Re: Is an oak tree a gametophyte?

This is a tricky question for seed plants like oaks. The oak tree is 
considered the sporophyte. However, when sexually mature, the oak sporophyte 
annually produces spores that develop into tiny gametophytes. The "male" 
gametophytes or microgametophytes are pollen grains. The "female" gametophytes 
or megagametophytes are the eight-nucleate egg sacs that occur in the ovary of 
the "female" flowers. Because the megagametophytes are retained by the oak 
sporophyte, it is a combination of sporophyte and gametophyte tissues during a 
part of each year. Before the pollen is released, the microgametophytes are 
also retained. A large oak tree in bloom consists of a single huge sporophyte 
plus millions of tiny gametophytes.

All plant species, including seedless plants (mosses, liverworts, hornworts, 
ferns, horsetails, whisk ferns, club mosses) and seed plants (conifers, 
ginkgo, cycads, gnetophytes, flowering plants) have an alternation of 
generations between a multicellular gametophyte and multicellular sporophyte. 
The term "alternation" might be a bit confusing in seed plants because a 
single long-lived tree, which is almost entirely sporophyte tissue, can 
annually produce new gametophytes for hundreds of years. The "alternation" is 
clearer with annual plants where the large sporophyte produces gametophytes 
once and then the sporophyte dies. Two gametes (egg and sperm) produced by the 
gametophytes fuse to produce a zygote. Each zygote develops into a new 
sporophyte.

Seedless plants often have separate gametophytes and sporophytes. Ferns are a 
good example. The large fern plant is the sporophyte. The fern gametophyte is 
a few millimeters across and heart-shaped.

Sporophytes are diploid (2n), and gametophytes are haploid (1n). Sporophytes 
use meiosis to produce haploid spores, which grow into gametophytes. The 
gametophytes produce gametes, sperm and eggs, via mitosis. In contrast, humans 
produce sperm and eggs directly via meiosis.

Most biology and botany textbooks have life cycle diagrams that show what 
parts of the plant life cycle represent the gametophyte and sporophyte. The 
second website listed below has excellent life cycle diagrams and discussion. 
Usually, moss, fern and flowering plant life cycles are considered because 
they represent three different patterns:

Moss: dominant gametophyte, dependent sporophyte
Fern: dominant sporophyte, independent gametophyte
Flowering plant: dominant sporophyte, dependent gametophyte

References


Re: what is alternation of generations?


Life Cycle Patterns: Human, Moss, Fern, Flowering Plant & Comparison