BIOLOGY 3404F EVOLUTION OF PLANTS Fall 2008 Lecture 20 Tuesday December 2, 2008 Review: Summary of Plant Adaptations to Life on Land

In Bryophyta, not Chlorophyta*: 1.presence of protective layer of cells surrounding the male and female gametangia, 2.retention of zygote and developing sporophyte within female gametophyte (= archegonium), 3.presence of a multicellular diploid sporophyte (multiple meioses per mating event), 4.multicellular sporangia (capsules) with protective layer of sterile cells, 5.drying-resistant spores with walls containing sporopollenin (a cyclic alcohol), which is also highly decay resistant. (*5 sporopollenin is found in zygote walls of Charophyceae)

Onwards and upwards … These five characteristics are also true of vascular plants. As we move up the evolutionary tree towards Anthophyta (Angiosperms), more and more adaptations to life on land are added. Protracheophytes 6.Dominant sporophyte (reduces dependence on a stage that requires free water) 7.Branched sporophyte with multiple sporangia (lots of spores from one mating event)

Rhyniophyta – 1 st true vascular plants 8.[Here or earlier] Lignin (allows toughening of secondary cell walls to allow greater height growth, reduces pathogen attack) 9.A vascular system based on tracheids (allows plants to grow tall and take advantage of light) 10. [Here or earlier] Development of waxy cuticle and stomata (keeps water in, allows for entry of CO 2 under controlled conditions)

Progymnosperms - wood 11. Secondary growth: a vascular cambium, produces wood (secondary xylem) internally and secondary phloem externally 12. [Here or earlier] Development of the periderm, which replaces the epidermis in stems and roots with secondary growth. Consists of cork cambium, producing phelloderm (living tissue) internally and cork (= bark) externally. Greatly protects from drought, including warm days in winter, and may protect from fire.

[Pteridosperms and] Gymnosperms 13. Seeds: encapsulation of the daughter embryo in a protective covering, the integument or ovule wall. This is like maternal investment (parental care) in animals - improves the odds of reproduction in harsh circumstances. There are many associated adaptations for seed dispersal. 14. Pollen: male gametophyte modified for air- dispersal. Motile sperm lost in most groups except Cycads and Ginkgos.

Gnetophyta (and possibly shared ancestor with Anthophyta) 15. Vessels: large volume, thick walled (with spiral or transverse ridges) cells with connecting pits at their ends, for much more efficient water transport upwards. Enables height growth under drier conditions. (Note that the world’s tallest trees are all gymnosperms that grow in rain-forest conditions)

Magnoliophyta – flowering plants 16. Carpels: Further protection of the ovule(s) by enclosing in a modified leaf that becomes the pistil (ovary, style and stigma). 17. Indirect pollination: pollen lands on stigma, germinates as pollen tube that travels down to individual ovule, 2 sperm nuclei travel to fertilize egg and polar nuclei

Angiosperms II 18. Double fertilization: fertilization of n+n polar nuclei results in triploid endosperm, the food reserves of angiosperm seeds (and principal carbohydrate source for most of the world) 19. Flowers: 4 whorls (sets) of modified leaves that: a) protect the developing flower (sepals  calyx), b) attract pollinators (petals  corolla), c) act as microsporophylls (stamens  androecium), d) and act as megasporophylls to enclose and produce the megasporangia (carpels  gynoecium).

and finally … 20. Numerous secondary adaptations to drought occur throughout the angiosperms - e.g., succulent growth forms, tap roots, CAM photosynthesis,... What other MAJOR adaptations to life on land have been left out of the list? Where would they fit in the historical perspective?