PinkMonkey Online Study Guide-Biology
CHAPTER 15 : CONQUEST OF LAND
Autotrophs are the vital base of food webs present on
Earth today. In the ocean, photosynthetic prokaryotes and bacteria
provide most of the energy. Today, photosynthetic plants support
life on land. Plants evolved from aquatic species not much different
from the primitive alge we see today.
Plants can be generally defined as multicellular photosynthetic
eukaryotes, although there are a few obviously related species which
rely instead on alternative modes of nutrition, such as Indian pipe
Bryophytes were the first organisms to invade land. They developed
certain adaptations which allowed them to cope with requirements
of a terrestrial environment. They retained some of the characteristics
of aquatic species, such as a soft body with the absence of root
and vascular systems. The gametophyte is the predominat phase in
the life cycle, and the sporophyte is totally dependent on the gametophate.
They still need a water environment for the opening of the sex organs
and fertilization to occur. Hence, bryophates are sometimes refferred
to as the "amphibians of the plant kingdome".
A terrestrial environment entails an entirely different set of environmental
conditions compared to an aquatic habitat. Water and nutrients are
available only through soil. Oxygen and carbon dioxide needed for
respiration and photosynthesis are available in gaseous form in
the atmosphere. Aerial structures are exposed directly to atmosphereic
conditions like air, light, temperature, wind, e.t.c. Unlike the
aquatic environment, the plant does not get natural support or buoyancy
from its environment. Thus, in order to survive on land, the terrestrial
plants had to develop stratagies, including some of the following
characteristics to address these environmental differences:
An extensive root system for efficient anchorage
Leaves as specialized organs for photosynthesis.
Specialized vascular tissues such as xylem
for conduction of water and minerals from roots to aerial parts, and
phloem for the conduction of organic solutes from leaves and other
green organs to non-photosynthetic portions of the plant.
Cuticle over aerial parts of the plant for protection
and to prevent surface evaporation.
Stomata to regulate gaseous exchange with the environment.
The increasing complexity of the plant body was accompanied
by "division of labor" among various organs according
to the requirements of the plant body, including metabolic and functional
Evolutionary ancient species produce independant, photosynthetic gametophyte
and sporophyte generations. Spores produced by the sporophytes are
of a singular type and develop into a single type of gametophyte bearing
both female and male reproductive organs. These are the female archegonium
and male antheridium, both found on a single plant.
Of the twelve phyla of the kingdome plantae, nine of them, known
as tracheophytes,.display developed "plumbing"
systems for transporting materials throughout the plant. The remaining
three phyla contain a very simple or no vascular system and are
referred to as non-tracheophates, which replaces the misnomer
Another remarkable change that occurred in the evolution of land
plants was the conversion of a predominant haploid, gametophytic
generation, as in Bryophyta, to a prodominant diploid, sporophytic
generation in the life cycle in pteridophytes, gymnosperms and angiosperms.
In bryophytes, the gametophyte is very elaborate, less so in pteridophytes,
and greatly reduced in gymnosperms and angiosperms. Fertilization
in land plants occurs within the protection of the female sex organ.
Later, heterosporous plants evolved, with megaspore spores developing
into female gametophytes yielding eggs, and microspore spores developing
into male gametophytes producing sperm.
Table of Contents
15.0 - Introduction
15.1 Pteridophyta : General Account
Angiosperms : Dicotyledons
Angiosperms : Monocotyledons
Development of seed habit
Development of Flower and Fruit