A woody plant is defined as a vascular plant that develops secondary xylem, also called wood. Wood is made of cellulose-reinforced cell walls in the xylem and provides strength. Woody plants evolved from seedless vascular plants and later gymnosperms in response to competition for light as plants colonized land. Today, all woody plants are seed plants, including both gymnosperms like conifers and angiosperm dicots. Woody plants are typically large perennials that develop wood, whereas non-woody plants are often herbaceous.

1. 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.

2. woody plants do not all fall into a single phylogenetic group

3. 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.

4. 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)

5. 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

6. 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)

7. 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.

8. 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

9. Gymnosperms Ginkgo biloba 2. gingkos 4. conifers Welwitschia mirabilis grows in SW Africa 3. gnetophytes 1. cycads Cycas circinales

11. 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

13. 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

14. Monocots Coconut palm (although palms are perennials, most lack secondary xylem growth and therefore, technically, they are not “woody plants”) rice

17. Distribution of woody plants

18. Forests tend to grow in areas that are relatively cool and moist compared

19. 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.