Cracking the AP Biology Exam

10

Plants

LIFE CYCLE OF PLANTS

Now that we’ve looked at the differences in plant classification, we’re ready to review the life cycles and reproduction of plants. One thing you’ll need to know for the AP Biology Exam is that plants have an alternation of generation. That is, they spend part of their lives as haploids and part of their lives as diploids.

The haploid plant is called a gametophyte because it produces haploid gametes. These gametes then combine to form a diploid plant. The diploid plant, the sporophyte, produces haploid spores by meiosis.

Take a look at the figure above. When the haploid gametes—eggs and sperms—unite during fertilization, a zygote is formed. This diploid organism, or sporophyte, begins the sporophyte generation. The zygote divides by mitosis and develops into a multicellular embryo that is supported and protected by the gametophyte plant.

The sporophyte plant divides and produces spores through meiosis. The spores, which are haploid, represent the first stage in the gametophyte generation. Each spore is capable of growing into a multicellular gametophyte plant. Once a gametophyte plant is produced the cycle continues again.

Here’s an example of the life cycle of a moss:

Notice from the bottom half of the diagram that the sporophyte plant grows on top of the gametophyte plant and depends on it for its survival.

Fortunately, you don’t have to know all the details of this process. What you do have to know is that there is a major difference between the two phyla:

• In bryophytes, the gametophyte stage is dominant.
• In tracheophytes, the sporophyte stage is dominant and the gametophyte generation is reduced in size.