Cracking the AP Biology Exam

9

Diversity of Organisms

V. KINGDOM PLANTAE

Plants are multicellular, photosynthetic eukaryotes with cell walls made of cellulose. In plants, the fertilized egg develops in a multicellular embryo within a protective archegonium (the female gametangia). They reproduce sexually and asexually, with alternating gametaphyte and sporophyte generations. Here’s a look at the phyla in this kingdom.

A. Bryophytes

Bryophytes are the more primitive plants and are characterized by the lack of true stems, roots, and leaves. Bryophytes anchor themselves in the soil by rhizoids (rootlike structures). Bryophytes have flagellated sperms within the antheridium (male-containing part) that swim to reach the egg, which lie within the archegonium (the female gametophyte). Common bryophytes are mosses, liverworts, and hornworts.

B. Pterophyta

Ferns are the “seedless plants.” They are among the earliest vascular plants to colonize land. The life cycle of ferns involves alternation of generation, in which the dominant stage is the sporophyte generation.

C. Sphenophyta

These vascular plants with hollow, ribbed stems and reduced, scalelike leaves are the horsetails. Although the extinct species were once as large as modern trees, the surviving species are small and found in wet, marshy habitats.

D. Lycophyta

Club mosses are small plants with rhizomes and short, erect branches. Like horsetails, they were common 300 million years ago when their extinct relatives were treelike plants.

E. Coniferophyta

Conifers are the woody plants that bear their seeds in cones (the seeds are not enclosed). They have tracheids (for the transport of water) and well-developed phloem (for carrying nutrients). Both the roots and the stems are capable of secondary growth. Fertilization doesn’t require a water source.

F. Anthophyta

These are known as the “flowering plants.” They have seeds enclosed within a fruit or nut. They are multicellular plants with highly specialized and developed conducting tissue for the transport of water and nutrients. Angiosperms can be further divided into two classes: monocots anddicots.

i. Monocots
Monocots have a single cotyledon (the embryonic seed leaf). The characteristics include flower parts in threes or multiples of threes, vascular tissues usually in scattered bundles, a fibrous root system, and leaves with parallel veins.

ii. Dicots
Dicots have two cotyledons. The characteristics include flower parts mostly in fours or fives, vascular tissue in distinct bundles arranged in a circle, a taproot system, and leaves with netted veins.