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

BryophytesMossesLiverworts

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

• Conifers are by far the most abundant gymnosperms. They date to the late carboniferous (~290 ybp).

• They include 50 genera and 550 species

• Conifers are wind pollinated and generally have many drought resistance features

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)

b. Flowers attract pollinators, resulting inmore efficient, more precise pollination

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: Gymnosperms (mostly conifers, but also

gingkos) and angiosperm dicots make up the woody species

Woody plants are perennials (but not all perennials are woody plants)

Woody plants have secondary xylem made of cellulose reinforced by lignin

Woody plants are big (compared to non-woody species)

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?

Distribution of woody plants

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.