Earth Science: Seed Plants – Angiosperms
Angiosperms (Magnoliophyta or Angiospermae) are divided into two categories: dicotyledons, which germinate with two cotyledons (seed leaves), and monocotyledons, which produce only one cotyledon.
The estimated number of angiosperm species is between 250,000 and 300,000. The ovules of angiosperms are not exposed on the carpels as seen in gymno- sperms. Instead, they are enclosed inside an ovary. The ovary consists of several carpels attached to each other. Once the seeds are ripe, they are released from the ovary, which coincides with fruit growth.
The cotyledons that are used as a characteristic for classification are already present in the seed and are the first part of the new germinating plant. They have a different appearance compared to the leaves that form later. Almost all of our cultivated plants belong to the angiosperms except for coniferous trees. The earliest angiosperms appeared during the Cretaceous about 110 million years ago.
Dicotyledons
Dicotyledons (Magnoliopsida or Dicotyledonae) include about three-quarters of the known Magnoliophyta species. Most have two cotyledons (rarely more) and their leaves usually have a distinctive stalk and midrib with lateral veins. Stipules are also common and the flowers are normally characterized by five whorls.
These plants predominantly show secondary growth like trees that increase in thickness. In fact, most trees and shrubs belong to this group. Their vascular tissue is normally in ring form and a cambium (a layer of active growth with new cells adding to the thickness of the plant shoot) is always present.
Monocotyledons
The smaller group of mono-cotyledons (Liliopsida or Monocotyledonae), include approximately 50,000 species, most of which are herbaceous. They are characterized by having only one seed leaf, irregularly placed vascular tissue, and no cambium.
Their leaves normally lack a stalk and their structure is less complex compared to the dicotyledons; the lack of stipules and roughly parallel veins are typical of these species. Liliopsida include grasses, orchids, lilies, irises, and palm trees.
INSECTIVORES
There are several plant species which use animals as additional nitro- gen sources. Such plants often live in habitats low in nutrients, for ex-ample, moors and bogs. They are equipped with various mechanisms to attract and catch prey, mainly insects.
These techniques can include sticky traps (e.g., sun- dew), jaw traps (e.g., Venus flytrap), and pitfall traps (e.g.. pitcher plants). Once a prey has been caught, its body is dissolved by special enzymes.