Problem 6 - Campbell Biology Concepts & Connections

Question

The diploid sporophyte stage is dominant in the life cycles of all of the following except a. a pine tree. b. a rose bush. c. a fern. d. a moss.

Step-by-Step Solution

Answer

d. a moss.

Step 1 - Understand the life cycle stages

Different plants have different dominant stages in their life cycles. The diploid sporophyte stage is when the plant has two sets of chromosomes. The haploid gametophyte stage has one set of chromosomes.

Step 2 - Identify sporophyte-dominant plants

Pine trees, rose bushes, and ferns all have a dominant sporophyte stage in their life cycles. This means that they primarily exist in the diploid stage.

Step 3 - Analyze moss life cycle

Mosses are different because their dominant stage is the haploid gametophyte stage, not the diploid sporophyte stage. The sporophyte in mosses is dependent on the gametophyte.

Step 4 - Identify the correct answer

Since mosses have a dominant gametophyte stage, they are the exception among the given options.

Key Concepts

Diploid Sporophyte

In the life cycles of many plants, the diploid sporophyte phase is prominent. A diploid cell contains two sets of chromosomes, one from each parent.

During this stage, the plant undergoes mitotic cell divisions, leading to growth and development. This stage results in the adult form of the plant, which we usually observe. For instance, in pine trees, rose bushes, and ferns, the mature plant we see is in the diploid sporophyte stage.

The sporophyte can produce spores through meiosis. These spores are haploid, carrying only one set of chromosomes and will eventually give rise to the next stage in the life cycle.

Haploid Gametophyte

The haploid gametophyte stage is when the plant has only one set of chromosomes. Unlike the sporophyte stage, the gametophyte is usually smaller and sometimes even microscopic.

In the gametophyte stage, gametes (sperm and egg cells) are produced through mitosis. When these gametes combine during fertilization, they form a diploid zygote, which will develop into a new sporophyte.

In mosses, the gametophyte is the dominant stage. The green, leafy parts of moss that we commonly see are the gametophytes, while the sporophyte emerges from the gametophyte and depends on it for nutrition. This makes mosses unique compared to many other plants.

Plant Dominance

Plant dominance in a life cycle refers to which phase (sporophyte or gametophyte) is the most prominent or long-lasting. In most vascular plants, the diploid sporophyte is dominant.

For example, in pine trees, rose bushes, and ferns, the sporophyte stage is the predominant form observed through their life cycles. The sporophytes are large, complex, and independent.

Conversely, in non-vascular plants like mosses, the haploid gametophyte stage is dominant. The gametophyte is the larger and more visible part of the plant's life cycle, and the sporophyte relies on it.

• Vascular plants: Dominant sporophyte
• Non-vascular plants: Dominant gametophyte

Thus, mosses are an exception as compared to pines, roses, and ferns.