Lecture 18: Seed-Plant Innovation

Seed plants are two large groups:

1. Gymnosperm

2. angiosperm

progymnosperms

trees that exhibited secondary growth (wood) but lacked seeds

seed ferns

were woody trees with fern-like leaves that bore seeds

four important innovations arose in the MRCA of all living Seed plants

-secondary growth

-reduced and dependent megagametophytes

-seed

-pollen

primary growth

the ability to increase the length of the plant by means of apical (shoot and root) meristems

secondary growth

the ability to increase the girth of the growing plant by means of vascular cambium

vascular camnium

an innovation of seed plants that is comprimised of a ring of meristematic cells between the primary xylem and primary phloemb

bifacial vascular cambium

produces secondary xylem to the inside of the plant (that gives rise to wood) and secondary phloem to the outside of the plant (gives rise to bark)

adaptive significance of secondary growth/ origin of vascular cambium

1. allows taller growth of the sporophyte, which facilitates competition for sunlight (enhanced photosynthesis)

2. taller growth also facilitates more efficient dispersal of pollen and seeds

   1. The formation of bark prevents water loss and infection and protects againt disease

secondary growth evolved independently with…

…lycophytes (in quillworts), but this involved unifacial vascular cambium that produces secondary xylem but not secondary phloem

In Seed Plants, the female gametophytes (megagametophyte)…

…greatly reduced, embedded within and nutritionally dependent upon sporophyte. This innovation enabled the evoultion of the seed

The Gymnosperm(“Naked Seed”) is comprimised of 3 nested components

1. diploid embryo (baby sporophyte): innermost component

2. haploid nutritive tissue (from the megagametophyte): middle component

3. diploid seed coat (from the integument): the outermost layer

adaptive significance of seeds

1. facilitates efficient dispersal of the next sporophyte generation

2. provides protection of developing sporophyte plant

3. allows for prolonged dormacy of the embryo so that development can resume when conditions are favorable, and is provisioned with nutrients to facilitate growth when development presumes

Euphyllophytes

-”true-leaved-plants”

-comprimised of Monilophytes and Seed plantsd

diagnostic features of Euphyllophytes

1. overtopping growth (sporophyte has a dominant axis and side branches)

2. megaphyll leaves (“true” leaves with more complex vascularization that evolved from side branches")

3. complex roots (with complex branching and root hairs that evolved from dermis)

overtopping growth involves…

asymmetric division of the apical meristem, such that the sporophyte has a main stem/axis and side branches

adaptive value of overtop growth

1. it allowed taller growth and enhanced competition for sunlight (by virtue of the main axis)

2. taller growth enhances ability to disperse spores

3. it allowed elaboration of novel organs from side branches (such as megaphyllous leaves and complex roots)

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