Environmental Engineering Reference
In-Depth Information
female and some that are male. Female cones produce spores that, after
fertilization, become eggs enclosed in seeds that fall to the ground. Male
cones produce pollen, which is taken by the wind and fertilizes female eggs
by that means. Unlike flowering plants, the gymnosperm does not form true
flowers or fruits. Coniferous tress such as firs and pines are good examples
of gymnosperms.
•
Angiosperms
—These flowering plants are the most highly evolved plants
and the most dominant in present times. They have stems, roots, and leaves.
Unlike gymnosperms such as conifers and cycads, the seeds of angiosperms
are found in the flowers. Angiosperm eggs are fertilized and develop into a
seed in an ovary that is usually in a flower.
The two types of angiosperms are monocots and dicots:
•
Monocots
—These angiosperms start with one seed leaf (cotyledon), thus
their name, which is derived from the presence of a single cotyledon dur-
ing embryonic development. Monocots include grasses, grains, and other
narrow-leaved angiosperms. The main veins of their leaves are usually
parallel and unbranched, the flower parts occur in multiples of three, and
a fibrous root system is present. Monocots include orchids, lilies, irises,
palms, grasses, and wheat, corn, and oats.
•
Dicots
—Angiosperms in this group grow two seed leaves (two cotyle-
dons). Most plants are dicots and include maples, oaks, elms, sunflowers,
and roses. Their leaves usually have a single main vein or three or more
branched veins that spread out from the base of the leaf.
l
eaVes
The principal function of leaves is to absorb sunlight for the manufacturing of plant
sugars in photosynthesis. The broad, flattened surfaces of the leaves gather energy
from sunlight, while apertures on their undersides bring in carbon dioxide and
release oxygen. Leaves develop as a flattened surface in order to present a large area
for efficient absorption of light energy. On its two exteriors, the leaf has layers of
epidermal cells that secrete a waxy, nearly impermeable cuticle (chitin) to protect
against water loss (dehydration) and fungal or bacterial attack. Gases diffuse in or
out of the leaf through
stomata
, small openings on the underside of the leaf. The
opening or closing of the stomata occurs through the swelling or relaxing of
guard
cells
. If the plant wants to limit the diffusion of gases and the transpiration of water,
the guard cells swell together and close the stomata. Leaf thickness is kept to a
minimum so that gases that enter the leaf can diffuse easily throughout the leaf cells.
Chlorophyll and Chloroplasts
The green pigment in leaves is
chlorophyll
. Chlorophyll absorbs red and blue light
from the sunlight that falls on leaves; therefore, the light reflected by the leaves is
diminished in red and blue and appears green. The molecules of chlorophyll are
large. They are not soluble in the aqueous solution that fills plant cells. Instead, they
