TISSUES AND THE PRIMARY GROWTH OF STEMS

Plant Cell Types:

• Parenchyma - thin-walled, metabolically active; the site of synthesis for many plant secondary metabolites, which are often the active ingredient in herbal remedies and drugs (ex. alkaloids and glycosides)

• Collenchyma - flexible support

• Sclerenchyma - thick, secondary walls, support fibers from stems; can be used to make surgical structures

Stem Organization:

External

• Nodes are where leaves and buds attack

• Internodes are the stem segments between nodes

Internal

• Epidermis is the outer protective layer

• Cortex is a region of ground tissue beneath epidermis

• Vascular Bundles contain xylem and phloem for transport

Primary Growth

• Increase the length of a plant, occurring at the shoots and root from Apical Meristems

Modified Stems:

• Stolons (Runners)

• Rhizomes (Underground Stems)

• Tubers

• Bulbs (Onions)

• Tendrils (Squash, Bitter Gourd)

• Spines

Secondary Growth of Stem:

Cambia

• Vascular Cambium - lateral meristem that produces secondary xylem and secondary phloem

• Cork Cambium - lateral meristem that produces the periderm which is the protective outer layer of the back

Wood

• Annual Rings - represents one year of growth

• Heart Wood - older, non-functional xylem at the center of the trunk

• Sapwood - younger, functional xylem that transports water and minerals

Roots:

Internal Structure:

• Epidermis - the outermost layer, often with root hairs that greatly increase the surface area for absorption

• Cortex - large region inside the epidermis for storage

• Vascular Cylinder - central core containing xylem and phloem for transport

Specialized Roots:

• Storage Roots - for storing carbohydrates

• Aerial Roots - for clinging to surfaces or absorbing moisture from air

• Pneumatophores - “breathing roots"; grow upward from waterlogged soil to obtain oxygen

• Haustoria - found in parasitic plants to pent rate host tissue and absorb nutrients