Lecture 16: Vascular Plant Innovations

what were the first vascular plants like?

Rhyniophytes are completely extinct fossil group that are sister to living vascular plants

-were relatively simple plants that lacked leaves and roots

two major innovations of rhyniophytes

1. branched sporophyte with apical sporangia(at the tips of the sporophyte)

2. vascular tissue (but lacked tracheid cells)

what did a branched sporophyte allowed:

1. production of more apical sporangia (and spores) per plant

2. the vascular system allowed more efficient transport of water and nutrients in the plant

Diagnostic features of Vascular Plants

1. the sporophyte is nutritionally independent of the gametophyte at maturity

   1. Tracheid cells with lignified cell walls that provide structural support and compromise the xylem

The vascular system includes two tissue types:

1. xylem

2. phloem

xylem

-conducts water and minerals from the roots to the aerial parts of the plant by a passive Transpiration-Cohesion-Tension system

-the tissue is comprimised of tracheid cells that are dead at functional maturity and have cells that are fortified with lignin

phloem

-conducts phloem sap (mainly the products of photosynthesis (carbs)) from source to sink regions of the plant via the Pressure Flow Model

-the tissue is comprimised of sieve-tube elements (which are alive at maturity but lack many cell components) and companion cells (which nurture the sieve-tube cells)

adaptive value of the vascular plants:

1. more efficient transport of water and nutrients, which allows plants to colonize more fully terrestrial environments

   1. rigid structural support allows plants to grow taller

      a. allows them to compete more effectively for sunlight

      b. allows them to more effectively disperse spores

the evolution of vascular plants was…

independ evolution (homoplasy) which included

-leaves

-roots

-heterospory

Leaves:

have evolved independentlytwice in vascular plants

-microphylls

-megaphylls

microphylls

leaves with a singular vascular strand that evolved from sterile lateral sporangia in Lycophytes

megaphylls

leaves with many branched vascular strands that evolved from modified side branches in euphyllophytes (enabled by overtopping growth)

Roots

evolved independently twice in vascular plants:

-simple roots

-complex roots

simple roots

comprimised of a dichotomously branching rhizome (underground stem) with “rootlets” derived from microphyllous leaves evolved in Lycophytes

complex roots

roots with complex branching and root hairs derived from dermis evolved in Euphyllophytes

heterospory

a sporic life cycle with two kinds of spores; evolved independently 3 times in Vascular Plants

1. with Lycophytes (in spike mosses)

2. with in Monilophytes (in some leptosporangiate ferns)

   1. in the MRCA of Seed Plants