Periderm and Stem Anatomy

rays and their functions

*radial files of cells in xylem* • move sugar laterally in stem • starch storage • move waste products laterally • defense *defense, lateral movement, storage*

xylem and its functions in stems

*wood* • transports water and elements from roots to growing leaves and stems (primarily upward) • provides structure and support to stem • stores starch in specialized cells (rays)

nested cones as growth increments *START STEMS*

As trees elongate and expand (grow) a new layer of xylem (a three dimensional elongated hollow cone) is build upon and over the immediate past layer of xylem. Over many years, growth increments produce a "nested" or "stacked" set of growth increment cones. A cross-section of these cone stacks in a stem or branch will show a series of growing season increments visible as rings. The thickness of each increment is dependent upon the amount of resources available and used for its construction at each specific location in a tree.

cross section

Between the phelloderm and vascular cambium, phloem ray cells expand or dilate to fill in the expanded space and form conical shaped areas

secondary growth and periderm development in twigs

The exterior of a twig is first covered with a primary tissue called an epidermis. This surface layer covers a primary cortex of simple cells. Once secondary growth begins (expansion in girth), the epidermis may grow to keep pace with stem circumference expansion, or a twig may form a periderm. Periderm is a secondary tissue set generated from a lateral meristem called a phellogen.

heartwood

a central core of internally shed dead xylem and rays (wood) which can have their cell volumes or walls filled with defensive or waste materials

phellem

a corky dead tissue surrounding stems and roots

phellogen

a lateral secondary meristem generating phellem to the outside and phelloderm to the inside

phelloderm

a tissue with many live cells providing support for phellogen function and serves as a portion of the secondary cortex

sapwood

an area of xylem (wood) containing living cells and dead resource transport tissues in both radial and longitudinal directions

rough periderm

can be... • warty (raised projections) • scaly (stiff plates curling and peeling away) • furrowed (either long ridges/indentations, flat rectangles, or square blocks)

both tracheids and vessels _____ when functional

dead

rhytidome

dead layers of old periderms to the outside of current periderm

lumen

empty center of dead xylem cell

epidermis

exterior of twig is first covered with this primary tissue which covers a primary cortex of simple cells

phellogen, phellem, and phelloderm =

periderm

twig and branchlet periderm is usually cited as...

smooth, papery, or rough

stem vs. root

stem = pith root = no pith

what makes annual rings visible?

transition from late wood one season to early wood the next season...this transition, within one growth increment, defines xylem porosity forms

phloem

vascular tissue that serves as food transport system

late wood/summerwood

xylem cells = thick walls and small lumens

secondary cortex

• A secondary cortex under periderm is an area of interlaced cells—inactive phloem and phelloderm. • These cells of the secondary cortex are derived from two different lateral meristems through formation and reformation of new phellogens.

secondary growth and periderm development in trees

• In new tree tissues, the outside is covered with an epidermis cell layer overlaying a primary cortex of simple cells. Once secondary growth (expansion in circumference or girth) begins, the epidermis may keep pace with tree growth. • In most trees species, periderm is generated beneath, and eventually replaces, the epidermis and primary cortex of young tree shoot tips.

why does this kill trees?

• Phloem loss stops sugar transport • Cambium damaged -- no new xylem (especially bad in ring-porous trees)

what is girdling?

• Removing circumference strip of tissue around stem into xylem • Periderm & phloem removed • Vascular cambium damaged (weak separation point)

when the periderm undergoes tension from being pulled apart by tree circumference growth...

• These physical stresses initiate sclerification and wall thickening within inactive phloem. • As girth continues to increase, micro-tears constantly occur between cells and as tears and faults continue, new phellogens are initiated

vascular cambium

• a mostly continuous zone of meristematic cells around the perimeter of stems and branches • generates secondary xylem (inward, toward pith) and secondary phloem (outward)

phloem and its functions in stems

• active (thin) and inactive (wide) • moves sugar from leaves for use or storage (primarily downward) • functional < one season

vessels

• angiosperms have vessels AND tracheids • large lumens and open on each end • form continuous tubes • carry more water volume at a faster rate

non-porous

• any type of boundary • gymnosperms: pines, spruces, firs

tracheids

• conifers only have tracheids • they are closed at each end • long fibers and tracheid cells from pines are good for strong paper • conifers can grow taller than hardwoods b/c of narrow tracheids which carry water higher up

ring porous

• early and late appear greatly different; sharp boundary • angiosperms: oak, ash, elm, chestnuts

diffuse porous

• early and late wood appear the same; no discernible boundary • angiosperms: maple, sycamore, sweetgum, yellow poplar, hickory • large vessels occur throughout annual increment (both early and late wood) • multiple increments functional

cell types in xylem

• fibers = wood strength • tracheids and vessels = water conducting

early wood/springwood

• first xylem developed in a growth period • xylem cells = thin walls and big lumens

smooth periderm

• no raised or sunken areas • appearance color and texture either even, striated (with thin lines), or mottled (color patches)

cells of phloem

• sieve elements--alive when functional • companion cells

periderm

• the most external of tree tissues providing protection, water conservation, insulation, and environmental sensing • protective tissue generated over live conducting and non-conducting cells of the food transport system--the phloem • generated by a secondary meristem called phellogen (a.k.a. cork cambium, "cork generator") • becomes thicker with increasing tree diameter and thinner with increasing tree height

papery periderm

• thin sheets peeling away in small flakes • same or different colors revealed beneath

secondary meristems

• vascular cambium-- (1)xylem (2) phloem • phellogen/cork cambium-- (1) cork/phellem = low density suberized tissue on exterior (2) phelloderm

vascular tissues

• xylem and phloem • moves things long distances