vascular plants - secondary growth (shoots)

secondary growth

The increase in diameter of the stem in regions that are no longer undergoing
elongation

what lateral meristems cause secondary growth

• cork cambium

• vascular cambium

what plants undergo secondary growth

• herbaceous plants undergo very little to no secondary growth

• trees and shrubs undergo lots of secondary growth (up to hundreds of years)

annuals 

Entire life cycle occurs in one growing season

biennials

• A type of annual plant that completes its life cycle over two growing seasons

• In the first season, form root system and a small rosette of leaves

• In the second season, the plant bolts, flowers, and produces seeds

perennials

• Plant vegetative structures live for multiple years

• Only flower when they become adult plants which varies

• Many undergo secondary growth

2 forms of meristematic cells/initials from the vascular cambium

1. fusiform initials

2. ray initials

fusiform initials

much longer than they are wide (vascular cambium initials)

ray initials

may be slightly elongated or square in structure (vascular cambium initials)

what do vascular cambium initials produce

Initials divide to produce secondary xylem and phloem

axial system

• formed through the division of the fusiform initials

• used to move water and nutrients up and down the plant

radial system

• formed from the ray initials

• Used to move nutrients and water back and forth between the xylem and phloem

• has some roles in storage

cambial zone

Collectively, the cambial initials and their immediate
derivatives are referred to as the cambial zone

is there more secondary xylem or phloem in woody plants

secondary xylem

periderm

• Replaces the ruptured epidermis as the protective covering

• follows the initiation of secondary vasculature

3 layers of the periderm

1. cork cambium (phellogen)

2. cork (phellem)

3. phelloderm

cork cambium

• aka phellogen

• meristematic

• produces periderm

cork

• aka phellem

• formed to the exterior and acts as a protective layer

phelloderm

resembles cortical parenchyma and formed interior

lenticels

facilitate gas exchange through the impermeable periderm

cork cell walls

• The inner walls of the cork cells are lined with either suberin or wax (often alternates) may also lignify

• Increases impermeability

where does periderm development occur

• Periderm development occurs just below the epidermis in the cortical layer

• in some species, may form deeper in the cortical tissue near the primary phloem

bark

refers to all tissue exterior to the vascular cambium

change to vascular cambium each growing season

• Each growing season, the vascular cambium adds secondary phloem

• Also adds secondary xylem interior of the bark

• Old secondary phloem generally crushed, pushed exterior by newly formed periderm and eventually shed

outer bark

• consists of all tissues outside the innermost cork cambium

• All periderms including cortex and phloem that is among them

• Separated from supply of nutrients and water, cells are dead at maturity

• Suberized cork cells cut them off

inner bark

• located interior to the cork cambium

• extends towards the vascular cambium

• living tissue

scale bark

forms due to periderm development that is discontinuous and in overlapping layers

ring bark

formed due to continuous rings of periderm formation around the axis

wood

• made up of secondary xylem

• one of the most important plant tissues to human survival throughout history

2 types of wood

1. hardwood

2. softwood

softwood

• conifers

• simpler than wood of angiosperms

• Lack vessels and have relatively small amount of axial parenchyma

• Mostly consist of long, tapering tracheids

• in some species like pine, the only parenchyma cells present are
  associated with resin ducts

hardwood

• angiosperms (eudicot and magnoliid)

• more varied than conifers

• lots of different cell type: vessel elements, tracheids, fibers, and parenchyma

growth rings

• Result from the periodic activation of the vascular cambium

• Most easily seen in the secondary xylem

• The width of growth rings are dependent on available resources

• Can be used to determine rainfall in a certain year

early wood

less dense and is produced early  in the growth season

late wood

have thicker cell walls and narrower cells

heartwood

• nonconduction wood

• no living cells

• infiltrated by substances such as oils, gums, resins, and tannins

sapwood

• still actively conducting

• has some living cells and reserve materials