Chapter 23: Plant Tissues & Systems Section 1

Specialized Plant Cells Remember plant cells have unique structures Cell wall Central vacuole 3 types of specialized plant cells Parenchyma Collenchyma Sclerenchyma

Paranchyma “puh-REN-kuh-muh” Usually loosely packed cube- shaped or elongated cells Contain large central vacuole Have thin, flexible cell walls Involved in many metabolic functions: photosynthesis, storage of water and nutrients, healing Usually form main part of nonwoody plants Ex. Fleshy part of apple

Collenchyma “koh-LEN-kuh-muh” Cells walls thicker than parenchyma Cell walls irregular in shape Thicker walls provide more support for plant Usually grouped in strands Specialized for supporting areas of plant that are still lengthening Ex. Celery stalks – lots of collenchyma

Sclerenchyma “skluh-REN-kuh-muh” Thick, even, stiff cell walls Support and strengthen plant in areas where growth is finished Usually dies at maturity Rough texture of pear is from presence of sclerenchyma cells

Tissue Systems Cells that work together to perform specific function make tissue In plants, arranged into systems Dermal system Ground system Vascular system Systems further organized into 3 major plant organs – roots, stems, leaves

Dermal Tissue System Forms outside covering of plants In young plants, made of epidermis “ep-uh- DURH-muhs” – the outer layer made of parenchyma cells In some species, epidermis more than 1 cell thick Outer epidermal wall often covered by waxy layer called the cuticle  prevents water loss

Some epidermal cells of roots develop hairlike extensions that increase water absorption Openings in leaf and stem epidermis are stomata  help regulate the passage of gases and moisture in and out of plant In woody stems and roots, epidermis replaced by dead cork cells

Ground Tissue System Dermal tissue surrounds the ground tissue system Has all 3 types of cells Functions in storage, metabolism, support Paranchyma most common cell Nonwoody roots, stems, leaves made mostly of ground tissue

Cactus stems have large amounts of parenchyma cells for storing water Plants growing in very wet soil have parenchyma with large air spaces to allow air to reach roots Nonwoody plants that need to be flexible to withstand wind have large amount of collenchyma cells Sclerenchyma found where hardness is advantage, i.e. seed coats, cacti spines

Vascular Tissue Vascular tissue supports the plant body and transports water and nutrients throughout the plant.   The two kinds of vascular tissue are xylem, a water-conducting tissue, and phloem, a tissue that carries dissolved food. Both xylem and phloem consist of long, slender cells that connect almost like sections of pipe, as shown in the figure.

All seed plants have xylem cells called tracheids. Xylem: Tracheids All seed plants have xylem cells called tracheids.   As they mature, tracheids die, leaving only their cell walls. These cell walls contain lignin, a complex molecule that gives wood much of its strength.

Xylem: Tracheids Openings in the walls connect neighboring cells and allow water to flow from cell to cell.   Thinner regions of the wall, known as pits, allow water to diffuse from tracheids into surrounding ground tissue.

Xylem: Vessel Elements Angiosperms have a second form of xylem tissue known as vessel elements, which are wider than tracheids and are arranged end to end on top of one another like a stack of tin cans.   After they mature and die, cell walls at both ends are left with slit-like openings through which water can move freely.

Xylem: Sieve Tube Elements Unlike xylem cells, phloem cells are alive at maturity. The main phloem cells are sieve tube elements, which are arranged end to end, forming sieve tubes. The end walls have many small holes through which nutrients move from cell to cell.  

Xylem: Sieve Tube Elements As sieve tube elements mature, they lose their nuclei and most other organelles. The remaining organelles hug the inside of the cell wall and are kept alive by companion cells.

Phloem: Companion Cells The cells that surround sieve tube elements are called companion cells.   Companion cells keep their nuclei and other organelles through their lifetime.

Growth in Meristems Plant growth starts in meristems “MER-i-stemz”  regions where cells continuously divide Apical “AP-i-kuhl” meristems  plant grows in length Located at tips of stems and roots

Some monocots have intercalary “in-TUHR-kah-ler- ee” meristems located above bases of leaves and stems Allow grass leaves to quickly regrow after being cut

Gymnosperms and most dicots also have lateral meristems  allow stems and roots to increase in diameter Located near outside of stems and roots 2 types Vascular cambium Cork cambium

Vascular cambium  produces additional vascular tissues Located between xylem and phloem

Cork cambium  produces cork Located outside phloem Cork cells replace epidermis in woody stems and roots Protects plant Cork  dead cells that provide protection and prevent water loss

Primary growth  increase in length Made by apical and intercalary meristems Secondary growth  increase in diameter Made by lateral meristems By vascular cambium and cork cambium