2.6.2.2: Conifers and Gnetophytes

Learning Objectives

• Use morphological features and life history traits to distinguish conifers from other plants.
• Connect the adaptations of conifers to dry and/or cold environments.
• Identify structures and phases in the Pinus life cycle; know their ploidy.
• Use morphological features and life history traits to distinguish gnetophytes from other plants.
• Describe the traits gnetophytes share with angiosperms.

Though the gnetophytes have been difficult to place, phylogenetically, recent genetic studies place them as sister to the Pinaceae (pine family, emerging from within the conifers. See this open-access paper for recent genetic work on the evolutionary relationships between gymnosperms.

Conifers

Conifers are the most species-rich lineage of gymnosperms. From the fossil record, we think there were over 20,000 species of conifers. However, their diversity declined with the dinosaurs. Currently, there are around 600 extant species. These amazing plants represent some of the oldest, tallest, and most massive organisms on the planet. Though currently low in diversity, these amazing plants make up 30% of Earth’s forests.

Conifers are the most widely known and economically important among gymnosperms. Conifers include the largest and the oldest of all living organisms. One redwood (Sequoia sempervirens) growing in California is almost 400 feet (122 meters) high. Bristlecone pines (Pinus longaeva, Figure \(\PageIndex{1}\)) growing in the mountains of eastern California some are more than 5,000 years old. Giant sequioas (Sequiadendron giganteum) Most of them are temperate evergreen trees, but some are deciduous, such as larch (Larix) and the dawn redwood (Metasequoia). The stem has a large amount of xylem, a small cork, and minute pith. Seeds are distributed by wind and animals.

Figure \(\PageIndex{1}\): A bristlecone pine in the White Mountains of California. Gnarly, CC BY-SA 2.0, via Wikimedia Commons

Note: The Pinaceae is currently the largest family of conifers, so many of the examples for this group of gymnosperms will be from the type genus Pinus (pines).

Xerophytic Leaves in Conifers

Xerophytic leaves are adapted to withstand drought conditions. In conifers, we see a wide range of xerophytic leaves with different morphologies that can be shaped by their local environment. Consider the leaves of the coast redwood and the giant sequoia (Figure \(\PageIndex{2-3}\)). Though these two trees belong to different genera--Sequoia and Sequoiadendron, respectively--they are sister taxa. However, the coast redwood has adapted to life on the coast, where the giant sequoia has evolved in inland, higher elevation forests with much more extreme climatic conditions. How can this be seen in the structure of their leaves?

Figure \(\PageIndex{2}\): The image on the left shows the adaxial (upper) side and the image on the right shows the abaxial (under) side of coast redwood leaves. Note that, particularly in the photo on the left, the leaves look shiny. This is due to the presence of a thick cuticle. The stomata are also concentrated on the underside of the leaf, as indicated by the black arrow in the image on the right. They are clustered in two silvery rows on each leaf, called a stomatal bloom. However, for xerophytic leaves, these are relatively flat, with a fairly standard surface area to volume ratio. Photos by Maria Morrow, CC-BY 4.0.

Figure \(\PageIndex{3}\): These giant sequoia leaves were retrieved from a tree planted in a coastal environment, so the differences here represent evolutionary history, alone. Much like the coast redwood, these shiny leaves have a thick cuticle. The big difference to note is the thickness of these leaves, maintaining a much lower surface area to volume ratio than the coast redwood. The second thing to note is the orientation of the leaves. They are held much more closely together, reducing exposure to the wider environment. Two stomata are indicated in the image, though there are many. They are present on both adaxial and abaxial leaf surfaces. Photo by Maria Morrow, CC-BY 4.0.

Figure \(\PageIndex{4}\)), a thick cuticle surrounds the outside, coating the epidermis. Beneath the epidermis are several layers of tightly packed, small cells: the hypodermis. Stomata are sunken, located within the hypodermis. Under the hypodermis are the highly invaginated mesophyll cells. Further to the inside is a suberized ring of cells called the endodermis, just like in the root, which surrounds the vascular tissue. Transfusion tissue is located between the endodermis and the vascular tissue.

There are also resin canals that ring the needle, appearing as holes surrounded by small cells. These secrete resin to protect the plant. Resin is a sticky fluid rich in compounds that protect the plant. It flows through canals in the stems, roots, and leaves and can rush to fill a wound. The resin can gum up the mouth parts of herbivorous insects, offer chemical defense against pathogenic bacteria and fungi, and harden to close the wound (much like a scab).

Figure \(\PageIndex{4}\): Most pine needles you see in botany are flat on one side, however, they also come in round. The tissues of this xerophytic leaf are labeled in the diagram. The epidermis is a single layer on the outside, coated by a thick cuticle. Under the epidermis, there are several layers of similar small, tightly packed cells (hypodermis). Within the hypodermis region, there are sunken stomata. Resin canals look like large holes and are present periodically around the cross section near the hypodermis. The highly invaginated mesophyll cells are between the hypodermis and endodermis. The endodermis is a single ring of cells that surrounds the vascular tissue. Within the endodermis, there is a single vascular bundle surrounded by transfusion tissue. Image by Fayette A. Reynolds M.S., Public Domain with labels added by Maria Morrow.

Reproduction in Conifers

Unlike other gymnosperms, conifers are monoecious, meaning megastrobili and microstrobili are produced on the same plant. In general, megastrobili tend to be larger and longer-lived, while microstrobili are smaller and ephemeral, disintegrating after pollen is dispersed (see Figure \(\PageIndex{5-6}\)). Some conifers, like junipers (Juniperus) and yews (Taxus), lack woody cones and have fleshy scales. In all, conifer life cycle takes up to two years. Conifers do not have flagellate spermatozoa; their non-motile male gametes (spermatia) move inside long, fast-growing pollen tube.

Figure \(\PageIndex{5}\): The two images above show the strobili of the genus Pinus. On the first image, a seed cone from a Jeffrey pine does not have visible bracts but it does showcase a pointy umbo (B) at the tip of the ovuliferous scale (A). Dual markings where two seeds developed on each scale (C). Pines tend to form woody megastrobili (seed cones) that decay slowly, the one pictured is likely over a year old. In contrast, the second image shows a cluster of microstrobili. They are small with papery scales that will fall off after the pollen has dispersed. First photo by Maria Morrow, CC-BY 4.0. Second photo by Beentree, CC BY-SA 3.0.

The female cones are larger than the male cones and are positioned towards the top of the tree; the small, male cones are located in the lower region of the tree. Because the pollen is shed and blown by the wind, this arrangement makes it difficult for a gymnosperm to self-pollinate.

Figure \(\PageIndex{6}\): This image shows the life cycle of a conifer. Pollen from male cones blows up into upper branches, where it fertilizes female cones. Examples are shown of female and male cones. Descriptive text: The conifer life cycle begins with a mature tree, which is called a sporophyte and is diploid (2n). The tree produces male cones in the lower branches, and female cones in the upper branches. The male cones produce pollen grains that contain two generative (sperm) nuclei and a tube nucleus. When the pollen lands on a female scale, a pollen tube grows toward the female gametophyte, which consists of an ovule containing the megaspore. Upon fertilization, a diploid zygote forms. The resulting seeds are dispersed, and grow into a mature tree, ending the cycle. Both the male and female cone are made up of rows of scales, but the male the female cone is round and wide, and the male cone is long and thin with thinner scales. (credit “female”: modification of work by “Geographer”/Wikimedia Commons; credit “male”: modification of work by Roger Griffith)

Seed Cones

The megastrobilus, or seed cone, is composed of spirally arranged megasporophylls called ovuliferous scales (Figure \(\PageIndex{7}\)). Each scale produces two megasporangia, which contain a diploid megasporocyte (also called a megaspore mother cell). Each megasporocyte undergoes meiosis. Only one of the four cells produced will survive to develop into a haploid megagametophyte and the other three will die. The megagametophyte is part of the ovule and contains archegonia, each with an egg cell inside (Figure \(\PageIndex{8}\)). The megagametophyte is retained within the megasporangium, which becomes the nucellus. Surrounding the nucellus is the integument, which is initially continuous with the ovuliferous scale and has a small opening called a micropyle.

Figure \(\PageIndex{7}\): A micrograph of a longitudinal section through a yearling Pinus megastrobilus, labeled as follows: A=Ovuliferous scale, B=Megasporangium, C=Cone axis. Many ovuliferous scales surround the cone axis. Each scale has a megasporangium at its base, where it connects to the axis. Scale=1.0mm. Image by Jon Houseman, CC BY-SA 4.0, via Wikimedia Commons.

Figure \(\PageIndex{8}\): The first image is a micrograph of a Pinus ovule, labeled as follows: A=Gametophyte, B=Egg cell, C=Micropyle, D=Integument, E=Megasporangium. The integument lines the outside with a small hole at the tip (micropyle). Inside the integument is the megasporangium. The megasporangium surrounds the gametophyte, which has two egg cells within it, on the end closest to the micropyle. Scale=0.8mm. The second image is a closer view, showing the eggs within the megagametophyte, labeled as follows: A=Egg cell, B=Egg nucleus. The eggs are large and one has a nucleus visible. Scale=0.31mm. Images by Jon Houseman, CC BY-SA 4.0, via Wikimedia Commons.

Pollen Cones

Microstrobili (pollen cones) are formed from overlapping microsporophylls that bear multiple microsporangia. Within the microsporangium, there are microsporocytes (also called microspore mother cells), diploid cells that undergo meiosis to produce haploid microspores. Microspores grow by mitosis into microgametophytes, AKA pollen, within the microsporangium. The microgametophyte in gymnosperms is the four-celled, "winged" pollen grain.

Within the pollen grain (Figure \(\PageIndex{9}\)), there is a generative cell, a tube cell, and two prothallial cells. On either side of the pollen grain, two wing-like structures called air sacs may help orient the pollen grain toward the ovule.

Figure \(\PageIndex{9}\): The first image is a long section through a pine microstrobilus that shows inside a microsporangium. Each microsporangium is produced on a leaf-like structure called a microsporophyll that emerges from the cone axis. Within the microsporangium, many mature pollen grains are visible. You can tell the pollen grains are mature at this magnification by the presence of the air sacs. In the second image, we see a closer image of pollen grains. The two ear-like air sacs flank the four cells enclosed in the center: the tube cell, generative cell, and two prothallial cells. The tube cell takes up most of the internal space and its nucleus is about the same size, if not a little bigger, than the entire generative cell. The two prothallial cells are squished against the edge opposite the air sacs and are not really distinguishable in this image. Photos by Maria Morrow, CC-BY 4.0.

A grain of pollen will be transported on the wind and, if lucky, it will land on a seed cone. The seed cone has a drop of sugary liquid (a pollen drop) that it secretes, then retracts, pulling the pollen in toward the ovule. This stimulates the tube cell to germinate a pollen tube, while the generative cell divides by mitosis to produce two spermatia (no flagella). These spermatia travel with the pollen tube, through the micropyle, and into an archegonium where one will fertilize an egg (Figure \(\PageIndex{10-11}\)). It takes approximately one year for the pollen tube to grow and migrate towards the female gametophyte! When fertilization occurs, the micropyle closes and the integument becomes the seed coat.

Figure \(\PageIndex{10}\): These series of micrographs shows male and female gymnosperm gametophytes. (a) This male cone, shown in cross section, has approximately 20 microsporophylls, each of which produces hundreds of male gametophytes (pollen grains). (b) Pollen grains are visible in this single microsporophyll. (c) This micrograph shows an individual pollen grain. (d) This cross section of a female cone shows portions of about 15 megasporophylls. (e) The ovule can be seen in this single megasporophyll. (f) Within this single ovule are the megaspore mother cell (MMC), micropyle, and a pollen grain. Descriptive text: Part a shows a cross section of a male cone, which is oval with a flattened bottom. A stem-like structure runs up the middle, and oblong microsporophylls radiate from either side. Migrograph b shows a blowup of a microsphorphyll, which is filled with round pollen grains. Micrograph C shows a blowup of a pollen grain, which is oval with two lobes and resembles Mickey Mouse. Part D shows a cross section of a female cone, which is similar in shape to the male cone but with a wider central structure. Megasporophylls radiate from either side of this structure. At the base the megasprophylls are narrow, and the widen out into a roughly diamond shape. Part E shows a blowup of the megasprophyll, which has an oval ovule at its base. Part F shows a blowup of the ovule. The megaspore mother cell is at the center. An opening called a micropyle allows entry of the pollen tube from the base. (credit: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Figure \(\PageIndex{11}\): The megastrobilus before (left) and after (right) fertilization. In the unfertilized cone on the left, there is an ovule on top of each ovuliferous scale. In the fertilized cone on the right, the ovule has become a seed, the cone has become woodier, and the sterile bract has been reduced. Image by Nefronus, CC BY-SA 4.0, via Wikimedia Commons.

The zygote will grow and develop as an embryo within the seed, nourished by the megagametophyte tissue, as well as the nucellus (Figure \(\PageIndex{12}\)). The scales of the cones are closed during development of the seed. Seed development takes another one to two years. Once the seed is ready to be dispersed, the bracts of the female cones open to allow the dispersal of seed; no fruit formation takes place because gymnosperm seeds have no covering. The seed will be dispersed by wind or animals and germinate to grow into a diploid pine tree once again.

Figure \(\PageIndex{12}\): A longitudinal section through a fertilized pine seed. The embryo is developing within the integument, labeled as follows: A=Root cap, B=Integument, C=Hypocotyl-root axis, D=Shoot apical meristem, E=Cotyledons. The integument forms the outer layer, surrounding the embryo. The embryo has a root cap at one end, connected to the shoot apical meristem by the hypocotyl-root axis. Two arm-like cotyledons flank the shoot apical meristem. Scale=1.3mm. Photo by Jon Houseman, CC BY-SA 4.0, via Wikimedia Commons.

The Full Life Cycle

Video \(\PageIndex{1}\) is an extremely helpful narrated animation of the pine life cycle. You should watch this video or some other walkthrough of the pine life cycle before attempting to interpret the complex diagram (Figure \(\PageIndex{13}\)).

Video \(\PageIndex{1}\): A narrated video of the pine life cycle, sourced from Youtube.

Figure \(\PageIndex{13}\): This is an illustrated diagram of the pine life cycle. Starting at the bottom of the image, there is a pine tree. This is the mature sporophyte (2n). It will produce both seed cones and pollen cones because it is monoecious. Seed cones will produce two megasporangia on each ovuliferous scale, each one surrounded by an integument. Within the megasporangium, a megaspore mother cell (2n) undergoes meiosis to produce four megaspores (n). Three die and one remains, developing by mitosis into the megagametophyte. The megagametophyte produces two archegonia, each with an egg. This trajectory occurs through the center of the diagram. Jumping back to the pollen cone, microsporangia are produced on each microsporophyll of the pollen cone. Initially, they are filled with microspore mother cells (2n) that then undergo meiosis to produce microspores (n). These microspores grow by mitosis (though only two rounds) into 4-celled microgametophytes: pollen. Each pollen grain is composed of two ear-like air sacs on the outside, a large tube cell, a small generative cell, and two tiny prothallial cells. The mature pollen grains are dispersed on the wind to seed cones where the tube cell will form a pollen tube into the megagametophyte by entering through a gap in the integument called the micropyle. The generative cell divides to produce to sperm cells, which travel down the pollen tube to fertilize an egg. Once fertilized, the integument closes, forming the seed coat. The embryo develops within the seed, consuming the megagametophyte and the megasporangium (now called the nucellus) as it grows. The seed is dispersed and, if in the right conditions, the embryo emerges from the seed coat and develops into a mature sporophyte. Diagram by Nikki Harris CC BY-NC with labels added.

Gnetophytes

Gnetophytes are a small group with only three genera that, excepting from their opposite leaves, seem not at all similar: Ephedra, Welwitschia, and Gnetum. Ephedra are horsetail-like desert leafless shrubs, Gnetum are tropical trees, and Welwitschia are strange desert plants that form two large, continuously growing leaves (Figure \(\PageIndex{14}\)). Gnetophytes represent an anatomically and genetically difficult group to classify. They have several traits in common with angiosperms, such as vessel elements in the xylem, double fertilization, and a covering over their seeds. Even their leaves are angiosperm-like, with netted venation. However, these traits are convergently evolved, meaning that angiosperms and gnetophytes each evolved these traits separately. Genetically, recent studies have placed the gnetophytes as a sister group to the Pinaceae (pine family) within the conifers. This would mean that pines, firs, and spruces are more closely related to strange gnetophytes like Ephedra than they are to other conifers like redwoods, cedars, and Pacific yew. However, the true nature of this evolutionary relationship remains murky and contentious.

Figure \(\PageIndex{14}\): These three genera of plants represent the gnetophytes: Ephedra (left), Gnetum (center two) and Welwitschia (right). Ephedra has thin stems with scale-like leaves. Gnetum has large, flat leaves on very angiosperm-like plants. Welwitschia has just two leaves emerging opposite each other, no stem is visible.

Ephedra has archegonia, but in Gnetum and Welwitschia they are reduced. On the other hand, Ephedra and Gnetum have double fertilization, a process that you will see in angiosperms where both male nuclei fuse with cells of the one female gametophyte. Double fertilization in gnetophytes results in two competing embryos, and only one of them will survive in the future seed.

Both Gnetum and Welwitschia have vessel elements (like angiosperms). Gnetum also has angiosperm-like opposite leaves with netted venation, like the coffee tree (however, this probably is a result of modification of dichotomous venation). Ovules are solitary and covered with an additional outer integument; the male gametes are spermatia moving with the pollen tube instead of swimming (no flagella).

Welwitschia is probably most outstanding among gnetophytes. There is only one species and it occurs only in the Namibian desert. The best way to describe this plant is an “overgrown seedling.” It has a small trunk with two wide leaves that have parallel venation. The secondary thickening is anomalous, the wood has vessels. This plant is insect-pollinated and its winged seeds are dispersed by the wind. Fertilization is not double, but, along with pollen tubes, involves some interesting structures: prothallial tubes which grow from female gametophyte and meet with pollen tubes.

Characteristics of Gnetophytes

• Angiosperm-like features: vessel elements, double fertilization, fruit-like ovule coverings
• Dioecious. Female plants have covered ovules, while male plants have pollen cones.
• Leaves xerophytic with opposite arrangement
• Primarily insect pollinated; brightly colored seeds are dispersed by birds

Welwitschia mirabilis

This strange plant grows in the desert of Namibia. It has two large leaves that grow from a basal meristem. As the plant gets older the leaves split and start to look like numerous long tentacles (Figure \(\PageIndex{15}\)). The tips of the leaves are ragged, as these are the oldest parts. The leaves are shiny and the setting is dry, indicating their xerophytic nature. In the center, where the two leaves meet, plants will produce either megastrobili or microstrobili (Figure \(\PageIndex{16}\)).

Figure \(\PageIndex{15}\): A Welwitschia at the petrified forest, west of the town of Khorixas, Namibia. Photo by Nanosanchez, CC BY-SA 3.0, via Wikimedia Commons.

Figure \(\PageIndex{16}\): Strobili of Welwitschia mirabilis. Megastrobili (first photo) are larger and rounder, composed of tightly overlapping megasporophylls. Microstrobili (second photo) are smaller and thinner than the megastrobili. First image by C T Johansson, CC BY-SA 3.0, via Wikimedia Commons. Second image by Krzysztof Ziarnek, Kenraiz, CC BY-SA 4.0, via Wikimedia Commons.

Ephedra

Ephedra spp. have scale-like, opposite leaves produced on tough, photosynthetic stems. They produce swollen megastrobili that look like fruits (Figure \(\PageIndex{17}\)), and microstrobili have extruded microsporangia, making them look like catkins (a type of inflorescence produced by some angiosperms, (Figure \(\PageIndex{18}\)). Some Ephedra species produce alkaloids that have been extracted for stimulant use, including ephedrine and pseudoephedrine.

Figure \(\PageIndex{17}\): A thin stem of Ephedra showing opposite leaf arrangement of two scale-like leaves. Megasporophylls are swollen and red, making the megastrobilus appear fruit-like. Seeds are produced within these structures. Descriptive text: Thin green stems end in swollen red structures that appear to be separated into a few different segments. Two tiny, scale-like leaves emerging from either side of a node. Photo by Germán Escorza, CC BY-SA 4.0, via Wikimedia Commons.

Figure \(\PageIndex{18}\): Microstrobili of a male Ephedra viridis. These small structures look like inflorescenses with anthers emerging. From between the microsporophylls, branching structures emerge, topped with microsporangia releasing pollen. Photo by Stan Shebs, CC BY-SA 2.5, via Wikimedia Commons.

Gnetum

Gnetum spp. could easily be mistaken for flowering plants. They produce leaves with netted venation and fruit-like megastrobili. These plants are restricted to tropical areas and generally take on a tree-like habit.

Figure \(\PageIndex{19}\): Gnetum strobili. The leaves have a larger central vein with smaller, branching side veins (netted venation). In the first photo, a "female" plant has megastrobilus is covered with swollen structures that look like berries. These will eventually redden, increasing the berry similarity. In the second photo, a thin microstrobilus is shown on a "male" plant. These plants look very much like angiosperms and very little like gymnosperms. First photo by BotBln, CC BY-SA 3.0, via Wikimedia Commons. Second photo by Kembangraps, CC BY-SA 3.0, via Wikimedia Commons.

Attribution

Curated and authored by Maria Morrow using the following sources:

• 19.1.5 Diversity and Evolutionary Relationships of the Plants from Biology by John. W. Kimball (licensed CC-BY)
• 26.2 Gymnosperms from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.
• 7.6 Spermatophyta – Seed Plants from Introduction to Botany by Alexey Shipunov (public domain)​

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