BIOLOGY Volume 1 - A Guide to General Biology - 2004

2. DIVERSITY OF LIFE ON EARTH

2.7. Plant Kingdom

2.7.4. Division Coniferophyta (Conifers)

The Main characteristics of Coniferophyta are summarized in Table 2.10.

Conifers (gymnosperms) are a thriving group of plants distributed globally, accounting for approximately one-third of the planet's forests. They are trees and shrubs, predominantly evergreen with needle-like leaves. The vast majority of species inhabit high latitudes and extend further north than any other trees. Conifers are of significant economic importance as "softwood," used not only for timber and lumber, but also for resin, turpentine, and wood pulp. Conifers include pines, larches (which shed their leaves in winter), firs, spruces, and cedars. A typical representative of conifers is the Scots pine (Pinus sylvestris).

This species is widespread throughout central and northern Europe, the former USSR, and North America. It has also been introduced to Great Britain, but grows in a wild state only in Scotland. This beautiful, majestic tree reaches up to 36 m in height with characteristic flaky pinkish- or yellowish-brown bark and is cultivated both for ornamental purposes and for timber and lumber. Pines most commonly grow on sandy or poor mountain soils, which is why their ROOT system usually spreads across the surface soil layers and branches extensively. The general appearance of the pine is shown in Fig. 2.39.

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Fig. 2.39. General appearance and Characteristic Features of the sporophyte Generation of the Scots pine (Pinus sylvestris).

Each year, a new whorl of branches grows from a whorl of lateral buds at the top of the trunk. The characteristic tapered, conical shape of the pine and other conifers is due to the whorls of shorter (younger) branches at the top gradually giving way to progressively longer (older) branches further down. With age, the lower branches die off and drop; consequently, in older trees, the crown is usually retained only at the top (Fig. 2.39).

The main branches and trunk continue to grow year after year As a result of apical bud growth. Therefore, conifers are said to exhibit indeterminate growth. Scale-like leaves are arranged spirally; in the axils of these leaves are buds that develop into short branches (2–3 mm long), known as short shoots. These are stems with determinate growth, at the apex of which two leaves grow. As soon as the SHOOT grows, the scale-like leaf at the base drops off, leaving a scar. The leaves resemble needles, which reduces their surface area and, consequently, Water loss. In addition, the leaves are covered with a thick waxy cuticle, and the Stomata are sunken deep into the leaf tissue—another adaptation for water conservation. These xeromorphic features help minimize water loss from evergreen leaves during cold seasons when water may freeze or be difficult to Histology/2.html">EXTRACT FROM THE soil. After two to three years, the short shoots drop off along with the leaves, leaving another scar.

The tree represents the sporophyte generation. In spring, male and female cones form on the same tree. Male cones are about 0.5 cm in diameter; they are rounded and located in clusters at the base of new shoots below the apical bud. They develop in the axils of scale-like leaves in place of short shoots. Female cones appear in the axils of scale-like leaves at the end of strong new shoots at some distance from the male cones and are arranged more sporadically. Since the complete development of a cone takes three years, the cones vary greatly in size, and cones ranging from 0.5 to 6 cm in size can be found on a single tree. Young cones are green, but in their second year they turn brown or reddish-brown. Both male and female cones consist of tightly packed sporophylls (modified leaves) arranged spirally around a central axis (Fig. 2.39).

On the lower surface of each sporophyll of the male cone are two microsporangia, or pollen sacs. Inside each pollen sac, meiotic division occurs, leading to The formation of haploid pollen grains, or microspores, which contain the male Gametes. The pollen grains have two large air sacs that facilitate their wind dispersal. In May, the cones turn entirely yellow due to the pollen released from them in a cloud. By late summer, they wither and drop off.

Each sporophyll of the female cone consists of a lower bract scale and a larger upper scale bearing the ovules. On the upper surface of the larger scale, two ovules lie side by side, inside of which female gametes are formed. Pollination occurs as early as the first year of cone development, but Fertilization is delayed until the following spring when pollen tubes grow. Winged seeds develop from the fertilized ovules. Seed maturation continues through the second year, and they are not shed until the third year. By this time, the cone has become relatively large and woody, and the scales bend outward, exposing the seeds before they are dispersed by the wind.