292 ■ CHAPTER 16 Plants, Fungi, and Protists
BIODIVERSITY
present-day bryophytes (liverworts and mosses),
grew as ground-hugging carpets of greenery.
These simple plants had relatively thin bodies,
often just a few cells thick, and absorbed water
through a wicking action. However, absorption by
direct contact cannot transport fluids effectively
in a plant that rises a foot or more aboveground,
so more complex plants, including cycads, have a
network of tissues, called the vascular system,
that is made up of tubelike structures special-
ized for transporting fluids. Roots, found in most
plants, also have an extensive vascular system.
The first land plant with a vascular system was
the ancestor of present-day ferns.
Like green algae from the protist kingdom,
plant cells have strong but flexible cell walls
composed of the substance known as cellulose,
which gives structural strength to the cells,
including those of the low-growing bryophytes.
Beginning with the ferns, plants evolved yet
another type of strengthening material, lignin,
that allowed them to grow even taller. One of
the strongest materials in nature, lignin links
together cellulose fibers in the cell wall to create
a rigid network. Lignin is the reason that some
cycads in Japan can grow over 20 feet (7 meters)
tall, though it takes them about 50–100 years to
achieve such heights.One reason rare cycads are so prized is that
it can take a long time to raise and grow them.
Cycads are gymnosperms, the first plants to
evolve pollen, a microscopic structure contain-
ing sperm cells that can be lofted into the air
in massive quantities. The evolution of pollen
freed gymnosperms from a dependence on water
for fertilization: instead of having to trans-
port sperm cells through water, they could do it
through air.
Gymnosperms were also the first plants
to evolve the seed, which consists of the plant
embryo and a supply of stored food, all encased
in a protective covering. The embryo uses this
stored food to grow until it is able to make its
own food via photosynthesis. Seeds also provide
embryos with protection from drying and from
attack by predators. Unfortunately, they don’t
provide protection from poachers. Today, the
San Diego Botanic Garden keeps not only its
prized cycads, but also their valuable seeds, in a
greenhouse under lock and key.Searching for Flowers
Botanical gardens also go to extreme mea sures
to keep flowering plants, or angiosperms, safe
for long-term conservation. Orchids, as we have
seen, are the most common illegally traded
angiosperms. Others include Galanthus, or
“snowdrop” plants, with delicate white flowers.
In 2012, a single bulb of a rare variety of snow-
drop fetched $945 at auction.
Compared to gymnosperms, angiosperms are a
relatively recent development in the history of life.
With about 250,000 species today, angiosperms
are the most dominant and diverse group of plants
on our planet. Nearly all agricultural crops are
flowering plants. These plants also provide humans
with materials such as cotton and pharmaceuticals.
Angiosperms’ key evolutionary innovation
is the flower, a structure that evolved through
modification of the cone-like reproductive
organs of gymnosperms. Flowers are structures
that enhance sexual reproduction in angio-
sperms by bringing male gametes (sperm cells)
to the female gametes (egg cells) in highly effi-
cient ways, by attracting animal pollinators
through scent, shape, and color (Figure 16.7).Figure 16.7
A flower attracts many pollinators
Pollination in the angiosperms is more efficient than in the wind-
pollinated gymnosperms. Flowers attract animal pollinators that carry
pollen to and from individual plants of the same species.