Botany - B.Ye. Yakubenko 2017

Part Three. Plant Kingdom (Planthae)
Chapter VIII. Division Pinophyta, Pine Family — Pinophyta
8.1. General characteristics of gymnosperms, classification

Gymnosperms differ from the preceding groups of archegoniate plants by the presence of ovules from which seeds are formed. Thus, the propagule that initiates the individual's developmental cycle is a seed containing a sporophyte embryo, rather than a spore, as observed in ferns.

An ovule is a sporangium modified during evolution, furnished with an integument and enclosing the female gametophyte (primary endosperm), which develops from a megaspore inside the sporangium. The ovule consists of an integument and a nucellus containing an archesporial Cell. This cell undergoes Meiosis to yield four megaspores; three of them degenerate, while the remaining one divides repeatedly to transform into a multicellular female gametophyte with archegonia at its apex. Ovules are formed openly on the seed scales of the female strobilus (cone), hence the name of the division — Gymnosperms.

All gymnosperms are heterosporous plants; their sporophytes are trees and shrubs with a well-developed ROOT system and a woody stem possessing a cambium. The xylem consists exclusively of tracheids, which perform both conducting and mechanical Functions. Stem branching is monopodial, and there are micro- and macrophyllous evolutionary lines. In most representatives, leaves are perennial, and resin canals are present within the Organs.

Spores are formed in strobili consisting of an axis and modified sporophylls. Microspores produced within microsporangia germinate right there into a reduced male gametophyte — pollen. The female gametophyte develops inside the nucellus and does not come into contact with the external environment; therefore, gymnosperms are classified as endoprotallic (endosporic) plants.

The Development of the male gametophyte is completed on the ovule, and the Fertilization process is independent of a liquid-drop medium. Male Gametes are transported to the archegonia via a pollen tube.

The seed that arises after fertilization is a multicellular, distinct part of the maternal sporophase. It consists of the primary endosperm (female gametophyte), the embryo, and a seed coat formed from the integument. The nucellus is absorbed by the embryo.

The appearance of gymnosperms in the course of evolution is associated with plant adaptation to conditions of insufficient moisture. The increasing continentality of the climate at the end of the Paleozoic contributed to the decline of spore plants. As a result of natural Selection, a group of plants emerged featuring internal fertilization and robust protection for the embryo and seed. Gymnosperms are divided into six classes, each representing a specific stage in the evolution of seed plants.

Class Seed Ferns, or Lyginopteridopsida (Lyginopteridopsida), comprises fossil plants that appeared in the Late Devonian and reached their peak development in the Carboniferous period. They are represented by trees with pinnately compound leaves. Microsporophylls are dissected into spore-bearing and sterile segments. The male gametophyte develops inside the microspore, and the spermatozoids are motile. Ovules were located at the tips of the leaves and possessed a pollen chamber. The female gametophyte was multicellular with three archegonia. No seed embryo has been detected. The discovery of these plants helped establish phylogenetic links between ferns and gymnosperms.

Class Cycadopsida (Cycadopsida) comprises tropical and subtropical plants that include 10 genera and 130 species. Species of the genera Zamia and Cycas are common in the evergreen forests and savannas of Africa. These are trees or epiphytes. The pith of the stem contains abundant starch. Cycads are dioecious plants. Male strobili are formed at the apex of the stem, and the pollen is tricellular. Megasporophylls alternate with vegetative leaves and bear six ovules. The seed has a fleshy seed coat and an embryo with two cotyledons.

Class Bennettitopsida (Bennettitopsida) consists of fossil plants that were widespread from the Triassic to the Cretaceous period. A distinctive feature of bennettites is the bisexual axillary cone (strobiluѕ). British paleobotanists Arber and Parkin regarded the strobilus of bennettites as the prototype of a flower.

Class Ginkgopsida (Ginkgopsida). The sole representative of this class is the relict plant Ginkgo biloba. It is a tree with fan-shaped leaves featuring dichotomous venation. The plant is dioecious; male strobili are catkin-like, whereas female strobili consist of a long stalk bearing two ovules, of which only one develops. The seed has a fleshy outer coat. Ginkgo has survived since the Triassic period. It possesses high ornamental value. In its natural state, it occurs in the forests of East Asia, while in cultivation it is widespread almost everywhere and is considered an adornment of botanical gardens.

Class Gnetopsida, or Gnetophytes (Gnetopsida). This class includes three orders: Ephedrales, Welwitschiales, and Gnetales. They all share certain common features: opposite leaves, dichotomous branching of unisexual strobili, the presence of sterile leaves surrounding them that resemble a perianth, and long micropylar tubes on the ovules. In Representatives of the genus Drimys, tracheas are present in the secondary xylem. Based on this convergent similarity, some researchers consider Gnetales to be the ancestors of angiosperms. Typical representatives are jointed pine (Ephedra distachya) for the Ephedrales, and Welwitschia mirabilis for the Welwitschiales.

The Gnetales encompass 40 plant species distributed in the humid tropical forests of Asia, Africa, and South America.

Class Pinopsida, or Conifers (Pinopsida), includes seven orders, 55 genera, and over 550 species of modern plants. The class is divided into two subclasses: Cordaitidae and Pinidae.

Subclass Cordaitidae (Cordaitidae) brings together fossil forms that reached the peak of their development in the Carboniferous period of the Paleozoic. These were tall trees with monopodial branching, long entire leaves, and catkin-like unisexual strobili. They are considered the ancestors of modern conifers.

Subclass Pinidae (Pinidae). Representatives of this subclass are widespread and cover vast areas in Europe, Asia, and America. Nearly 90% of the planet's forests consist of conifers or include them as a major component. In the Northern Hemisphere, conifers mostly form pure stands, whereas in the Southern Hemisphere they are distributed in Australia and South America; relict genera such as redwood (Sequoia), dawn redwood (Metasequoia), Japanese cedar (Cryptomeria), and others have survived along the perimeter of the Pacific Ocean. Conifers are dominated by tall and slender trees that are sometimes true giants of the plant world, such as the giant sequoia (Sequoiadendron giganteum), standing over 100 m tall and 10–12 m in diameter. It is among conifers that the longest-living tree on Earth is found — the bristlecone pine (Pinus longaeva), which reaches an age of 5,000 years.

Branching in conifers is monopodial. In some species, alongside elongated shoots, shortened (dwarf) shoots are also present. Leaves are mostly needle-like or scale-like, perennial, with a homogeneous mesophyll and Stomata deeply sunken into the leaf tissue. The leaves are arranged spirally, less frequently oppositely or in whorls, and exhibit a pronounced xeromorphic Anatomical Structure. The xylem consists 95% exclusively of tracheids. Resin canals are present in the bark, wood, and leaves of most species. Conifer strobili (cones) are unisexual, and the plants are monoecious or dioecious. Female cones possess small bract scales, in the axils of which lie megasporophylls (seed scales) bearing ovules. The microstrobilus (male cone) has an axis to which microsporophylls bearing microsporangia are attached. Different conifer genera have their own structural features and cone arrangements, but the developmental cycle is common to all and can be traced using the Scots pine as an example, as detailed in the practical course (see Laboratory Class 13).

Within the conifer subclass, 7 families are distinguished, a brief description of which is given below.

Family Araucariaceae (Araucariaceae) includes 2 genera and about 40 species, distributed predominantly in the Southern Hemisphere. These are mostly large trees with strobili up to 25–30 cm long, and needle-like or broadly triangular scale-like leaves. Within their natural range, the genera Araucaria and Agathis play a forest-forming role, while in Ukraine they are sometimes cultivated as ornamental plants.

Family Pinaceae (Pinaceae) includes 10 genera and about 250 species distributed in the Northern Hemisphere; many species are forest-forming trees in the forests of Eurasia and North America. These are monoecious, rarely dioecious, trees or shrubs. In some species (spruces, firs, hemlocks), shoots are exclusively elongated, whereas in others (pines, cedars, larches), both elongated and shortened shoots are present. Leaves are needle-like or linear; on shortened shoots they are gathered in bundles, while on elongated shoots they are solitary. Microsporophylls are membranous, with two parallel microsporangia; pollen often possesses air sacs, and microstrobili are solitary or gathered in groups. Female cones are usually solitary or paired, consisting of an axis, bract scales, and seed scales with ovules. They are wind-pollinated.

The most widespread genera are: pine (Pinus), with about 100 species — Scots pine (Pinus sylvestris) and Crimean pine (Pinus pallasiana) grow naturally in Ukraine; larch (Larix), with 20 species — European larch (Larix decidua) occurs naturally in the Ukrainian Carpathians; spruce (Picea), with about 50 species — Norway spruce (Picea abies) grows in Ukraine, and other species are cultivated as ornamentals; fir (Abies), which numbers 45 species, of which only silver fir (Abies alba) grows in Ukraine, forming pure or spruce-mixed forests in the Carpathians.

Family Taxodiaceae (Taxodiaceae) comprises 10 genera and 14 species of plants that were widespread in the Tertiary period of the Cenozoic and have survived to this day in small patches in North America and East Asia. These are the genera Taxodium, Metasequoia, and Cryptomeria. In Ukraine, they are preserved in botanical garden collections.

Family Cupressaceae (Cupressaceae) brings together 20 genera and about 150 species distributed in both hemispheres. These are shrubs and trees with tiny scale-like or needle-like leaves. Male cones are formed at the ends of shoots or in leaf axils. Microsporophylls bear 3–6 microsporangia each, and pollen lacks air sacs. Female cones are woody, leathery, or fleshy. The seeds and bract scales fuse and bear 1–3 ovules. If the female cones had fleshy scales, upon seed maturation they form so-called "galbuli" or cone-berries (e.g., in junipers). The central genus of the family is Cupressus, which encompasses 15 species; Mediterranean cypress (Cupressus sempervirens) is cultivated as an ornamental plant along the Black Sea coast of Ukraine. Widespread in the Northern Hemisphere and in Ukraine is the genus Juniperus, which numbers 70 species, of which 17 species grow naturally in Ukraine and another 15 species are cultivated as ornamentals. North American representatives of the genera Thuja and Biota from this family are also cultivated for ornamental purposes in Ukraine.

Families Podocarpaceae (Podocarpaceae) and Cephalotaxaceae (Cephalotaxaceae) are distributed in the Southern Hemisphere and the mountains of Southeast Asia; some species are cultivated here only in greenhouses.

Family Taxaceae (Taxaceae) includes 20 species distributed in the Northern Hemisphere. This is one of the most ancient families of the class; only a single species grows in Ukraine — English yew (Taxus baccata), which is listed in the Red Data Book of Ukraine.