Unit 3: Plant Morphology

Basic Plant Body Structure

Organ systems - shoot & root Shoot system - aerial stems & leaves (vegetative) and sometimes cones or flowers (reproductive parts) Stems - transport water and nutrients and provide support Apical bud or shoot apical meristem (SAM) is where new shoot system cells are produced, allowing plant to grow upwards Stems are divided into nodes & internodes Nodes are where leaves attach to a stem & where axillary buds develop Axillary buds are typically dormant & controlled by terminal bud but also have SAMs and contain potential to develop into a branch Internodes are stem areas that elongate to spread the leaves apart and optimize exposure to sunlight and air circulation Leaves - photosynthesis & gas exchange Cells are rich in light-harvesting chlorophyll pigment, predominantly green Composed of a flat, thin blade responsible for capturing sunlight and are webbed with veins that act as internal plumbing Covered in waxy cuticle to prevent drying Have pores called stomata on surface responsible for capturing CO2 and releasing oxygen Leaf blades often attached to main stem via thin, sturdy petiole responsible for holding blade upright   Shoot variations Shape & structure of leaves vary from species of plants depending on their adaptation to climate & available light Leaves represent heavy investment on the part of the plants bearing them Leaf arrangement - shoot systems have wide variation in phyllotaxy or leaf arrangement on the stem Alternate - one leaf at each node, alternating sides Opposite - two leaves at a node, on opposite sides Whorled - three or more leaves arranged around same node Basal - leaves occur in tight rosette at base of plant Leaf form Two forms of broad leaves based on the way the leaf blade (also called lamina, widest portion of the leaf) is divided: simple and compound Simple - undivided blades Compound - blades that are fully divided into leaflets, seperated along main or secondary vein Leaflets may have petiolules and stipels Important to recognize where petiole occurs (where leaf attaches to a stem node) to identify compound leaves Compound leaves can be pinnately (leaflets on both sides of the petiole) or palmately (radiating from a single point of the petiole) compound Third form differs from broad leaf types - needles Very low light absorptive surface area Well-suited to drier climates by water conservation Last three to four years, while broad leaves only live for growing season Evergreens have advantage over broad-leafed deciduous plants since cost of leaf synthesis can be recovered via phosynthesis over several growing seasons Leaf venation Veins provide transportation of water and nutrients between leaf and stem and play crucial role in maintenance of leaf water status and photosynthetic capacity Also play role in mechanical support of the leaf Angiosperms possess two main types of leaf venation - parallel and reticulate Parallel (all veins run the same direction), monocots are generally parallel venated  Reticulate (veins form a net-like webbing throughout the leaf) dicots are generally reticulate venated Pinnate - single midrib and secondary veins branching off at intervals Palmate - primary veins radiate from a single point near the base Leaf margins Edges of leaf blades come in many forms - entire, lobed, toothed Entire - smooth, complete edge Lobed - indentation of the edge towards the midrib Create a smaller leaf since deep division makes it easier for wind to carry away heat and water vapor Toothed - saw-like edge with small teeth Leaf shape  Flat shape maximizes contact with surrounding air, promoting cooling Long leaves help with temperature regulation - good for hot conditions Thin leaves help prevent water loss and are good for dry conditions Large, broad leaves can harvest lots of light and are good for gloomy conditions Long term changes are adaptations Short term changes are plasticity Leaves that receive lots of sunlight tend to be smaller to reduce light-absorbing area and tend to have more complex edges and lobes to disperse heat rapidly Leaves in more shaded environments are usually larger to increase light-absorbing surface area and tend to have reduced expressions of lobes and edges Leaf surfaces - glabrous to pubescent Glabrous - smooth; help encourage shedding water in wet environments to prevent interference with light capture or gas exchange Pubescent - hairy; help insulate the leaf in cold or heat, prevent excessive water loss in arid conditions and provide UV protection under high light   Root system - organs of the plant found below-ground Can modify environment through growth and chemical interactions with organisms, unlike shoots Anchor the plant body in place Responsible for absorption of water and minerals Storage space for extra food and nutrients Each root tip contains equivalent to apical bud, root apical meristems (RAM) where new root cells are produced allowing them to grow downwards Roots only absorb water at their tips through root hairs, rest of the root develops a thick, protective outer layer and transports water form tips to shoots Root variations Categories - coarse and fine Coarse roots provide stability, storage, transport, exhibit branching and sometimes secondary (thickening) growth Fine roots are most distal part of root system, responsible for water and nutrient uptake due to their highly permeable membranes Systems - tap and fibrous Tap roots - single, stout primary root growing from stem base with thinner lateral roots growing from it Best for anchorage, reaching deep sources of water, storage of extra food Fibrous roots - primary root is replaced by a mass of equally sized roots developing from main stem base Water and mineral absorption and plant stabilization Shallow netting of roots prevents soil erosion Special roots develop from shoot system called adventitious roots Above or underground Increase plant stability, or allow plant to reproduce asexually Architecture   in well-drained uplands, roots grow deep to reach down to the level of rainwater & snowmelt in waterlogged lowlands, roots stay shallow In between, high growth rates and drought can send roots many meteres down to the saturated zone just above the groundwater table    

Body Modifications

Plant organs have become secondarily modified via natural selection to deal with abiotic & biotic challenges Abiotic factors are non-living parts of environment Water, sunlight, wind, temperature, inorganic nutrients, soil Biotic - living things in ecosystem Other plants, animals, fungi, bacteria

Root modifications Four categories: food storage, aeration, support, absorption Lack nodes & won't have leaves or buds Aerial or underground   Storage Root tubers are fleshy adventitious roots or tap roots with increased storage capacity Store food and water for plant to help them persist through bad conditions High amount of food molecules (starch) within cell of roots act as antifreeze and protects form cold damage Aeration Pneumatophores are air-breathing roots that grow vertically upwards from submerged roots Typically in plants inhabiting swampy areas Internal air spaces that help get oxygen to the connected, submerged portions Support Buttress roots are huge, wedge-shaped portions of roots that give support in soft, wet soil Often found in tropical forests Prop roots serve the same supportive purpose and are adventitious roots that arise some distance up the stem growing downward to ground Floating roots are spongy adventitious roots with large air pockets that help keep a plant buoyant in aquatic environment Climbing roots are adventitious roots that emerge from the stem in contact with a supportive surface Often found in vines Absorption Epiphytic roots are hanging aerial roots of epiphytes covered by a thin greyish layer of tissue known as velamen that can absorb moisture from air Parasitic roots are found in parasitic plants that use them to penetrate tissue of their host plant and absorb food and moisture Nodules are small structures forming along roots that house nitrogen-fixing bacteria Bacteria provide nitrogen directly to roots in exchange for house  

Stem modifications Aerial or underground Four categories: storage, protection, support, reproduction Have nodes and internodes Have buds and sometimes leaves and adventitious roots Storage Stem tubers are swollen, underground stems with small depressions called eyes where a node with one or more buds occurs Leaf scars seen around eyes, adventitious roots are usually absent Rhizome is a non-green, horizontal stem growing at or below soil surface Thick, woody stem that stores large quantities of food and water Distinct nodes and internodes Dry, scale-like leaves near axillary buds Adventitious roots on lower side Corms are condensed forms of rhizomes growing in vertical direction Rounded with flat base Distinct circular nodes & internodes At or above ground Adventitious roots that arise form their base or all over body Bulbs are short, disc like stems with thick scale leaves surrounding terminal bud on top side and a cluster of adventitious shoots arising from bottom Swollen with stored water and food Sometimes covered with dry sheath of leaf tissue Cladodes are thickened green stems swollen with water-storing tissue Allows plants in arid climates to have supply of water during drought Succulent stems can be rotund or flattened and leaf-life Main photosynthesizing organ Nodes are visible on their body and leaves are often modified into spines or scaled down to reduce water loss & protect plant form herbivores seeking moisture Common in desert plants Protection Thorns are modified short branches grown from axillary buds Terminate in sharp hard point Often still bear leaves & have visible buds Protect plant from animals trying to eat leaves or fruits Prickles are not a full stem modification but come from the epidermal tissue of the internode Arranged in irregular patterns and are often recurved to hinder progress of small animals or insects trying to climb up to leaves   Support  Twiners are stems of vines and are modified to be thin and flexible Spiral their entire bodies around upright objects to elevate their leaves Tendrils are short branches that are modified into thread (spiral)-like, leafless structures that are meant for climbing Sometimes branch but will always have small scale-like leaf at the point of branching Reproduction Runners and stolons are creeping stems with long internodes that run horizontally above soil surface Each node bears axillary buds, leaves, adventitious roots Nodes can break off and grow into individual plants, aid in asexual reproduction Bulbils are modified vegetative flowers or buds with stored food that help a plant reproduce asexually As bulbil develops into mini plant, they detach from parent and take root

Leaf Modifications Aerial Five categories: storage, protection, support, reproduction, nutrient acquisition Absence of nodes, buds, and adventitious roots   Storage Succulent leaves are common in desert plants Thick leaves provide ample water storage during drought Protection Spines are modified leaves or parts of leaves that have become woody, sharp, non-photosynthetic Often paired with cladode Protect plant from herbivores  Small surface area reduces water loss Leaf hairs are projections of epidermis and can vary in shape and how thickly they cover leaf surface Protect leaves from excess light Offer protection against herbivores Reduce water loss Support Leaf tendrils look similar to stem tendrils and serve the same function Appear next to distinct nodes and won't have any small scale like leaves next to them Floating leaves provide buoyancy for aquatic plants via large air filled cavities occuring in leaf or in petiole Reproduction Bracts are brightly colored leaves occurring next to small, nondescript flowers that play the role of petals in attracting pollinators Reproductive leaves have ability to form tiny plant clones at edges of their leaves from adventitious buds similar to bulbils As they develop into mini plant, they detach from the parent and take root Nutrient acquisition Carnivorous leaves are modified to form a trap that catches insects or small animals and then digests them using special enzymes Occurs in plants growing in nitrogen-poor environments such as bogs and marshes Plant still phosynthesizes but relies on prey to provide nitrogen Cupped leaves are rosettes of smooth textured leaves that collect water in their center Found in some epiphytes who rely on the water  Refuges for small animals Waste & rotting vegetation that collects in them supply mineral nutrients to the plant