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Vascular plants without seeds (Lycopodium (Common in forests from tropical…
Vascular plants without seeds
Dibiontic life cycle
Monobiontic
Interpolation hypothesis
postulates that a small sporophyte came into existence when a zygote germinated mitotically instead of meiotically
Transformation theory
Postulates that after the dibiontic lifecycle originated, both gametophyte and sporophyte became larger , more complex, and more vascularized, in a life cycle with an alternation of isomorphic generations
Rhyniophytes
Earliest vascular plants
Cooksonia
genus of extinct plants
Upright stems with no leaves
Equal dichotomous branching
Homosporus
A reconstruction of
Cooksonia caledonica
, the earliest known plant that had xylem- tracheids with annular secondary walls. Important features are its sporangia at the end of its branches , the lack of leaves, and the dichotomous branching.
Xylem structure
Both have a center of a solid mass of xylem with no pith :Protostele
Endarch protostele
Protoxylem is located in the center
Metaxylem differentiates on the outer edge of the xylem mass
Exarch protostele
Metaxylem located in the center of the xylem mass
Protoxylem on the edges as several groups next to the phloem
Siphonostele
Pith is present in the center, as occurs in the stems of ferns and seed plants
Zosterophyllophytes
Small herbs without secondary growth
Sporangia were lateral, not terminal
Xylem was an exarch protostele
Grew as small branches approx. 15cm high
Upper portions of stems had a cuticle, ordinary epidermal cells, and stomata but lower portions did not
Stems were naked and dichotomous
Homosporus
Zosterophyllum rehenanum
plants were quite similar to those of rhyniophytes, but the ends of fertile branches bore numerous lateral sporangia , not a single terminal one.
Lycophytes
#
Lateral sporangia and exarch protostele
Genera:
Drepanophycus
and
Baragwanathia
Enations were large up to 4cm long , and contained a singular well- developed trace of vascular tissue
Sporangia are clustered together
Cones or strobili
Sporangium dehisced, megaspore wall cracked
Lycopodium
Common in forests from tropical regions to the arctic
Approx. 200 living species
Small herbs with prostrate rhizomes that have true roots and short upright branches
Microphylls are spirally arranged on their stems and secondary growth never occurs
Sporangia may be arranged in cones or distributed along shoots
Homosporous
Lycopodium obscurum
with sporangia clustered into cones at the tips of the branches. (yellow)
Trimerophytes
#
Proposed in 1968 for 3 genera of extinct plants:
Trimerophyton, Psilophyton, Pertica
.
Overtopping
Pseudomonopodial branching
Pertica quadrifaria
had one main trunk from which grew small branches; the smallest twig still shows dichotomous branching, but larger stems branched pseudomonopodially. This branch pattern also evolved separately in lycophytes, but here in the trimerophytes it is the ancestor to the stem structure of seed plants. The globular structures are clusters of sporangia.
Origins of megaphyllus
Leaves
Leaves on gametophytes of nonvascular plants
Enations/microphylls of zosterophyllophytes and lycophytes
Megaphylls
Telome theory
Euphyllophytes
Ferns
#
Leaves may be leathery or delicate
Homosporus
Eusporangiam
Several surface cells undergo periclinal division resulting in a small multilayered plate of cells.
Fundamental type of sporangium
Large sporangium with many spores
Leptosporangia
Initiated when a single surface cell divides periclinally and forms a small outward protrusion
Few spores are produced
Perennial and herbaceous
Vascular cryptograms
Have vascular tissues and because they lack seeds their reproduction is hidden
Bryophyte
May have came from Zosterophyllem type of ancestor
Fossils strongly resemble Rhyniophytes
Fern sporophyte bears both true roots and megaphyllous leaves
