THE WORLD OF LIFE with Douglas Drenkow

Conifers live on land, especially in cool climates with little rain (although there is often much snow) -- most conifers have needle- or scale-like leaves, less prone to losing moisture than the foliage of "broad-leaved" plants, which is an advantage in areas with little liquid water (snow and ice, as solids, typically above-ground, are less available to plant roots).

OVERALL STRUCTURE

Cell walls, composed primarily of cellulose, give shape to individual cells.

Conifers are typically large trees and shrubs. They have woody roots, trunks, and branches (Trees have one trunk; shrubs have more). Most conifers have needle- or scale-like "evergreen" leaves, often in bunches; and most have woody cones (The cones of a few are fleshy, like berries). Note that the needles or scales of "evergreens" do not persist indefinitely; rather, they tend to be shed and replaced constantly.

ENERGY CAPTURE

Light-energy is captured, for photosynthesis, by chloroplasts within the cells in the leaves.

EXCHANGE OF MATERIALS WITH THE ENVIRONMENT

Water vapor and gases flow especially through "stomata" pores (each regulated by a pair of "guard cells") in leaves. A waxy "cuticle" helps prevent water loss from leaves. Bark helps prevent water loss from woody stems, and it also helps defend the plant against attacks by pathogens, parasites, and predators.

Water with dissolved substances is absorbed especially by the fine "root hairs", at the tips of young, "primary" roots.

INTERNAL TRANSPORT

The stems and roots of conifers, like other "vascular" plants, are composed of various layers and tissues, the arrangement in young, typically fleshy "primary" stems and roots maturing into a different pattern in older, woody "secondary" stems and roots.

Typically in the young, primary stems of conifers, the "epidermis" covers the "cortex", composed primarily of "parenchyma" cells (thin-walled, undifferentiated cells, which store water and food). In the very center of primary stems, there is "pith", somewhat similar to the cortex (For more details, of anatomy similar to conifers, see flowering plants).

Typically embedded within the cortex of the primary stems of conifers, there is a circular arrangement of "vascular bundles", alternating with "pith rays" (of cortex) in between the bundles. Within each bundle, there is primary "phloem" (food-conducting tissue) to the outside, a narrow band of "meristematic" tissue (active in cell division) in the middle, and primary "xylem" (water-conducting tissue) to the inside.

Typically, secondary, woody growth in the stems of conifers is laid down by a cylinder of "vascular cambium", which is formed from the meristematic tissue between the primary phloem and xylem, within each vascular bundle, plus some of the parenchyma cells in the pith rays, between the vascular bundles: In cross-section, the vascular cambium forms a ring inside the stem. To the outside, the vascular cambium forms "secondary phloem", which (with corky tissues produced from "cork cambiums", arising even more to the exterior) forms the bark. To the inside, the vascular cambium forms "secondary xylem", consisting of young, active "sapwood", which eventually matures into inactive "heartwood" (which eventually crushes the central pith, of primary growth). In Temperate climates, the seasonal variation in xylem production can be noted in the "annual growth rings" within the wood.

The primary and secondary growth of the roots of conifers is somewhat different from that of the stems.

Typically in the young, primary root of conifers, there is a "vascular cylinder" in the center of the root, instead of pith. In cross-section, the primary xylem, within the vascular cylinder, looks like a star; in between the "arms" of the star is the primary phloem, with a thin layer of meristematic tissue separating the xylem from the phloem. There is also a thin layer of cells, the "pericycle", encircling the vascular cylinder and lying just to the inside of the "endoderm" (the innermost layer of the cortex, filtering materials entering the vascular cylinder).

Typically, secondary, woody growth of the roots of conifers is laid down by a cylinder of vascular cambium, formed from the meristematic cells in between the primary phloem and xylem plus some of the cells in the pericycle (namely, those lying at the tips of the primary xylem "arms"). As in the secondary growth of the stem, the vascular cambium forms secondary phloem to the outside, which (with corky tissues from cork cambiums, arising even more to the exterior) forms the bark; and to the inside, the vascular cambium forms the secondary xylem, the wood.

The food-conducting phloem tissue in conifers includes "sieve cells". Like "sieve tube members", of flowering plants, the sieve cells of conifers are alive; but unlike the sieve tube members of flowering plants, the sieve cells of conifers and not connected end-to-end via perforated "sieve plates" into "sieve tubes" and are not accompanied by "companion cells".

The water-conducting xylem tissue in conifers typically includes non-living "tracheids" (communicating via "pit pairs" in their side walls) but no "vessel elements" (connected via perforations in their end walls into water-conducting "vessels").

In healthy trees, a copious production of "resin" (carried by resin ducts) and of sap (the mineral-rich, watery solution carried by the xylem and/or the food-rich, watery mixture carried by the phloem) can kill or repel invading pests, such as wood-boring grubs.

DEVELOPMENTAL CONTROL

The growth and development of conifers is under genetic and undoubtedly hormonal control.

ASEXUAL REPRODUCTION

Conifers can reproduce asexually, via vegetative body parts.

SEXUAL REPRODUCTION

In conifers, as in other plants, there is an "alternation of generations" in the life cycle, between "diploid" forms (with both sets of chromosomes) and "haploid" forms (with just one set of chromosomes). As in other higher plants, the haploid "gametophytes" (producing the "gametes", sperms and eggs) are dependent upon the dominant, diploid "sporophytes" (the typical plant bodies).

Although some conifers are "dioecious" (with separate sexes -- that is, with male and female plants), most conifers are "monoecious" (producing both sperms and eggs, typically in separate cones).

Typically, "staminate" (male) cones are small, borne on the lower branches, and composed of many staminate scales ("microsporophylls", modified leaves). On the underside of each scale in a male cone are borne a pair of capsule-like "microsporangia". Within each microsporangium, many "microspore mother cells" each produce four haploid "microspores", by "meiosis" (cell division that cuts the number of chromosomes in half). Within each microspore, two haploid nuclei (and the remnants of other cells) are formed, by "mitosis" (cell division without a reduction in the number of chromosomes): The resulting "pollen grains" (the young male gametophytes), typically hard-shelled and winged, are released to be dispersed by the wind.

Typically, "ovulate" (female) cones are large, borne on higher branches than the male cones (thus discouraging self-pollination), and composed of many woody "ovuliferous" scales ("megasporophylls", modified leaves). On the upperside of each scale in a female cone are borne a pair of "ovules". Each ovule is composed of "nucellus" tissue (the "megasporangium") covered by a protective "integument". At the inside end of the integument there is a "micropyle" opening, into a "micropylar chamber". Within the nucellus, one to several "megaspore mother cells" each produce four haploid "megaspores", by meiosis. Three of these megaspores typically die; the surviving megaspore develops into the generally round female gametophyte (completely embedded within the nucellus, which is covered by the integuments, of the ovule). As the female gametophyte develops, two or more reduced "archegonia" typically develop within its end nearest the micropylar chamber (from which they are separated by some nucellus tissue).

Pollen grains that land within the scales of the ovulate cones are caught
by a "glue" secreted by the ovule and are drawn into the micropylar chamber. There, each grain germinates into a "pollen tube" (the mature male gametophyte), which grows through the nucellus and into an archegonium, as it forms two "sperm nuclei" (Note that unlike in lower plants, the gametophyte does not produce an "antheridium", a separate structure for the production of sperms). The sperm nuclei are then released into the egg cell within the archegonium (Note that unlike in lower plants, the sperms are not flagellated -- they do not swim through water, environmental or otherwise, in order to reach the eggs). One of the sperm nuclei fertilizes the egg; the other contents of the pollen tube decompose. As the diploid "zygote" (fertilized egg) develops into an embryo sporophyte, an attached "suspensor" pushes it well within the also enlarging female gametophyte (which will serve as food for the seedling when it germinates, after a period of dormancy). The nucellus becomes a thin, food-storing "perisperm", surrounding the female gametophyte; and the integuments develop into a papery covering and a hard, sometimes winged coat around this true, "naked" seed (termed "naked" because it is not borne within a fruit, as in flowering plants).

Atypically, junipers bear seeds in "berries", which are actually ovulate cones with fleshy scales; and yews bear seeds not in cones but in a berry-like pulp, which develops from the integuments -- although these fleshy structures can function like true fruits in attracting hungry animals, to spread the seeds within, there are no true "ovary" walls, as in flowering plants.

The seedling of a conifer typically bears many "cotyledons" (seedling leaves).

The entire life cycle of a conifer often takes years.

Green Plants (Viridaeplantae)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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