PLANT STRUCTURE: PARTS (ORGANS) Roots Leaves Stems
ROOTS
El Hiquieron. Strangulating Plant Ficusjimenezii The trees you see growing on the wall are the Higueron. The Higueronsare plants that can grow in many different places: the ground, on rocks, walls, or on other trees. The fruits of this plant are favored by many diverse species of birds. The excrement from the birds contribute to the dispersion of the seeds. When the seeds fall, and good conditions for germination exist; if they fall on a tree, the roots grow until they reach the ground, wrapping around it little by little until it strangles it.
Root functions 1. uptake of water and mineral nutrients 2. anchorage of shoot 3. storage 4. communication with shoot; hormone synthesis
Design problems: 1. soil resources (water, nutrients) are dispersed throughout soil volume 2. resources must be transported to the shoot 3. sugar must be transported to growing root 4. uptake system must be functional throughout plant development Solution: A growing systems of pipes connected to absorptive surfaces A branching system is the most efficient way to fill space (volume)
Two basic branching structures 1. Taproot gymnosperms and dicots, primary root grows downward, lateral roots grow out from tap root. 2. fibrous root system monocots (eg grasses) primary root short lived, multiple adventitious roots arise from stem base. 30.13
Both types of roots can go very deep and produce large absorptive surfaces -a mesquite (desert shrub) had roots > 50m deep -a rye grass plant (4 months) in 6 liters of soil had root surface of 640 sq m (400 sq m were root hairs) (130 times shoot) Fig. 44.--Top view of surface roots of Comanche cactus (Opuntia camanchica). Scale in square feet Fig. 43.--Wire grass (Aristida purpurea) from the short-grass plains From J.E. Weaver 1926. Root Development of Field Crops
Soil often resists root growth -root tips small -able to follow least resistance -root growth occurs at the tip (apical meristem) -root cap continually sloughed off and replaced protects meristem and lubricates root passage cap cells slimy hydrated polysaccharide 35.13/35.12
3 zones of root development 1. division at tip 2. elongation behind tip cell growth 3. maturation tissue differentiation and root hair development 35.13/35.12
Mature Tissues: Epidermis, Cortex, Vascular cylinder 35.14/35.13
35.3 Epidermis absorbs water and nutrients aided by root hairs (trichomes) increased surface secrete root exudate promotes activity of soil microbes near root trade carbohydrate for nutrients helps create rhizosphere Initiate contact with root symbionts in soil Mycorrhizae(fungi) N-fixers (bacteria) 37.10 37.3/37.6
Cortex water & nutrients move through and between these cells many air spaces some starch storage 35.14/35.13
(Cortex) Endodermis compact inner layer Casparian strips suberin bands in walls forces water movement through membrane osmotic barrier plant inputs from soil highly controlled 35.14/35.13
36.12/36.9
Vascular Cylinder 1. Pericycle layer of parenchyma cells around conducting tissues origin of lateral roots (new meristems) 2. Xylem water and nutrients up to shoots 3. Phloem carbohydrates down from shoots 35.15./35.14
Root Modifications Storage Roots (and also stems) often used to store carbohydrates usually in parenchyma cells of vascular cylinder e.g. carrots, sweet potatoes, beets, radishes cultivars selected for food, from ancestors that store for their own benefit may be important in human evolution
Functions of storage 1. Sustain plant during stress 2. Recover from damage (herbivory) 3. Allow rapid burst of growth or reproduction spring growth (competition) rosette species (reproduction) skip life stages (longleaf pine)
Other specialized roots Aerial roots support prop roots, buttresses aerial uptake
Aerial uptake
Pneumatophores air roots (Mangroves) supply oxygen to roots in water saturated soils and sediments
35.4
LEAVES
Leaves Roots site of water and nutrient uptake Leaves site of CO2 uptake and light absorption Together, this is the complete recipe of essential resources 36.2/37.2
Leaf Design Problem leaf resource supply 1. Light is directional (from above) 2. CO2 is a gas, obtained by diffusion (H2O lost) 3. Leaf requires water and nutrients (from root) 4. Leaf products must be transported out Solution leaf structure chloroplasts arrayed in a flat plane surface impermeable to water stomates to control gas exchange (CO2, H2O) near chloroplasts, often underneath system of vasculature throughout Is flat design to maximize light or CO 2 uptake?
Angiosperms leaves Dicots blade photosynthetic surface petiole stalk (sometimes reduced) Variation in size and structure- 1. Simple leaves 1 blade, 1 petiole 2. Compound leaves blade divided into leaflets a. pinnatelycompound leaflets along axis b. palmatelycompound leaflets from petiole tip 35.6
Monocots (grasses, palms) Blade Sheath (leaf base around stem) Meristem at base of blade creates parallel structure enhances recovery from grazing damage Also much variation in size and structure Function of size variation? Larger continuous blades have thicker boundary layer more resistance to diffusion higher temperatures
Leaf internal structure tissues epidermis, mesophyll, vasculature 35.18/35.17
Epidermis compact array of epidermal cells surrounds leaf covered with cuticle to reduce H2O loss
stomates (guard cells) concentrated on lower surface except floating leaves, submersed leaves scattered in dicots, in monocots in parallel rows
(Epidermis) arid habitat modifications thicker epidermis and cuticle why? often lower stomatal density why?
Mesophyll ground tissue of leaf parenchyma contains chloroplasts air spaces for gas exchange 1. Palisade parenchyma elongated cells below epidermis high chloroplast concentration 2. Spongy parenchyma -large air spaces, enhance diffusion
Leaf Vasculature Veins vascular bundles throughout mesophyll both xylem and phloem Major veins conspicuous ribs with support tissue mainly for rapid transport Minor veins small veins in contact with mesophyll important for exchange supply water and collect photosynthate(sugars) surrounded by bundle sheath cells exchange layer
Dicots netted venation central midrib branches into smaller veins Monocots parallel venation large major veins connected by smaller veins
Leaf Modifications 1. Spines (not thorns ) protection 2. Tendrils support 3. Water storage 4. Plantlets reproduction (asexual) 5. Insect traps food, reproduction (sexual) 35.7/35.6
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