JIB 115 Plant Diversity assignment 2

Debbra Marcel JP/8544/13

1 JIB 115 Plant Diversity: Assignment 2

Gymnosperms: The Naked Seed Plants

Seed is known as a significant remark for adaptation of the terrestrial plants. It is a

reproductive structure which consists of embryo that protected with seed coat. There are two

major monophyletic groups of seed-bearing plants recognized: the gymnosperms and the

angiosperms. The term “gymnosperm” is derived from ancient Greek words means the “naked-

seed” where the seeds develop on the surface of the reproductive structures. Gymnosperms

were evolved from a group of fossil plants which believed to be the first seed producing plants

that dominated many Mesozoic terrestrial ecosystems called the seed ferns (pteridosperms).

Unlike gymnosperms, the seeds and ovules of flowering plants (angiosperms) are protected by

a layer of tissue called a carpel which consists of stigma, pistil and ovary. In comparison,

gymnosperms have cones or strobili, while angiosperms have flowers.

There are four major phyla of plants that make up the gymnosperms: Pinophyta,

Ginkgophyta, Cycadophyta and Gnetophyta. Most studies put cycads and ginkgo as the

simplest of a gymnosperm evolutionary tree while conifers and gnetales as more evolutionarily

advanced. All the four groups are considered as heterogeneous group of plants and

characterized by the production of the ovule (which becomes the seed) that is naked

(unprotected) prior to fertilization. The key features of a typical gymnosperm life cycle are the

dominance of the sporophyte generation, the development of seeds from fertilized ovules, and

the role of pollen in transferring sperm to ovules. In the alternation of generations, the

sporophyte phase of gymnosperms is mainly tree or shrubs with few vine-like species whereas

the gametophyte (gamete-bearing phase) is relatively short-lived and develop on the

sporophyte.



Although gymnosperms are similar in not having their seeds enclosed in ovaries and

fruits, their seed-bearing structures vary greatly. For instance, cycads have large cones,

whereas some gymnosperms, such as ginkgo and gnetum, have small cones that look

somewhat like berries, even though they are not fruits. Their leaves shape also varies greatly,

from the needles of many conifers to the palm-like leaves of cycads to gnetum leaves that look

like those of flowering plants.

A. Phylum Pinophyta or Coniferophyta (the conifers)

Phylum Pinophyta or Coniferophyta (from the Latin conus, cone, and ferre, to carry) is

the largest and the most conspicuous group of gymnosperms, containing 7 families and more

than 600 species of conifers. The tallest, thickest and oldest living trees in the world are all

conifers. The largest genus of conifers is Pinus (pines), which more than 100 living species

identified develop the coniferous forests. Bristle-cone pines known as the oldest living

organism and mainly occur in White Mountain of California. Other conifers include California

nutmeg (Torreya california), yews (seed with fleshy, cup-like covering called aril), podocarps

(ornamental trees), junipers and redwoods (e.g Giant Redwoods = Sequoia giganteum and

Coastal Redwoods = Sequoia sempevirens).

The pine cone is best known as representative of gymnosperm reproductive structures.

Even though known as a highly variable group, the pollen cones of conifers are always simple,

that is, the organs that produce pollen are attached directly to the cone axis without other

associated leaves or bracts. Conifer seeds generally developed inside a protective cone called a

strobilus. The cones require four months to three years to reach maturity and vary in size from



2 mm to 600 mm long. Many conifers posses resin canals which secrete scented resin to

protect the tree against fungal infection and insect infestation. The sizes of mature conifers are

varying from less than one meter to over 100 meters.

The ever-present thin and needle-like leaves (typically occur in clusters of two or more)

of conifers allow them to conserve moisture thus enable them to do so well in Northern

Hemisphere. The presence of hypodermis, thick cuticle, sunken stomata and no air space

system in the mesophyll of the pine leaves enable them to adapt the harsh environments (dry

season, windy season and even in cold winter months). Unlike the hardwood of broadleaf tree,

the wood of pines is classified as softwood due to the absence of vessel members or fibers and

only tracheids are found. The heavy-duty xylem (where annual rings can be seen by transverse

section) and thick bark allows these kinds of trees to grow large and very tall (e.g Giant

Redwoods). The unique root system of pines also associated with fungus called mycorrhizae.

Most conifers are monoecious, wind-pollinated, evergreen woody plants and have

monopodial growth form with strong apical dominance. The pine life cycle is called

heterospory (bears two types of reproductive structures) as ovulate cones (female cones)

produce megaspores and pollen cones (male cones) produce microspores. The sporophyte-

dominance of pine consists of an underground network of roots supporting the trunk which

bears whorls of branches, whereas the reduced gametophyte develops from the megaspore

which solely depends on sporophyte for nutrition. In each scale of the male cones of pines,

structures called microsporangia bear microspore mother cell (microsporocyte) which produces

microspores that will develop into yellowish pollen grain through meiosis. Meanwhile, the

female cones of pines posses structures called megasporangia which bear microspore mother



cell (microsporocyte) and produces megaspores that will develop into archegonia and eggs

through meiosis. The released haploid pollen will be carried by the wind to the female cones

and are drawn into a tiny opening on the ovule called the micropyle. Then, a pollen tube

formed and seeks out the haploid female gametophyte to fertilize the eggs. When fertilization

success, diploid embryo (seed) developed and drops out of cone onto the ground or disperses

via wind or animals. Finally, the seed germinates and seedling will turn into a developing

sporophyte. When the plant is mature, the adult plant produces cones and the cycle

continues.

B. Phylum Cycadophyta (the cycads)

Cycads are the next largest group of gymnosperms after the conifers. These members of

phylum Cycadophyta have cylindrical, unbranched trunk with the leaves all bunched together in

a crown at the top of the plant that look somewhat like palms, but eventually the plant gets a

big cone in the center for reproduction. Of all the gymnosperm lineages, cycads are considered

as living fossils which occur about 280 million years ago even though there are only about 130

species survive today. The cycad taxa growing today are the survivors of a once very common

across most of the planet and were a prominent plant group in the age of the dinosaurs, but

they have since retreated to the tropics and sub-tropics such as Mexico, Cuba and Africa. The

common well-known cycad is that typically grown as a house plant called sago palm (Cycas

revoluta).

Cycads are dioecious and produce seeds on specialized leaves. The cones of cycads are

typically large, with many fertile, leaf-like organs (sporophylls) that are aggregated into cones.



Naturally, both the seed and pollen cones are important to the evolution of the cone in cycads.

The pollen cones of cycads are similar to seed cones, and pollen is born on the lower surface of

scale-like structures. These fertile leaves then evolved into a condensed and simplified form,

the cycad cone. The Cycas cone represents the best example of a naked ovule within the

gymnosperms before pollination as the ovules are mainly exposed to the air. Other members

of Cycadophyta have ovules born on scale-like structures, some with leaf-like structures along

the margin, but many without any leaf-like morphology at all. The leaf-like structure of most

cycads was retained, but in more advanced cycads have further reduction and elimination of

the leafy parts, resulting in the scale-type cones (e.g Zamia sp.). This shows that the cycad cone

and the conifer cone had evolutionarily distinct beginnings.

Cycads are known as slow growing plants. Of all cycads, the genus Cycas produce

megasporophylls that looks lightly like modified vegetative leaves. While others like Zamia and

Dioon produce highly modified megasporophylls in simple strobili (sporangia-bearing structures

densely aggregated along a stem). However, all cycads bear their microsporophylls in simple

staminate strobili. In the initial life cycle of cycads, the mature seed started with the formation

of the ovule with its megaspore mother cell and the microsporangia with microspore mother

cells. The microsporocytes (microspore mother cells) undergo meiosis to form microspores,

with are released as three-celled pollen. Then, the pollen through micropyle and germinates in

the pollen chamber of the ovule. The microgametophyte matures and forms a haustorium

(called a pollen tube), that slowly feeds on the megasporangial wall. Meanwhile, the

megasporocyte (megaspore mother cell) undergoes meiosis and bears three daughter

megaspores which then grow and develop into a megagametophyte.



C. Phylum Ginkgophyta (the ginkgo)

Ginkgo biloba is the only surviving species of the once widely distributed order

Ginkgoales. Also well-known as the Maidenhair Tree or Silver Apricot, the most distinctive

feature of ginkgoes are the deciduous fan-shaped, thick and leathery leaves with parallel veins,

and turn bright yellow in autumn that grow on the plant’s long shoots (typically two-lobed).

Ginkgo trees are considered as very primitive and were first found high in the mountains of

China, all but extinct. Even though ginkgoes are dioecious (separate male and female plants) as

can be seen in cycads, the adult trees of ginkgoes are heavily branched and have a broad

crown. The male trees produce inconspicuous reproductive structure while the female trees

produce plum-like seeds and have rank odor. These plants survive only because of cultivation

as the ginkgo trees are popular as ornamental tree in cities because it tolerates air pollution

well.

The fertile structures in ginkgoes are unique which can be easily distinguished with

either the cones of cycads or conifers. The most common interpretation of the female

reproductive structure of ginkgo is that it is an extremely reduced and the modified branch, so

highly reduced. The female cones (strobili) of Ginkgo biloba are generally thought to have

evolved from a branch, arise at the base of leaves on the short shoots and consist of a stalk and

two terminal ovules (seeds) with a thin fleshy covering. Meanwhile, the male ginkgo cones

(strobili) are simple structures that arise at the base of leaves on the short shoots which bear

many pollen-producing organs along a central stalk. They have longish stalks with lots of pollen

producing organs attached directly to the stalk.



D. Phylum Gnetophyta (the gnetales)

Plants in the phylum of Gnetophyta, called gnetophytes consist of 70 species that found

among only three genera, namely Gnetum, Ephedra, and Welwitschia. Some species live in

tropical region, whereas others live in deserts. Although very different in appearance, they are

grouped together based on molecular data. This order of plants is made up of 3 families;

Ephedraceae, Gnetaceae, and Welwitschiaceae, each with a single genus.

Ephedraceae comprises about 35 species of Ephedra and mostly found in drier regions

of the world. Generally, Ephedra sp. are small, spindly shrubs which make them called as joint

firs, while the leaves are mainly scalelike and all are joined at the base to form a sheath around

the stem. Most species of Ephedra are dioecious and their pollen cones have a pair of bracts at

the base of the cones, which makes these cones compound structures in the same fashion as

the seed cones of conifers. Meanwhile, the female cones are also compound.

Gnetaceae has only one genus called Gnetum which most species are tropical vines,

though one of the most widely studied species called Gnetum gnemon, is a tree that grows up

to 10 meters tall. Since Gnetum can only be naturally found in parts of Asia, South America,

Africa and some pacific Islands, there is no English name for this plant. Their leaves are broad,

flat, and have netlike veins, making them look much more like flowering plants. The cones of

Gnetum are also very different from typical conifer cones and they form fleshy seeds that look

like berries. The Gnetum gnemon for example, are long and have distinct nodes where the

fertile structures are formed. The pollen cones of Gnetum have bracts that cover the nodes,

and underneath these pollen organs are enclosed within two fused structures. Meanwhile, the



seed cone also is on a long axis, with the fertile structures occurring on the nodes. There are

bracts that cover a ring of 8 to 10 ovules which each is surrounded by 3 bract-like structures

that form envelopes around the ovule.

One of the strangest plants on the planet is Welwitschiaceae, which consists of only one

species named Welwitschia mirabilis. It confined only to Namibian desert of Angola and

produces just two huge leaves from a short, woody, unbranched stem. Most of these plants

have unusual appearance. This type of plants survives in the desert by developing a huge

taproot with two-strap-like leaves grow an average of 8 to 15 centimeters (3 to 6 inches) per

year, and often are split and twisted at their ends, forming a tangled mass. The unique features

of Welwitschia mirabilis enable this plant to retain moisture by obtaining water from fog. The

seed cones are similar in design to the pollen cone; the outer bracts are not fused and inner

bracts are long and fused and form an envelope over the ovule in the middle of the apex.

Gnetales especially Gnetum and Welwitschia lend themselves to comparison with

flowers because of having both seed and pollen structures within a single cone. In gnetales,

vessels are presence in the xylem, while all other gymnosperms have tracheids. The presence

of bracts that envelope the ovule also means that the ovule is not necessarily naked, as in the

rest of the gymnosperms. Even so, an equally valid interpretation is the placement of gnetales

within the gymnosperms as sister to the conifers, which makes comparisons of the bracts and

scales of conifers relevant. Significantly, gnetales sit in the phylogeny of seed plants because

their placement affects the evolutionary concepts for all of the shared features of the

gymnosperm cone.



References

1. Campbell, N.A., & Reece, J.B. 2008. Biology. 6th ed. San Francisco (CA): Benjamin

Cummings. p. 1247

2. Bidlack, J. E., Jansky, S. H. copyright 2014. Stern’s Introductory Plant Biology. 13th ed.

U.S.A: McGraw-Hill Education, International Edition. p. 517

3. Internet source: http://en.wikipedia.org/wiki/Gymnosperm

http://en.wikipedia.org/wiki/Gymnosperm

JIB 115 Plant Diversity assignment 2

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