Plant Structure and Growth Chapter 35

n n Objectives F F List the differences between dicotyledons and monocotyledons F F Describe the basic structure of all plants F F Describe how the structure of plants is an adaption to the basic function of plants in terrestrial environments F F Name the six types of cells and three types of tissue systems found in plants and describe their distinguishing characteristics

F F Differentiate between primary and secondary growth, and describe where and how each kind of growth occurs

Introduction n n Angiosperms are largest group of plants u u make up 90% of world’s plant species u u two major groups of angiosperms F F monocotyledons include orchids, bamboos, palms, lillies and grasses distinguishing characteristics include – –single seed leaf; cotyledon – –leaves usually parallel-veined – –scattered vascular bundles in stems – –floral parts in multiples of three – –fibrous root system

F F dicotyledons includes most angiosperms including most shrubs and trees (except conifers) and many herbaceous plants distinguishing characteristics include – –two cotyledons – –net-veined leaves – –vascular bundles in ring in stems – –floral parts in multiples of four or five – –taproot system

Plant Structure n n Plant body consists of roots and shoots u u allows plants to function in terrestrial environment F F take up water and minerals from soil F F absorb light F F take in CO 2 from air F F create plant bodies from molecules assembled from these raw materials and products of photosynthesis

u u root system F F anchors plant F F absorbs and transports minerals, water and stores food F F ultimate site of absorption is root hair outgrowth of epidermal cells increases absorptive surface area

u u shoot system F F consists of supporting stems, photosynthetic leaves and reproductive structures F F composed of nodes-point where leaves, flowers and other stems attached internodes-stem between nodes leaves composed of photosynthetic blades and short stalks (petioles) that join blades to nodes

u u buds F F undeveloped shoots contain potential nodes, internodes and leaves two types – –terminal bud at plant apex; source of growth in height – –axillary bud in angle of petiole and stem; usually dormant but can produce new branches

u u apical dominance F F results from release of hormones from terminal buds inhibits growth of axillary buds removal of terminal bud stimulates development of axillary buds – –basis for pruning

n n Many plants have modified roots and shoots u u modified tap roots of some dicots used for food storage F F stored as starch examples-root crops u u stems can be modified for several purposes F F asexual reproduction runners-strawberries F F food storage rhizomes-irises tubers-potatoes

u u leaves can be modified F F food storage leaf bases of celery F F grasping and climbing tendrils of cucumbers F F protection spines of cactus

n n Plant cells and tissues diverse in structure and function u u cells have unique features F F photosynthetic and contain chloroplasts F F often have large central vacuole; maintain turgor F F bounded by cell wall composed of cellulose F F many cells have additional secondary wall hardened with lignin F F plasmodesmata provide continuous cytoplasmic link between adjacent cells

u u six types of plant cell; based on wall morphology and chemistry, shape and function F F parenchyma abundant and unspecialized; primary cell walls food storage, photosynthesis and aerobic respiration F F collenchyma similar to parenchyma but have thicker primary wall provide support for young growing parts of plant

F F sclerenchyma have rigid cell walls hardened with lignin provide support and protection – –seed coats – –gritty texture of pear

F F vessel elements and tracheids found in xylem elongated and secondary wall thickening; dead and open ended when functioning as water conducting cells connected end-to-end tracheids – –tapered ends – –covered with open pits vessel elements – –wider and shorter; completely open ends

F F sieve-tube members found in phloem relatively thin primary walls, no secondary wall; alive but lack nucleus and ribosomes when functioning contain numerous pits with plasmodesmata associated with at least one companion cell – –provides nucleus functions for sieve-tube member

n n Three tissues make up plant body u u epidermis F F composed of single, surrounding layer of cells F F first defense against infection and damage u u vascular tissue F F composed of xylem and phloem F F conducts water and nutrients through plant

u u ground tissue F F fills space between epidermis and vascular tissue F F composed mainly of parenchyma F F functions include photosynthesis, storage and support u u each system continuous from organ to organ

u u roots F F surrounded by epidermal cells with root hairs; no cuticle F F ground tissue (cortex) conducts material from root surface to central vascular tissue F F inner layer of cortex (endodermis) forms selective barrier; regulates flow into vascular tissue

u u stems F F epidermal cells covered by waxy layer-cuticle F F dicots-vascular tissue bundles in outer ring of ground tissue cortex surrounding parenchyma pith F F monocots-vascular tissue bundles scattered in uniform ground tissue

u u leaves F F also have cuticle F F lower epidermis includes pores (stomata) surrounded by guard cells-gas exchange F F ground tissue arranged in two mesophyll layers lower loose layer (spongy mesophyll) for gas exchange upper compact layer (pallisade mesophyll) for photosynthesis F F branches of vascular tissue enter leaf and provide transport to and from photosynthetic cells

Plant Growth n n Primary growth lengthens roots and shoots u u indeterminate growth-grow during entire life u u three seasonal growth patterns F F annuals-complete life cycle in one year F F biennials-complete life cycle in two years F F perennials-live and reproduce for many years

u u indeterminate growth results from presence of meristems F F unspecialized cells that continue to divide F F apical meristems at root and shoot tips and in axillary buds F F differentiation controlled by master control genes (homeotic genes)

u u apical meristem in root tip divides cells downwards F F root cap protects meristem; abraided by soil F F other cells grow upward, forming three rings of tissue-become epidermis, cortex and vascular cylinder above meristem cells elongate; force root tip down above this region, cells differentiate

u u apical meristem of shoot forms three downward-forming cylinders of embryonic tissue F F contains zones of elongation and differentiation F F some meristem cells remain in lateral position form meristem of axillary buds

n n Secondary growth increases girth of woody plants u u involves meristems that grow laterally in stems F F most evident in trees, shrubs and vines

u u vascular cambium-cylindrical meristem F F develops from parenchyma cells between xylem and phloem of shoots F F cells dividing inwards form new secondary xylem outside primary xylem F F cells dividing outwards add secondary phloem inside primary phloem u u secondary xylem cells larger during favorable growth periods and smaller at other times F F annual growth rings

u u new layers of phloem do not accumulate F F sloughed off in bark at same rate produced F F in secondary phloem, meristematic cells (cork cambium) produce cork cells dead when mature thick, waxy walls-protect stem surface

u u wood divided into two layers F F heartwood-nonfunctioning xylem, plugged with resin acts as endoskeleton; strong, rigid but flexible core F F sapwood-functioning secondary xylem F F wood rays are collections of parenchyma cells forming connections between heartwood and sapwood