growth (growth in stem girth) is necessary to support primary growth (growth in stem length) and occurs at the vascular cambium.  This tissue layer develops between the vascular bundles and produces secondary phloem to the outside.  Secondary xylem is what is commonly referred to as wood and contains the familiar annual growth rings. Think about how this arrangement of tissues helps provide support for the growing tree.

Primary and secondary xylems contain two different xylem elements.  Gymnosperms (conifers and others) only contain tracheids while angiosperms (flowering plants) contain tracheids and vessels.  Vessels are capable of transporting more water than tracheids because they are bigger and contain perforated walls with less resistance to water flow.  Tracheids are tapered at their ends and contain only pits for water movement. 

Examine the prepared slides of monocot and dicot stems, including those slides that show secondary growth.  Again, make a good sketch of a cross section and longitudinal section for monocots and dicots.  Make sure the picture you draw is big enough to allow for enough detail, especially the elements of secondary growth. 

C.     Leaves

Leaves are the most diverse organs among plant species.  Leaves can be found in nearly any size and shape.  In most cases, the varied morphologies of leaves are due to responses to environmental conditions.  For example, small leaves provide less surface area for heating and losing water.  Large leaves can provide more photosynthate to the plant body than small leaves.

      The internal anatomy of leaves is less varied among species and is related to being monocot, dicot, C₃ or C₄.  In dicots, secondary veins arise from the primary vein of vascular tissue along the midrib and branch out to supply the rest of the leaf.  In monocots, the veins are parallel to the long axis of the leaf.  In C₃ plants, three tissue layers are present.  The epidermis covers the leaf.  The palisade layer beneath the epidermis is composed of columnar cells rich in chloroplasts.  Below this layer is the spongy mesophyll.  This layer contains numerous air spaces that allow gas exchange in the interior of the leaf.  In C₄ leaves, epidermis is outside and spongy mesophyll inside, but the chloroplasts are concentrated in bundle sheath cells.  These cells surround the vascular bundles.  This is known as "Kranz" anatomy.  We'll get into the physiology of this anatomy in Lab 5. 

      Last pictures!  Draw a cross-section of the three different kinds of leaves: mesophytic, xerophytic, and hydrophytic. 

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