# 7.4: Conifers

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## Conifers

Conifers are the most species-rich lineage of gymnosperms. From the fossil record, we think there were over 20,000 species of conifers. However, their diversity declined with the dinosaurs. Currently, there are around 600 extant species. These amazing plants represent some of the oldest, tallest, and most massive organisms on the planet. Though currently low in diversity, these amazing plants make up 30% of Earth’s forests. Conifers share the following characteristics:

Note: The Pinaceae is currently the largest family of conifers, so many of our examples for this group of gymnosperms will be from the type genus Pinus (pines).

### Seeds & Pollen

#### Seed Cones

The megastrobilus, or seed cone, contains diploid megasporocytes that are produced within a megasporangium.

Each megasporocyte (also called a megaspore mother cell) undergoes meiosis. Only one of the four cells produced will survive to develop into a megagametophyte and the other three will die.

The megagametophyte is part of the ovule and contains archegonia, each with an egg cell inside. The megagametophyte is retained within the megasporangium, which becomes the nucellus. Surrounding the nucellus is the integument, which is initially continuous with the ovuliferous scale and has a small opening called a micropyle.

A grain of pollen will be transported on the wind and, if lucky, it will land on a seed cone. The seed cone has a drop of sugary liquid that it secretes, then retracts, pulling the pollen in toward the ovule. This stimulates the tube cell to germinate a pollen tube, while the generative cell divides by mitosis to produce two sperm. These sperm travel down the pollen tube, through the micropyle, and into an archegonium where one will fertilize an egg. When fertilization occurs, the micropyle closes and the integument becomes the seed coat.

The zygote will grow and develop as an embryo, nourished by the megagametophyte tissue, as well as the nucellus. If you look in a long section of a pine seed, you can see the embryo’s RAM and SAM. The seed will be dispersed by wind or animals and germinate to grow into a diploid pine tree once again.

#### Pollen Cones

The microgametophyte in gymnosperms is the four-celled, "winged" pollen grain. Within the pollen grain, you can distinguish the generative cell and the tube cell nucleus. The two prothallial cells are not apparent under the microscope. On either side of the pollen grain, two ear-like structures emerge. These air sacs may help orient the pollen grain toward the ovule.

### Xerophytic Leaves

Xerophytic leaves are adapted to withstand drought conditions. In conifers, we see a wide range of xerophytic leaves with different morphologies that can be shaped by their local environment. Consider the leaves of the coast redwood and the giant sequoia, shown below. Though these two trees belong to different genera--Sequoia and Sequoiadendron, respectively--they are sister taxa. However, the coast redwood has adapted to life on the coast, where the giant sequoia has evolved in inland, higher elevation forests with much more extreme climatic conditions. How can this be seen in the structure of their leaves?

This page titled 7.4: Conifers is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Maria Morrow (ASCCC Open Educational Resources Initiative) .