21.5 Gymnosperms

Introduction

It took more than three thousand years to make some of the trees in these western woods ... Through all the wonderful, eventful centuries since Christ's time—and long before that—God has cared for these trees, saved them from drought, disease, avalanches, and a thousand straining, leveling tempests and floods; but he cannot save them from fools.

Giant sequoia

The redwood trees and Douglas firs referred to in Muir's quote are indeed ancient, and among the largest living things on the planet. They are members of the group we call gymnosperms , meaning “naked seeds,” which are a diverse group of seed plants and are paraphyletic . Paraphyletic groups are those in which not all members are descendants of a single common ancestor. Gymnosperm characteristics include naked seeds (not enclosed in an ovary), separate female and male gametes, pollination by wind, and tracheids (which transport water and solutes in the vascular system).

Gymnosperm seeds are not enclosed in an ovary (they are "naked"); rather, they are exposed on cones or modified leaves. Gymnosperms were the dominant phylum in the Mesozoic era. They are adapted to live where fresh water is scarce during part of the year, or in the nitrogen-poor soil of a bog. Therefore, they are still the prominent phylum in the coniferous biome or taiga, where the evergreen conifers have a selective advantage in cold and dry weather. Evergreen conifers continue low levels of photosynthesis during the cold months, and are ready to take advantage of the first sunny days of spring. One disadvantage is that conifers are more susceptible than deciduous trees to infestations because conifers do not lose their leaves all at once. They cannot, therefore, shed parasites and restart with a fresh supply of leaves in spring.

The life cycle of a gymnosperm involves alternation of generations, with a dominant sporophyte and reduced gametophytes that resides within the sporophyte. All gymnosperms are heterosporous. The male and female reproductive organs can form in cones (strobili). Male and female sporangia are produced either on the same plant, described as monoecious (“one home”), or on separate plants, referred to as dioecious (“two homes”) plants. The life cycle of a conifer will serve as our example of reproduction in gymnosperms.

Life cycle of a conifer

Pine trees are conifers (cone bearing) and carry both male and female sporophylls on the same mature sporophyte. Therefore, they are monoecious plants. Like all gymnosperms, pines are heterosporous and generate two different types of spores: male microspores and female megaspores. In the male or staminate cones, the microsporocytes give rise to pollen grains by meiosis. In the spring, large amounts of yellow pollen are released and carried by the wind. Some gametophytes will land on a female cone. Pollination is defined as the initiation of pollen tube growth. The pollen tube develops slowly, and the generative cell in the pollen grain divides into two haploid sperm cells by mitosis. At fertilization, one of the sperm cells will finally unite its haploid nucleus with the haploid nucleus of a haploid egg cell.