PLANT DIVERSITY II. THE EVOLUTION OF SEED PLANTS
OVERVIEW OF SEED PLANT EVOLUTION The most important reproductive adaptations of seed plants are: 1. Continu Continued ed reduct reduction ion of the the gameto gametophy phyte te 2. Evol Evolut utio ion n of the the see seed. d. 3. Evolu Evolutio tion n of of the the polle pollen. n. The reduced gametophyte can develop from spores retained within the sporangia of the parental sporophyte. •
Retaining the female gametophyte within the sporangium protects it from environmental stresses. The gametophyte otains food from the sporophyte.
!ll seed plants plants have have two different different "inds "inds of spores. spores. They They are heterosporous. heterosporous . 1. Megasporangu! produces Megasporangu! produces !egaspores# !egaspores # which produces produces "e!a#e ga!etoph$tes. ga!etoph$tes . 2. M%rosporangu! produces M%rosporangu! produces !%rospores# !%rospores # which produces !a#e ga!etoph$tes. ga!etoph$tes . $n seed plants# the megaspores and the female gametophyte are retained on the parent sporophyte. %ayers of sporophyte tissue envelop the megasporangium. These tissues are called ntegu!ents. ntegu!ents. The megasporangium# megaspore and integuments are called the o&u#e. o&u#e. The female gametophyte develops inside the megaspore and produces one or more egg cells.
Megasporangu! & Megasporangu! & !egaspores ' "e!a#e ga!etoph$tes ' egg %e##. M%rosporangu! & M%rosporangu! & !%rospores & !%rospores & !a#e ga!etoph$tes (po##en) & (po##en) & sper!s. 'ollen does not need a li(uid for fertili)ation. Po##naton is Po##naton is the transfer of pollen to ovules. *ind and animals carry out pollination. 'ollen grains are well protected y sporopo##enn. sporopo##enn. +poropollenin contains only caron# hydrogen and o,ygen- it is an isoprene polymer. The seed is a sporophyte emryo pac"aged along with a food supply within a protective coat. +eeds can e very resistant and can e dispersed widely y animals and wind.
1. ulticellular emryo
2. /ood supplied y tissue
/ood only in the cell
3. ulticellular seed coat
Covering not cellular
. iploid sporophyte
. 'roduct of fertili)ation
'roduct of meiosis
There are two groups of seed producing plants# g$!nosper!s and g$!nosper!s and angosper!s. angosper!s . •
'roduce seeds. 4ascular tissue: *$#e! for *$#e! for water and mineral transport and ph#oe! for ph#oe! for dissolved sugars.
5ametophyte is much reduced and totally dependent on the sporophyte.
eterosporous: microspores and megaspores.
+YMNOSPERMS There are aout 620 species found in all terrestrial haitats grouped into four divisions 7phyla8. They have great economic importance: lumer# paper# and chemicals. 9y the late evonian period 736 million years ago8 some plants had egun to ac(uire some adaptations that characteri)e seed plants. The first fossils of seed earing plants appeared in the Caroniferous# some 3;0 million years ago. The 'ermian was warmer and drier and the gymnosperm egan to diversify ecause they were etter adapted than %ycopods to this type of climate. 5ymnosperms dominated the landscape during the eso)oic era# which egan aout 20 million years ago. The eso)oic ended ; million years ago and the Ceno)oic egan. ! great e,tinction too" place at the end of the eso)oic# which included the elimination of the dinosaurs. The Ceno)oic is dominated y flowering plants ut gymnosperms remain an important component of the flora.
,HARA,TERISTI,S OF +YMNOSPERMS 1. *ood *oody y tree trees s and and shru shrus. s. 2. +eeds +eeds are orne# orne# e,pos e,posed# ed# in cones cones 7megas 7megastro troilus ilus8. 8. 3. 'ollinat 'ollinated ed y y wind# wind# seldom seldom y y insect insects. s. . +ingl +ingle e fertili fertili)at )atio ion: n: sperm sperm < egg egg
. ost ar are !onoe%ous: !onoe%ous : male and female organs on the same individual.
There are four phyla studied in this te,too": •
5in"gophyta# Cycadophyta# 5netophyta# Coniferophyta
See the Life Cycle of a pine on page 597
AN+IOSPERMS There are aout 23#000 species of flowering plants. This is the dominant group in terrestrial haitats. =ur survival depends on them. •
/ood# medicine# lumer# etc.
There are two groups of angiosperms: !ono%ots and !ono%ots and -%ots. -%ots. /irst appeared in the Cretaceous aout 130 million years ago. I. ,HARA,TERISTI,S 1. *ood *oody y or her hera ace ceou ous. s. 2. 'rod 'roduc uce e flow flower ers. s. 3. 'olli 'ollina nated ted y wind wind or or anima animals. ls. . oul oule e fertili fertili)a )atio tion: n: egg egg < sperm sperm
emryo and 2 polar nuclei < sperm
. +eed +eeds s enclo enclosed sed in a fruit. fruit.
II. THE FLOWER The flower is a reproductive shoot or ranch.
$t has four parts arranged in whorls or circles on a stal" or peduncle. The parts of the flower are the sepa#s 7caly,8# sepa#s 7caly,8# peta#s 7corolla8# peta#s 7corolla8# sta!ens and sta!ens and %arpe#s. %arpe#s . • •
+tamens consist of a filament and an anther. Carpels are also referred to as pst#s. pst#s. They consist of an ovary# a style and a stigma.
/lowers may e orne singly or in clusters called n"#ores%en%e. n"#ores%en%e . /lower parts are considered modified leaves. III. DOULE FERTILI/ATION. FERTILI/ATION . $t is characteristic of flowering plants. oule fertili)ation results in the formation of a -p#o- 0$gote and 0$gote and a trp#o- en-osper!. en-osper! . The female ga!etoph$te or e!1r$o sa% has sa% has an egg nucleus and two polar nuclei. =ne sperm fertili)es the egg nucleus and forms the )ygote# 2n. !nother !nother sperm >oins the two polar polar nuclei nuclei forming the the triploid 73n8 73n8 nutritive tissue called called the endosperm. IV. FRUIT ,LASSIFI,AT ,LASSIFI, ATION ION.. ! fruit is a mature mature ovary. ovary. A%%essor$ "ruts include "ruts include other parts in addition to the ovaries# e.g. strawerry# apple# and pear. S!p#e "ruts develop "ruts develop from a single carpel or several united carpels. Aggregate "ruts are "ruts are produced from separate carpels in one gynoecium# e.g. lac"erries# magnolia. •
The individual parts of the aggregate fruit are "nown as fruitlets.
Mu#tp#e "ruts consist "ruts consist of several female structures and accessory flower parts that ecome fused into one fruit# e.g. pineapple# =sage orange. See the angiosperm life cycle on page 600. V. EVOLUTION OF AN+IOSPERMS !ngiosperms !ngiosperms evolved evolved some 1 1 million years years ago during during the early Cretaceous Cretaceous 7eso)oic8 7eso)oic8 or possily in the late ?urassic. !ngiosperm !ngiosperm egan to dominate dominate the landscape landscape at the end end of the eso)oic and and they form now the dominant plant form.
There are different opinions aout the origin of the angiosperms and their flower. [email protected]
=C=T+ have floral parts in multiples of three and the seed contains one cotyledon. The endosperm provides the food for the emryo. 4enation is usually parallel 7there are e,ceptions8. Their vascular undles are scattered throughout the ground tissue. The root system is firous. $C=T+ have floral parts in multiples of four or five# and their seeds contain two cotyledons. The cotyledons usually asor the food from the endosperm first# and then provide the food for the emryo. 4enation is netted. The vascular undles in the stem crossAsection are arranged in circles 7rings8. They usually have a taproot system for at leas part of their life. Ths s not a !onoph$#et% group. Current research supports the hypothesis that monocots are monophyletic and form a single clade. The dicots# however# appear to e a polyphyletic grouping. ost of the dicots elong to the eu-%ots. Three lineages of the remaining dicots form what is called the 1asa# angosper!s ecause angosper!s ecause they include what are considered to e the oldest lineages. !nother !nother group is called called the !agno#-s. !agno#-s .
VI. ,OEVOLUTION OF FLOWERIN+ PLANTS AND ANIMALS. ,oe&o#uton is ,oe&o#uton is the process y which two or more species act as selective forces on one another and each undergoes evolutionary change. The earliest seedAearing plants were pollinated. 'ollen ecame a source of food for insects and those flowers that could etter attract insects were visited more often thus increasing the chances of passing those characters to the ne,t generation. !ny mutation mutation that made those those visits more more fre(uent fre(uent offered offered a selective selective advantage. advantage. 9ise,ual flowers have the advantage that visiting insects can pic" up pollen from the anthers and deliver pollen from a neighoring plant at the same time.
$n the early part of the Tertiary# 0 A ;0 million years ago# speciali)ed groups of flowerAvisiting insects ecame more diverse. VII. Hu!an 2e#"are ost of our food comes from angiosperms. +i, crops produce B0 of the calories consumed y humans: A *heat# rice# mai)e# potatoes# cassava and sweet potatoes. =ur food crops are derived from wild ancestors that have een domesticated very much li"e we have domesticated animals. A The domestic dog is derived from the wild wolf. any of our medicines are derived from plants or ased on plant natural products. aitat destruction is the iggest threat to plant e,tinction.