Plants have a different life cycle from animals, known as alternation of generations life cycle:
Land plants have two multicellular stages in their life cycles, one diploid and one haploid.
The diploid generation produces spores and is called sporophyte.
The haploid generation produces gametes by mitosis and is called a gametophyte.
The haploid phase of the plant life cycle begins of the sporophyte, where spores are produced by
A gametophyte’s function is to nourish and protect the forthcoming sporophyte generation.
Each generation gives rise to the otherhence the name alternation of generations for this life cycle.
earliest land plants had neither a waterproof cuticle nor tissues with sufficient mechanical strength
to allow for upright growth.
Modern day plants were poikilohyrid( variable, relating to water), meaning that they have little
control over their internal water content and do not restrict water loss. So when the habitat dries
out, so did their tissues, and their metabolic activities cease. But when external moisture levels rise,
they rehydrate again
Poikilohydric plants are drought tolerators that can drying out, while vascular plants, which regulate
their internal water content and restrict water loss, are drought avoider with numerous adaptations
to avoid drying out.
The early land plants did have specialized water conducting cells that transported water through
the plant body, called nonvascular tissues. Vascular plants have apical meristems, regions of constantly dividing cells near the tips of shoots
and roots that produce all tissues of the plant body.
Rootsanchoring structures that also absorb water and nutrients in association with mycorrhizal
fungiwere the eventual solution to these problems.
Root systems general consist of underground, cylindrical absorptive structures with a large surface
area that favors the rapid uptake of soil water and dissolved mineral ions.
The root system has been called ‘the hidden half’ of a plant: ‘half’ refers to the fact that there is as
much plant biomass below ground as there is above ground. ‘Hidden’ refers to the root system is
hidden from out sight below ground.
Shoot Systems have stems and leaves that arise from apical meristems and that function in the
absorption of light energy from the sun and CO2.
The sporophyte generation begins after fertilization, when the zygote divides by mitosis to produce
a multicellular diploid organism.
Its body will develop called sporangia(angiumvessal or chamber, sporeproducing chambers;
The sporophyte phase of that plant is diploid and so has a backup copy of the DNA that can
continue to function normally even if one strand is damaged.
When a plant makes only one type of spore, it is said to be homosporous.
Usually, a gametophyte that develops from such a spore is bisexual it can produce both sperm
Gymnosperms and angiosperms are heterosporous. They produce two types of spores in two
different types of sporangia.
The smaller spores are microspores, which develop into male gametophytes, and the larger
megaspores will develop into female gametophytes.
Heterospory and the development of gametophytes inside spore walls are important steps in the
evolution of the seed, as we will see further on.
24.3 The bryophytesliverworts, hornworts, and mosses are important both ecologically and
Because bryophytes lack cells strengthened by lignin and are poikilohydric(can live in inhospitable
habitats), it is not surprising that they are small and commonly grow on wet sits.
bryophyte produced flagellated sperm that must swim through water to reach eggs, which is
another reason they are small: the sperm must be able to swim between plants in water.
They also lack xylem and phloem and vascular tissue.
Bryophytes’ roots are rhizoids that serve only to anchor the plant to its substrate and do not take
up any water or nutrients from the substrate.
Bryophytes gametophytes produce games inside a protective organ called a gametangium.
The gametangium where bryophytes eggs form are flaskshaped structures called archegonia.
flagellated sperm form in rounded gametangia called antheridia.
The sperm swim through water to the archegonia to fertilize eggs.
Each egg gives rise to a diploid embryo sporophyte, which stays attached to the gametophyte and
Unlike Vascular plants: the gametophyte is much longer lived than sporophyte and it
photosynthetic, whereas the sporophyte remains attached to the gametophyte and depends on the
gametophyte for much nutrition.
Bryophytes are not a monophyletic group( do not evolve from a common ancestor)
liverworts make up the phylum hepatophyta, so called because thought they were shaped like the
lobes of a human liver.
There simple body, called a thallus, is the gametophyte generation.
Evidence shows that liverworts were probably the first land plants.
Liverwort production; a sperm released from the antheridia swim through water to the eggs inside
archegonia. Once egg fertilized, diploid sporophyte develops inside the archegonium, and
produces haploid spores
In liverworts, the spores develop inside jacket sporangia that split open to release the spores unlike
meiosis in other plants.
Liverworts can also reproduce asexually by way of gemmae. 24.3b
100 species of homoworts make up the phylum Anthocerophy