Lecture 16: Vascular plant innovation

First Vascular plants: rhyniophytes

extinct fossil sister to living vascular plants, lacked leaves and roots but had two major innovations

two major innovations:

branched sporophytes with apical sporangia: at the tip of the sporophytes

vascular tissues: lacked tracheid cells

The branched sporophytes allowed production of more apical sporangia and spores per plant

the vascular system allowed for a more efficient transport of water and nutrients in the plants

Diagnostic features of vascular plants

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

2. Tracheid cells with lignified cell walls that provide structural support and comprise the xylem

Vasular tissues of vasular plants: two types: xylem

Xylem: conducts water and minerals from the roots to the aerial parts of the the plant by a passive transportation cohesion tension system. the tissues is made up of the tracheid cells that are dead at functional maturity and have cell walls that are fortified with lignin

Vasular tissues of vascular plants: two types: phloem

conducts phloem spa mainly the products of photosynthesis carbohydrates from sources to sink region of the plant via the pressure flow model. The tissues is made up 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 system to plants include

more effiecient transport of water and nutrient: allows the plant to colonize more terrestrial environments

rigid structure support allows plant to grow taller which: more sunlight and disperses spores

Evolution of vascular plants involves homoplasy of different innovations

leaves: have evolved twices in the vascular plants

roots: evolved independently twice

heterospory: sporic life cycle with 2 kinds of spores evolved independently 3 times

Leaves

Microphylls: leaves with a single 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 the overtopping growth form

Roots

simple roots composed of a dichotomous branching rhizomes(underground stem) with rootlets derived from microphyllous leaves evolved in lycophytes

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

heterospory evolved 3 times:

within lycophytes:spikes mosses

monilophytes: some leptosporangiate ferns

in the MRCA of seed plants