Gymnosperms and Vascular Plants Help

Introduction to Gymnosperms and Vascular Plants

The great majority of vascular plants (other than ferns and their fern-like relatives) reproduce by means of seeds. One important group are the gymnosperms ( JIM -nuh- sperms ) or vascular plants with “naked” ( gymno -) “seeds” ( sperms ). By naked, it is meant that the seeds of the gymnosperms are exposed, and not enclosed within fruits.

As the Ancient World’s climate became cooler and drier, many of the vast fern swamps of the Carboniferous Period died and dried up. Arising in their place were the conifers ( KAHN -uh- fers ) or “cone-bearers.” The conifers, as their name suggests, produce cones that hold their naked seeds. Most of the conifers are evergreen trees or shrubs that bear cones. These conifers are truly “ever-green,” because they keep their green, needle-like leaves all year. The thin leaves are an adaptation to cold, dry conditions in that their needle shape gives them a small surface area exposed to the air, thus reducing evaporation or drying of the conifer plant.

The conifers include pine trees, fir trees, spruce, larch, hemlock, and yew bushes. Today, much of the still-natural land of the Northern Hemisphere is covered by beautiful coniferous (kuh- NIF -er-us) forests, graced with tall spruce, fir, and pine trees.

The Life Cycle Of A Pine Tree

Since pine trees and other conifers are called gymnosperms, it is important to examine their typical life cycle, wherein their “naked seeds” play an important role. Each of the major steps are described below. (Consult Figure 9.4 for a summary.)

Fig. 9.4 The life cycle of a pine tree.

Step 1 A healthy mature sporophyte (pine tree)

An entire pine tree, itself, is technically called a mature sporophyte ( SPOR -uh- fight ). It is literally a “spore” ( sporo -) or seed “plant” ( phyte ) that has matured. The same individual pine tree has two different sexes of cones hanging from its branches – small male cones, and not far away, big female cones. The small male cones are called pollen ( PAHL -un) cones , while the much larger female cones are termed the seed cones . (It is the large female seed cones that we usually notice on the tree branches.)

Both types of cones produce spores for reproduction. Hence, when the male fertilizes the female, an interaction between their spores eventually results in a seed, which grows into a mature pine tree or sporophyte.

Step 2 Meiosis and spore production in both male and female cones

Pine cones produce spores by the process called meiosis (my- OH -sis). Meiosis means “a condition of lessening,” wherein one cell divides into two daughter cells, each containing only half the number of chromosomes found within the original parent cell.

The small, male pollen cones consist of many loose, oval scales fastened together. Within each scale, meiosis produces tiny microspores ( MY -kroh-spores). The microspores, each containing half the number of chromosomes of the parent tree, soon develop into gametes (sex cells). The male gametes become pollen granules – a “fine flour” ( pollen ) of tiny, yellow-colored grains.

The large, female seed cones consist of many tough, pear-shaped scales arranged into a spiral shape. The scales are woody and tilted outward at an angle. Each of these female scales holds two ovules ( OH -vyools) or “little eggs.” Each of these ovules (little eggs) produces a megaspore ( MEH -gah-spore) – a “large” ( mega -) spore.

Step 3 Pollination (followed by fertilization) of female seed cones

When the male cones have matured, their scales pop open and release a cloud of pollen (millions of microscopic, yellowish grains). During pollination ( pahl-uh - NAY -shun), a pollen grain lands on a female cone and enters one of its ovules. After pollination, meiosis is finally triggered within the ovule, and the megaspores eventually develop into female gametes, the egg cells. Fertilization finally occurs when the pollen grain from the male grows out a tiny tube and releases a sperm cell into an egg cell.

Step 4 Zygote, embryo, and seed formation

The result of fertilization is a zygote. The zygote undergoes repeated mitosis, adds many cells, and becomes an embryo. The original ovule within the female cone eventually develops into a pine seed, which contains the embryo. The seed provides continuing nourishment for the developing pine tree embryo, and it is covered by a tough seed coat. But the seed is still “naked” in the sense that it is not buried within a fruit. Finally, the seed falls to the ground, germinates (sprouts) under favorable conditions, and grows into a pine tree.