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How do conifer pollen grains develop into male gametophytes?

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conifer A giant sequoia (*Sequoiadendron giganteum*) estimated to be between 1,900 and 2,400 years old, Grizzly Giant is the oldest tree in the Mariposa Grove, Yosemite National Park, California.

In conifers, the development of the male gametophyte begins within the microsporangium, where a microspore undergoes successive mitotic divisions to form an immature pollen grain.12

In the Pinaceae, for example, this process results in a four-celled grain containing two prothallial cells, a tube cell, and a generative cell. These pollen grains are then released and transported by wind to the female ovule.13

Following pollination, the male gametophyte continues its development:
* The tube cell develops into a pollen tube that grows through the megasporangium toward the archegonium.12
* The generative cell divides to form a sterile cell and a spermatogenous cell.1
* Prior to fertilization, the spermatogenous cell divides again to produce two male gametes (sperm).12

While this sequence is typical, the number of prothallial cells varies significantly among different conifer groups, ranging from none in families such as Cupressaceae and Taxaceae to as many as 40 in some species.1