All green plants including the green algae ancestors of
plants alternate between sporophyte and gametophyte generations.
Alternation of
generations is the name given to this cycle. The sporophyte on a spore
producing plant such as a fern is composed of stems, fronds (leaves),
rhizomes,
and roots. The sporophyte on a seed plant such as a pine is composed of
a trunk,
branches, needle-like foliage, and roots. Gametophytes from both spore
and seed
plants produce eggs and sperm.
On a spore producing fern,
haploid spores release from a diploid
sporophyte soon after they form by meiosis. The spores land on the
ground and
grow into haploid gametophytes. When enough moisture is present, sperm
swim to eggs
on the same or nearby gametophytes and fertilize them. The fern has
fertilized
gametophyte eggs that are analogous to seeds on a seed plant sporophyte
such as
a pine. Fertilized fern eggs and pine seeds germinate. Fern
sporophytes, grow
out of fertilized eggs on the gametophyte. The eggs are comparable to
‘seeds’
on the ground. Pine sporophytes grow out of seeds after they fall to
the ground.
On a seed producing pine
tree, a delay occurs in the release
of the spore from the sporophyte. Instead of producing releasable
spores, the
sporophyte produces haploid gametophytes called microspores and
macrospores. They
remain attached to the sporophyte. The pine’s gametophytes grow
into eggs or
sperm on the sporophyte and not on the ground as do spore releasing
plants such
as ferns. When sperm fertilize the pine’s gametophyte eggs, they
form seeds on
the sporophyte instead of ‘seeds’ on the ground. When the
seeds fall to the
ground, they germinate and grow into new sporophyte pine trees.
The key difference between
a seed and a spore is the timing
of release from the sporophyte. The sporophyte of a spore producing
plant releases
spores soon after they develop. They land on the ground or some other
substrate, and
become fertilized gametophytes. The sporophyte of a seed producing
plant delays
the release of spores. This enables them to develop into seeds on the
sporophyte.
An evolutionary change in a
regulatory gene of a late Devonian
period spore plant could have caused the delay in the spore release
that led to
the first seeds. The spore plant could have been similar to the
‘almost seed
ferns’, moonworts and grapeferns of the extant Botrychium genus.
The first seed
plant (seed fern) could have been Elkinsia Polymorpha or some other
Devonian
seed plant. The first Devonian seeds probably had spore-like
characteristics
that disappeared soon after seeds evolved into modern forms. The change
that
turned spores into seeds led to the diversity of seed plants we see
today.
