What is a “woody plant”? “ Wood” is secondary thickening of xylem: cellulose and hemicellulose in the cell walls is reinforced by lignin for strength. Non-woody plants only have ‘primary’ xylem. A cross section of an oak stem. See the link to “wood” on the course web page for information about different types of wood.
woody plants do not all fall into a single phylogenetic group
Woody plants in an evolutionary context Note that the evolution of photosynthesis occurred LONG before plants invaded terrestrial environments. “Woodiness” evolved after plants invaded land in response to intense competition for light.
All Plants Non-vascular plants Bryophytes Mosses Liverworts Vascular Plants (All woody plants are vascular plants. “wood” is part of the vascular system) (see the link to The Plant Kingdom on the course web page for more information)
Seedless vascular plants Vascular Plants Seed plants 4 divisions: Club mosses, horsetails, Ferns, psilophytes (extinct) 4 divisions of gymnosperms and 1 division of angiosperms All woody plants alive today are seed plants, but this hasn’t always been the case
Trees of the carboniferous period: these seedless vascular plants had much wider distribution than modern seed plants – spores were easy to disperse in the wet, mild climate and relatively flat terrain Lepidodendron , a lycophyte (a seedless vascular tree). Lycophytes comprised about 2/3 of plants producing modern “coal” Medullosa noei , a carboniferous seed fern (gymnosperm)
Seed Plants: the oldest known seed plants -- about 360 million ybp The seedless vascular plants disappeared as the climate became more variable, due to lifting of mountain ranges, and drier.
Perennial woody gymnosperms Perennial woody angiosperms Perennial herbaceous angiosperms Annual herbaceous angiosperms Evolutionary progression: Note that in an evolutionary sense, herbaceous angiosperms are more “advanced” than woody angiosperms or gymnosperms
Gymnosperms Ginkgo biloba 2. gingkos 4. conifers Welwitschia mirabilis grows in SW Africa 3. gnetophytes 1. cycads Cycas circinales
<ul><li>Conifers are by far the most abundant gymnosperms. They date to the late carboniferous (~290 ybp). </li></ul><ul><li>They include 50 genera and 550 species </li></ul><ul><li>Conifers are wind pollinated and generally have many drought resistance features </li></ul>
Flowering plants: the angiosperms Angiosperms evolved from a primitive gymnosperm ca. 120 million ybp. By 80-90 million ybp angiosperms were more numerous than any other plants
Key features of angiosperms : a. Specialized vascular cells: sieve tube members (in the phloem) and vessels (in the xylem) <ul><li>Flowers attract pollinators, resulting in </li></ul><ul><li>more efficient, more precise pollination </li></ul>c. Huge chemical diversity d. Deciduous habit allows survival in climates with periodic drought/freezing cycles (some conifers are deciduous, but most deciduous trees are angiosperms)
Dicots: all true “woody” angiosperms are dicots, but not all dicots are woody species (likewise, not all perennials are woody species) Saguaro cactus Hepatica americana
Monocots Coconut palm (although palms are perennials, most lack secondary xylem growth and therefore, technically, they are not “woody plants”) rice
Woody plants: <ul><li>Gymnosperms (mostly conifers, but also gingkos) and angiosperm dicots make up the woody species </li></ul><ul><li>Woody plants are perennials (but not all perennials are woody plants) </li></ul><ul><li>Woody plants have secondary xylem made of cellulose reinforced by lignin </li></ul><ul><li>Woody plants are big (compared to non-woody species) </li></ul>
<ul><li>Given the differences in structure and life history between woody and non-woody seed plants, what kinds of differences might you expect in their physiology? </li></ul>
Forests tend to grow in areas that are relatively cool and moist compared
About half of the forest that was present under modern (i.e. post-Pleistocene) climatic conditions, and before the spread of human influence, has disappeared (see map), largely through the impact of man's activities. The spread of agriculture and animal husbandry, the harvesting of forests for timber and fuel, and the expansion of populated areas have all taken their toll on forests.