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Ulva, and other algae growing in H2O can absorb carbon for
photosynthesis directly from H2O. In Marcantia the upper
surface has numerous pores which opens into air chamber
which is partially filled with photosynthetic cells. Through
these pores exchange of gases takes place and CO2is absorbs
by photosynthetic cells. Evaporation of H2O can occur from
the wet surfaces of these cells at the same time. This
evaporating H2O must be replaced or plant will soon die.
This evaporating H2O is replaced by presence of long
filamentous cells known as rhizoids in Marcantia.
Sexual reproduction which is important feature of all land
plants is also helpful. Large non-motile eggs produced after
Hetrogamy is full of stored food which is used to nourish
the new offspring. In isogamy in chalamydomnas supply of
stored food is very small and food required for further
development is manufactured by zygote which retains its
photosynthetic a capacity.
Land plants e.g. Mosses have their sex organs at the tip
being protected by clusters of leaves or protective
coatings. These land plants show al of G, which is also
found in Ulva in which both gametophyte and sporophyte are
identical. But in mosses sporophyte is dependent on
gametophyte. In evolution of life on land sporophyte became
partially dependent on gametophyte while gametrophyte became
the dominant generation.
Vas, plants are also evolved from algae because we see all
these plants are similar in their photosynthetic pigments
stored food. Primitive vas. Plants had no leaves. These
consisted of branching system. Some of branches grow
underground functioning as roots while some grew upwards
functioning as stem.
Later on, leaves are evolved. Leaf originated as an sub
growth, lacking vas. Tissue from the naked branches of plant
with increase in size, a vas strand was required to supply
the leaf of H2O and to support it. Another possibility is
the single veined leaf originated by reduction in size of
part of leafless branching system of primitive vas-plants.
Roots are evolved from the rhizoids.
In primitive plants branches which grew upwards become stem.
In simple types in vas-tissue there present a central solid
rod but in complex types stem is supported by one or more
rings of vas-tissue.
Evolution of pollen tubes brought evolution of seeds because
sporonjium is protected inside the ovule. So it is difficult
for sperm to enter the egg. Therefore pollen grain produces?
a long tube which pierces the walls of ovule and reaches the
egg.
In vas plants sporphyte became pre-dominant and gametophyte
is subordinate. So we van say that during evolutionary
history, sporophyte has evolved towards complexity and
gametophyte towards simplicity.
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