Forest Biology Exam 2

phloem/inner bark

living tissue that transports the sap from photosynthesis passively

created by the vascular cambium

gradient moves from high to low sugar concentration

xylem

transports water up the tree

phloem anatomy

phloem cells are alive and have thin walls

active part has specialized connective cells called sieves

in angiosperms, sieve tubes have companion cells that provide support to cell

in gymnosperms, there are no sieve plates, and no companion cells Gymnosperms have tracheid-like sieve cells that overlap.

why is the phloem a thin layer?

the phloem is a thin layer because not much is made and its typically only active for a year

bark

outermost layer of a woody stem, includes all living/nonliving tissues external to the vascular cambium

what is the purpose of bark

Protection from:

• Bacteria/fungi

• Insects

• Animals

• Some barks contain chemicals

that reduce attack:

• Cinnamon

• Tannins

• Juglone

what is outer bark composed of

mainly old inner bark, phloem, cork is added to waterproof protection

how does outer bark develop

In a young stem, the epidermis acts as the

protective, outer coating.

• The epidermis is later replaced by a corky

layer called the periderm.

• A new meristem (cork cambium) arises on

the outer layer of the phloem.

• Dead phloem cells get “pushed” outward and

incorporated into the bark

how does bark respond to growth

Pholem starts growing before xylem

• Xylem growth puts strain on the phloem

(somewhat absorbed by the expansion of ray

cells)

• Phloem dies, becomes “suberized” by the

cork cambium, and then becomes the

innermost layer of the dead portion of bark

• Outermost layers of bark split or shed.

gas exchange/lenticels

Cork makes the bark mostly

water- and gas-proof.

• But the living cells of the

cambium need oxygen!

• Solution: Lenticels!

why does bark vary so much?

Differences in cork

cambiums

• Periderm characteristics

• Continuous periderm =

smooth or peeling bark

• Growth allocation?

strobeli

gymnosperm cones

angiosperms

produce fruits (seeds

enclosed in an ovary) that arise from

flowers.

gymnosperm

pollen produced and seeds housed in structures called strobil

meristems responsible for bark growth

vascular cambium, cork cambium.