Class Note for HORT 101 with Professor Bosela at IPFW
Class Note for HORT 101 with Professor Bosela at IPFW
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Date Created: 02/06/15
PLANT DIVERSITY AND ADAPTATIONS FOELLINGER FREIMANN BOTANICAL CONSERVATORY Introduction The focus of this lab is on plant diversity with an emphasis on morphological variability The concepts of plant life forms and life histories will be introduced as a heuristic aid to facilitate your comprehension and interpretation of the wide range of plant types in the Conservatory collection The lab will include an evaluation of adaptations that are characteristic of the ora of tropical rainforests and deserts Horticultural plants of tropical or desert origin will be emphasized I Plant Life Forms Life forms ecomorphs are morphological adaptations towards the environment or one or more stress factors The oldest and most popular plant life form classification system is that of Raunkier 1907 1934 who recognized five life forms based on the position of the overwintering buds relative to the ground level In most subsequent life form classifications the life form classes are defined based on multiple criteria Box 1981 organized plants into life forms based on the siX characteristics structural type tree shrub herb etc plant size leaf type relative leaf size leaf structure and photosynthetic habit evergreen deciduous etc with nearly eighty life forms being recognized in total Raunkier s Life Form Classes Phanerophytes Trees and shrubs 7 Regenerative buds more than 50 cm above ground level Chamaeophytes Small shrubs 7 Regenerative buds within 1050 cm of the ground level Hemicryptophytes Perennial herbs 7 Regenerative buds located at or near the ground level Cryptophytes Perennial herbs 7 Regenerative buds located below ground on specialized regenerative organs rhizomes bulbs etc Raunkier recognized two subtypes of cryptophytes based on whether the plants were terrestrial geophytes or aquatic hydrophytes Therophytes Annuals 7 No regenerative buds regrowth from seeds with the plants completing their lifecycle with a year In our climate the distinction is typically made between summer annuals which germinate in the Spring or Summer and set seed by the Fall and winter annuals which germinate in the Fall and overwinter before owering in the Spring Generalized Plant Life Form Classes 1 Woody plants characterized by the production of perennial stems and roots that grow in diameter via the annual production of new increments of secondary xylem wood and secondary phloem bark a Tree 7 a woody perennial with a single stem trunk with growth to 30 ft or higher b Shrub 7 a woody perennial 220 ft tall with multiple stems from its base A subshrub is a trailing or horizontally spreading shrub c Liana 7 a woody vine see below 2 Herbaceous plants 7 woody aboveground stems are not produced Variable in longevity annual biennial or perennial For perennial herbs new shoots develop each year from basal buds the root system or modi ed subterranean stems bulbs corms rhizomes etc a Graminoids 7 grasses and grasslike plants including sedges and rushes monocots b Forbs 7 broadleafed herbaceous plant dicots c Succulents 7 plants typically herbaceous with stem leaves or roots that are specialized for water storage and swollen in appearance when hydrated c Vines 7plants that are selfrooted but not selfsupporting Vines may grow horizontally along the ground but most have specialized mechanisms for climbing over other plants Vines are mechanical parasites A woody vine is termed a liana d Aquatics herbaceous plants that are adapted for growth in water Aquatics may be free oating submergent or emergent e Epiphytes 7 plants that grow upon other plants rather than in the soil 11 Life History Life histories are patterns of resource allocation that have evolved via natural selection Thought to be manifestations of a plant s evolutionary strategy Some of the most important traits evaluated in life history analyses are listed below 1 Phenology 7 the timing of growth and reproduction resource allocation as evaluated temporally on a per year basis For example plants may ower in the Spring Summer or Fall all year termed ever blooming In terms of their leaf phenology native plants in Indiana may be evergreen e g pines summergreen bearing leaves in the Spring and Summer alone and Wintergreen 2 Type of Reproduction 7 many plants are able to reproduce both sexually and asexually In asexual reproduction syn vegetative reproduction new plants are produced independent of meiotic reduction gamete production and sexual recombination Root sprouting and branch layering are common examples of vegetative reproduction However asexual reproduction via seeds is also possible 3 Mating System 7 plants may reproduce sexually via selfcrossing selffertilization or out crossing For some plants both types of crossing are possible but with one type being preferred ie facultative selfcrossing vs facultative outcrossing For other plants only one type of crossing is possible In the case of obligate selfcrossers selfcrossing is ensured via the production of owers that fail to open termed cleistogamous owers The type of mating system affects the ratio of pollen to ovules seeds produced in the owers as indicated in Table 2 below Mating system Sample size n Mean pollen to ovule ratio Obligate selfcrossing cleistogamous owers 6 47 i 07 Obligate selfcrossing 7 277 i 31 Facultative selfcrossing 20 1655 i 221 Facultative outcrossing 38 7966 i 877 Obligate outcrossing 25 58592 i 9365 4 Pollination Syndrome 7 plants may be wind water animal or insect pollinated with distinct oral adaptations being shown for each pollination syndrome For example moth pollinated owers tend to be large and fragrant with ower opening occurring at night and white to grey in color In contrast bird pollinated owers are generally brightly colored red orange but generally only weakly fragrant 5 SeedFruit Dispersal Mechanism 7 seeds and fruits may be dispersed by wind water animal or insect Each type of dispersal mechanism has selected for the evolution of different sets of fruit traits 6 Seed Ecology 7 seeds may be initially dormant and if so they may vary widely in their germination requirements light cold treatment water soaking etc In addition seeds vary widely longevity Life History Classi cation R C and S selection Grime 1977 Grime recognized three basic types of selective habitat l temporary habitats with abundant resources which would be expected to select for rapidly colonizing ruderal species termed Rselection 2 stable habitats with abundant resources that favor species with superior competitive abilities termed Cselection and 3 resource deficient habitats that would favor species that persist under severe stress termed Sselection with the associated species allocating most of their resources to reproduction growth or maintenance respectively Table 1 Table l Traits correlated with competitive stress tolerance and ruderal plant species Trait type Competitive Stresstolerator Ruderal Life form Variable Variable Herbs Leaf form Variable Leathery needles Variable Shoot morphology Dense canopy Variable Small stature Leaf longevity Deciduous Evergreen Deciduous Plant longevity Long or short Very long Very short Flowering Annual Intermittent Annual Reproductive maturity Late Late Early Reproductive allocation Small Small Large Perennation Buds seeds Leaves roots Seeds Growth rate Rapid Slow Rapid Seed longevity Short Long Long Stress response Rapid Slow Reproduces Palatability herbivores Variable Low Often high III Horticultural Importance of Life Form and Life History Analyses A plant s life form and life history traits affect is horticultural utility particularly in ornamental horticulture For example plants that are bird or mothpollinated are generally be more desirable than plants that are windpollinated as a result of their larger owers Similarly plants that are perennial compact and everblooming offer distinct advantages as ornamental horticulture crops A competence for vegetative propagation is also important in ornamental horticulture since vegetative propagation facilitates the use and multiplication of sterile mutants and genetic mutants which are generally not seedstable For horticultural crops that are grown for consumption foods species a basic knowledge of life history traits particularly those related to sexual reproduction eg mating system pollination syndrome etc is crucial for the development of successful breeding programs and also simply for the optimization of fruit yield IV Tropical Rain Forest The tropical rainforest biome represents a nearly ideal environment for plant growth There are no cold or dry seasons and light and water are both abundant with 200400 cm of rainfall per year The temperatures are generally warm and the degree of temperature variation is minimal on both a daily and seasonal basis Under these conditions growth and reproduction are possible on a yearround basis making tropical rainforests the most productive and also most diverse plant communities in the world However rainforest soils are generally less nutrientrich than the soils of more temperate biomes as a consequence of their greater age lack of glaciation and the more rapid weathering In addition light and nutrient stresses are generated secondarily as a result of the vigorous plant growth Trees which are competitively superior to herbs as a result of their large size account for up to 95 of the plant species in a tropical rainforest a larger percentage than for any other biome Most trees are evergreen and broadleaved and compared with temperate forests the level of tree diversity is much greater with up to 40 to 50 species per hectare Tropical rainforests also exhibit a high degree of vertical strati cation with multiple tree layers typically being recognized including a canopy layer and several subcanopy layers In contrast vertical strati cation of the tree layer is absent or dif cult to detect in temperate forests The ratio of leaf area to ground area summed across all vegetation layers and termed the leaf area index LAI is signi cantly greater for tropical rainforests 10 12 than for temperate forests 56 In fact in a mature a tropical rainforest less than 1 of the sunlight may penetrate to the lower canopy Herbaceous plants which are not able to directly compete with the trees for canopy positions have developed two contrasting strategies to deal with the low light levels of the rainforest understory Some herbs have evolved an ability to grow and reproduce under such low light conditions Aside from tending to have large thin leaves these shade tolerant rainforest herbs exhibit several novel leaf traits including high frequencies of variegation blue iridescence velvety upper surfaces and red pigmented undersurfaces that have been hypothesized to contribute to their light harvesting ability Others plants epiphytes and lianas have developed a shade avoidance strategy which is based on their functioning as mechanical parasites Since the stems of vines are not selfsupporting they contain both a higher proportion of water conducting cells and largerdiametered water conducting cells than are typical of related tree of shrubs and thus are able to conduct water more ef ciently and to grow at faster rates In contrast with vines epiphytes lack a connection with the soil Accordingly most epiphytes contain few roots or highly modi ed roots Most epiphytic orchids for example produce specialized roots termed velamen roots with a multilayered epidermis that absorbs and temporarily stores rainwater The velamen roots of orchids are also typically green interiorly and presumably photosynthetic Epiphytic bromeliads produce few roots however their leaves merge at the base so as to form tanks that can hold up to 45 liters of water which is subsequently absorbed across the leaf surface Horticultural Relevance A large number of important nut and fruit crops are of tropical rainforest origin including guava Macademia nuts pomegranates oranges Kumquat pineapples a bromeliad bananas and mangos Similarly several of the most common plant spices and fragrances are derived from plants that evolved in the tropical rainforest biome including pepper nutmeg vanilla cinnamon and camphor among others In addition many poplar house plants are derived from the rainforest understory These plants are wellsuited for indoor growth as a result of their adaptation to low light environments and more or less uniform temperatures In addition the foliage of many of these herbs is large and relatively showy Genera that are especially popular for interior growth include Dracaena Calathea Codiaeum Ficus H elicom39a Acanthus Philodendron Tradescantia and Peppermom39a among others Most orchids and bromeliads that are of value as horticultural commodities for their oral displays are also of tropical origin V Deserts Deserts are characterized by low rainfall less than 25 cm per year and the rain is frequently con ned to short periods ie large downpours etc Other stresses include large diurnal temperature variations as a consequence of the low humidity and the limited plant cover little heat trapping The soils of deserts are generally rocky or sandy with limited organic material and high pH levels In addition desert soils are frequently saline as a consequence of water evaporation and salt precipitation from the soil surface With the exception of desert annuals and some deeprooted trees and shrubs which are localized to river edges and outwash basins desert plants are generally slow growing and longlived Sselection In addition in contrast with tropical rain forests the interactions between desert plants are frequently facilitative more so than competitive with established plants moderating the microenvironment via shading and water collection with rainwater being funneling to the plant base so as to facilitate additional plant germination and growth at the site One of the most conspicuous morphological adaptations of desert plants is that of tissue succulency Two primary types of succulents are typically recognized based on whether the water is stored in the leaves or stems with stem succulents being further differentiated based on whether the entire stem is adapted for water storage or just the stem base In the former case the stems are generally lea ess and photosynthetic and in the latter case the stems are typically woody and non photosynthetic with the swollen stem base being termed a caudex The roots of succulents are generally widespreading but shallow to maximize water absorption following desert rains In addition most succulents exhibit a specialized form of photosynthesis termed CAM photosynthesis in which carbon dioxide is absorbed at night thus limiting the degree of water loss from the leaves or stems Adaptations to deter herbivory are also conspicuous among desert succulents which can tolerate little biomass loss as a result of their intrinsically slow growth rates Common modifications include the production of physical deterrents ie thorns spines prickles however some desert plants produce toxic secondary metabolites that presumably serve a similar function Desert succulents of the Lithops genus which resemble rocks as indicated by their common name living stones have been suggested to avoid herbivory by camou age Annuals though not routinely seen represent the predominant growth form in most deserts Most desert annuals do not exhibit any obvious morphological or physiological adaptations to the desert environment Their strategy is rather one of drought avoidance ie most produce large number of seeds that only germinate in response to periods of sustained rainfall Most deserts also include and shrub and tree component Aside from being deeprooted the leaves of desert trees and shrubs are generally small andor highly divided presumably to increase air circulation around the leaf thus limiting leaf heating and the rate of water evaporation The leaves of desert trees and shrubs are also frequently distinctly hairy pubescent with the hairs limiting water loss by re ecting light and creating a layer of still air around the leaf In addition some desert trees and shrubs produce vertically oriented leaves limiting light interception and heating and many desert trees and shrubs are able to shed their leaves in response to periods of extreme drought Horticultural Relevance With the exception of prickly pear desert plants are of limited importance as a source of nuts or fruits However in the Southwestern US desert perennials especially cactuses are becoming increasingly important as landscape plants as a part of the trend towards producing landscaped areas that require less irrigation termed xeriscaping In addition cactuses and other desert succulents are popular houseplants Although they generally require direct sun exposure as a result of their slow growth rates and water conservation mechanisms they are hardy and easy to grow Many desert plants are also of interest for exotic plant collectors as are result of their novel adaptations and growth forms Questions I Rainforest Room 1 List the scienti c and common names of at least three epiphytes 2 What is the scienti c name ofthe cocoa plant 3 What is the scienti c name and life form of Staghorn fern 4 Several species of bromeliads that are native to the tropical rainforests exhibit CAM photosynthesis an adaptation that is normally associated with desert environment Can you provide an explanation for the evolution of CAM photosynthesis in these plants based on their life form 5 The leaves of many tropical rain forest trees are drawn out at their apex so as to form structures termed drip tips that are thought to prevent the accumulation of lms or puddles of water on the leaves limiting the growth of fungi algae or lichens Provide the common name and scienti c name of one tree in tropical rainforest room with drip tips 11 Desert Room 1 What is the scienti c name for the creosote bush and what is the shape of its leaves 2 What is the scienti c name of the Saguaro cactus 3 What is unique about the bark of Palo Verde Cercidium microphyllum Is this trait an adaptation to the desert environment Please explain
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