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