Gymnosperma (Pine)

Characteristics of the life cycle The female and male inflorescence of the pine tree develop as cones on the sporophyte. The female cones (female strobili, singular strobilus) consist of megasporophylls that carry uncovered (Gymno = naked / exposed, hence Gymnosperms) ovules. Inside the ovule the megasporocyte undergoes meiosis, producing four haploid cells. One of these cells survive as a megaspore. The megaspore goes through mitotic divisions and forms a small female gametophyte, in which two or three archegonia are present, each with one egg cell. De male cone is in fact composed of numerous strobili. Each strobilus contains a short appendix and a larger and broader microsporophylls that contains two microsporangia. In the microsporangium microspores are formed after meiosis. These microspores give rise to pollen grains (the proper male gametophyte) after undergoing mitotic divisions. When the microsporangia disrupt a huge number of pollen grains is released into the atmosphere. The pollen grains can be dispersed by the wind over long distances thanks to their air sacs. In fist instance the pollen grain consists of two male prothallium cells (which quickly degenerate) and one antheridium cell: together the gametophyte. A pollen tube cell and a generative cell are formed from the antheridium cell. The generative cell divides in a sterile cell and an immobile sperm cell which reaches the archegonia through the pollen tube and leads to fertilization of the egg cell. The embryo resulting from fertilization enters a long developmental path that after several years will results in the formation of a mature seed. A Megasporangium with megaspore mother cell (2n), B After meiosis of the megasporocyte: only one of the four haploid cells persists. The pollen grain is germinated, C The megagametophyt develops within the megaspore. D The megagametophyt contains two (or three) archegonia, each with one egg cell. The pollen tube is grown allowing the sperm cell to reach the archegonium . The nucleus of the sperm cell fuses with that of the egg cell: fertilization! E An embryo develops, surrounded by rests of the megasporophyte and there around nucellus tissue F The seed is nearly ripe Male organs for reproduction A Habitat of a pine tree (Gymnosperms) with male inflorescence. Each male inflorescence consists of a large number of strobili arranged in a spiral. Pollen is formed in these cones. Male inflorescence (2) with strobili (1) B Detail of the male inflorescence. C The microsporophylls of the male cones carry two oval pollen sacs. the pollen sac is the microsporangium, in which microspores are formed after meiosis of the microsporocyte. The microspores then undergo mitosis and give rise to pollen grains. D Longitudinal section through the male cone. 1 microsporophyll, 2 pollen sac with pollen  E Detail of a longitudinal section through the male strobilus of pine (Pinus). 1 microsporophyll (2n), 2 wall of the microsporangium (2n), 3 pollen grains (contain haploïde cells). The wall of the microsporangium disrupts upon drying and the pollen grains are releaed and transported by the wind (anemophylly).  F Detail of the male sporangium (male carrier of spores) of pine. The sporangium contains sporogenic tissue (microsporocytes). Each microsporocyte forms four microspores through a meiotic division. Subsequently, each microspore forms a pollen grain (the microgametophyte) through mitosis. 1 pollen , 2 rest of sporogenic tissue G Detail of pollen grain of pine 1 air sacs, 2 generative cell H Detail of a germinated pollen grain of pine, with a pollen tube. 1 air sacs, 2 generative cell, 3 pollen tube, 4 tube nucleus female reproductive organs The female strobili (singular strobilus) of Gymnosperms contain ovules. The ovule can be found on a megasporophyll, which itself is located on a scale. On each megasporophyll of the female strobilus two seeds can develop after fertilization. In each megasporangium (the female carrier of spores) a megasporocyte is present which leads to four megaspores after meiosis. three of these megaspores degenerate, only one megaspore is functional and forms the megagametophyte with two or three archegonia containig each one egg cell. A Longitudinal secion through a female cone 1 Sterile scale 2 Megasporophyll, 3 Integument, 4 Megasporangium, 5 Micropyle B Female cones. The ovule can be found on megasporophylls, arranged in a strobilus. 1 female strobili, 2 Seeds with wings C On each megasporophyll of the female strobilus two seeds can develop following fertilization. 1 Seed, 2 Wing (outgrowth) D Cross-section through the megasporophyll, megasporocyte and megasporangium (see labels in E). E Section through the female sporangium 1 Megasporophyl, 2 Megasporangium wall, 3 Integument, 4 Megasporocyte, 5 Tissue of the megasporangium, 6 Micropyle F section through the megasporangium with egg cell 1 Integument, 2 Megagametophyte, 3 Archegonia, 4 Egg cell nucleus, 5 Rest of megasporangium G Detail of section through the (female) megasporangium with egg cell 1 Integumenten, 2 Rests of megasporangium, 3 Archegonia, 4 Egg cell nucleus H Detail archegonium with egg cell I and J Ripe embryo of pine. 1 Storage product 2 Embryo last modified: 1 Oct 2013

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Cycadofilicales: Features and Classification | Gymnosperms |

Origin of Cycadofilicales:

The Cycadofilicales or Pteridosperms are popularly known as ‘seed ferns’. They were trees or small plants bearing fern-like foliage. They flourished greatly in the Carboniferous and the period was so very rich in fossil fern leaves that it was thought to be the ‘Age of Ferns’.

Owing to the presence of fern-like leaves they were formerly believed to be a kind of fern, but subsequently when fossil leaves with attached seeds were discovered (1903) the Cycadofilicales were designated as Pteridosperms by Oliver and Scott. They are regarded as the primitive group of seed-plants.

In the year 1877, Grand Eury first discovered a few species of fern leaves which later on went by the name of the form genus myeloxylon. In 1887 Williams pointed out that Heterangium (stem) and Kaloxylon (root) combined characters of both ferns and cycads, which led potonie (1899) to create a new systematic group the Cycadofilicales.

With the discovery of the fact that Lagenostoma lomaxi belongs to the stem of Lynginopteris oldhamia by Oliver and Scott in 1903, the name Pteridospermae was pro­posed for the seed-bearing Cycadofilicales. The seed-bearing habit of the Pteridospermae was again verified next year by Kidston in Neuropteris heterophylla.

The Cycadofilicales or Pteridospermae are popularly known as ‘seed ferns’. Though a few petrified stems have been discovered in the Devonian, yet no positive seed has been obtained before the Lower Carboniferous is reached. The plants first appeared in the Devonian, became very prominent in the Carboniferous, but remained wide-spread during the Permian, but a few survived even up to Mid-Jurassic. The best known Cycadofilicales are reported from Great Britain.

Characteristic Features of Cycadofilicales:

(a) Vegetative Organs:

1. Plants were trees or small plants with monopodial bran­ching and presence of ramentum.

2. Leaves were large and fern-like bearing micro-and megasporangia.

3. Leaf traces relatively large, single or double.

4. Leaves possessed resistant cuticle.

5. Primary xylem well-developed, usually mesarch or endarch.

6. Secondary xylem and phloem present, tracheids bearing numerous multiseriate bordered pits on the radial walls, and a large pith.

(b) Spore-Producing Organs:

1. Pollen-producing microsporangia are of marattiaceous type. They are exannulate, sometimes grouped into synangia and difficult to distinguish from the fructifications of ferns, but never grouped into cone.

2. The ovule had an integument differentiated into three layers, the vascular supply was divided into two sets of branches, a well-developed nucellar beak and a pollen chamber.

3. Seeds were not in cones or inflorescences, but on slightly modified and undistinguished foliage and they resembled those of Cycads.

Classification of Cycadofilicales:

The Cycadofilicales have been divided into two major groups:

i. The Paleozoic Cycadofilicales and

ii. The Mesozoic ones.

The former is further divided into three families, viz., Lyginopteridaceae, Medullosaceae, and Calamopityaceae, while the latter also consists of three families, viz., Peltaspermaceae, Corystospermaceae, and Caytoniaceae. Of the six families mentioned above, the most exten­sively studied one is Lyginopteridaceae.

A brief account of each of the families is given below:

1. Lyginopteridaceae:

In the family the leaves are of the filicoid type, and the stem is of monostelic organization. The leaf, traces and foliar bundles are single or double; the xylem is mesarch. Seeds are of cycadean type, and the cupule is present. The best known lyginopterid is Calymmatotheca, hoeninghausi. This is the plant to which both Lyginopteris and Lagenostoma belong.

2. Medullosaceae:

In this family the stem is polystelic, and many bundles are scattered in the petiole. Seeds are large.

3. Calamopityaceae:

This is the oldest and the largest of all the families of Pteridosperms. The stem is monostelic, and scattered bundles are present in the petiole.

4. Peltaspermaceae:

The leaves in this family are small, bipinnate, and the rachis is characterized by swellings. The microsporophyll is branched, and microsporangia open longitudinally, and smooth pollens are discharged.

5. Corystospermaceae:

The leaves in this family are linear-lanceolate and the seeds are characterized by ovoid, elliptic shape, and curved bifid micropyle.

6. Caytoniaceae:

This family is characterized by leaves formed usually in two (occasionally three) pairs. Anatomically it has transfusion tissue and mesophyll with palisade layer. The pollen grains show a symmetrical bilobed structure, and these are winged.

The ovules are orthotropous, and the nucellus is free from the integument except at the base. This is an advanced character. Formerly the Caytoniales were compared with the Angiosperms, but later on it was found out that members of this family are essentially gymnospermous.

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Bennettitales: Classification and Features | Gymnosperms

Bennettitales: Classification and Features | Gymnosperms 

Contents:

1. Origin of Bennettitales
2. The Sporophyte and the Flower of Bennettitales
3. Characteristic Features of Bennettitales
4. Classification of Bennettitales
5. Comparison between Williamsoniaceae and Cycadeoideaceae
6. Genera of Bennettitales
7. Indian Bennettitales 
8. Affinity of Bennettitales

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1. Origin of Bennettitales:

Bennettitales resembled modern Cycads but had a more diversi­fied habit with tuberous or columnar branched trunks with per­sistent leaf-bases. There was a crown of pinnate leaves. The re­productive organs were borne terminally on short axillary branches arising from the trunk. The strobili were typically bisporangiate unlike those of living Cycads.

The most common fossils among the Cycadophytes are leaves and it is difficult to distinguish the leaves of Mesozoic Cycadales and Bennettitales from their external features. Their stomatal characters are quite distinctive.

The Bennettitales, or the Cycadeoideales, appeared first in all probability in the Triassic flourished very luxuriantly in the Middle or Late Jurassic, and then went on declining in the Creta­ceous. It is believed that this order arose side by side with the living Cycadales from a pteridospermous stock. The fossil remains are distributed extensively throughout the United States, Mexico, different parts of Europe, as well as India.

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2. The Sporophyte and The Flower of Bennettitales:

The plant body of W. sewardiana is a small, stout tree resem­bling a miniature living Cycas. The trunk is columnar, bearing a cluster of pinnate compound leaves at its top. Intermingled with the leaves are scales, which leave small scars on the stem. There are lateral shoots, which are constricted at their bases. This suggests that they broke away, and served as propagative shoots. The leaf-bases formed an armour around the stem.

Internally, the pith of the stem is enclosed by a xylem ring. Tracheids of scalariform type are noted, and multiseriate bordered pits are present. The xylem rays are variable from uni- to multi- seriate type. The protoxylem is endarch, and secretory sacs are present in the cortex as well as in the pith.

The Flower of Bennettitales:

Flowers are produced on short-branched lateral shoots, which project beyond the armour of leaf cushion. Each flower is unisexual and ovulate, having a receptacle bearing the stalked ovules inter­mingled with interseminal scales. The nucellus of the ovule is en­closed by an integument, which prolongs into a micropylar beak.

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3. Characteristic Features of Bennettitales:

(a) Vegetative Organs:

1. Plants were un-branched trees, usually 1-2 metres in height, with compressed internodes and ramentum, bearing a crown of large pinnately compound leaves on the top.

2. The trunks were covered with an armour of persistent leaf-bases.

3. The outer walls of the epidermal al cells are sinuous.

4. Guard cells and subsidiary cells are syndetocheilic, i.e., are produced by the division of the same initial.

5. The anatomical structure of the stem shows a thick cortex, a broad pith and narrow vascular cylinder composed of collateral bundles with exarch xylem.

(b) Spore-Producing Organs:

1. Flowers unisexual or bisexual in strobili.

2. Microsporophylls frond-like forming a loose cone.

3. Megasporophylls are not leaf-like, but found in specialised cone-like structure

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4. Classification of Bennettitales:

Bennettitales are divided into three families, viz.,

(1) William­soniaceae,

(2) Wielandiellaceae and

(3) Cycadeoideaceae.

(1) Family. 1. Williamsoniaceae:

Plants are all older and primitive in age, mostly Triassic and Jurassic. Flowers were borne on relatively slender stems or pedicels and fully exposed. Materials are preserved as compressions.

Genera:

Williamsonia (whole plants, male and female flowers), Ptilophyllum (fronds).

(2) Family. 2. Wielandiellaceae:

These plants are also primitive and older like Williamsoniaceae. The chief difference is that their trunks are not covered by leaf armours as in Williamsoniaceae. Branches are slender and dichotomously branched.

Genus:

Wielandiella, Williamsoniella (fronds).

(3) Family. 3. Cycadeoideaceae:

Plants are younger and more advanced, mostly Cretaceous. The trunk is short, ovoid or columnar with an armour of leaf-bases. Flowers are embedded within leaf- base layer.

Genus:

Cycadeoidea (= Bennettites).

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5. Comparison between Williamsoniaceae and Cycadeoideaceae:

The valid name for Bennettitales is Cycadeoideales as explained by Arnold in 1947, but the former name is still frequently used by many workers. They were the characteristic flora of the Mesozoic era, and were distributed throughout both the hemispheres. They had many resemblances with Cycadofilicales and Cycads in their general habit.

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6. Genera of Bennettitales:

The following genera of Bennettitales are found in India:

i. Williamsonia:

Several species of Williamsonia are represented in India. Of these W. sewardiana is the most completely worked out species, which is found in the Rajmahal Hills as reconstructed by Prof. Sahni.

Other Indian species of Williamsonia are W. indica, W. microps, W. blanfordi, etc. But, the vegetative parts of these species are not well preserved.

ii. Ptilophyllum:

It is a leaf genus belonging to Williamsonia and occurs in the Rajmahal and Jabalpur series of India.

The two widely distri­buted species are:

a. P. acutifolium and

b. P. c.f. cutchense.

The leaf is pinnate having the pinnae linear, straightly falcate with parallel or sub-parallel veins. The pinnae are characterised by their attach­ment to the upper surface of the rachis covering it almost com­pletely. The base of the lamina is rounded.

iii. Bucklandia:

This is stem genus belonging to Williamsonia. It has only one species, viz., B. indica. It is regarded as the stem of Williamsonia.

iv. Otozamites:

It is also a leaf genus belonging to the Family Williamsoniaceae, and occurs in Upper Gondwana, Rajmahal and Jabalpur series. There are four species of the genus, viz., O. bengalensis, O. parallelus, O. pecten and O. bunbaryanus. The leaves resemble those of Ptilophyllum but differ in the points that the pinnae have acute base and also spreading venation.

v. Dictyozamites:

It is also a leaf genus belonging to the Family Williamsoniaceae and occur in the Upper Gondwana of India. It has two Indian species D. falcatus and D. indica. The leaf resembles Otozamites, but differs mainly in anastomosing veins.

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7. Indian Bennettitales:

The group is well represented in the Rajmahal Hills and other Upper Gondwana localities in India. No member of Cycadeoidaceae is known from India. Williamsoniaceae is represented in India by several species of Williamsonia. W. sewardiana found in Rajmahal Hills is the most completely known species.

This, as reported by Sahni is a small tree with a columnar trunk, covered with scars of foliage and scale leaves and a crown of ;slender pinnate leaves. Female flowers only are known and they are borne on short lateral branches, which are covered with leaf-bases and scales.  

The flower consists of a conical fertile portion enclosed by linear heavy bracts and scales. The ovules are borne on long stalks and are surrounded by club-shaped interseminal scales. The trunk of W. sewardiana is known as Bucklandia indica and its leaves Ptilophyllum acutifolium.

Other Indian Williamsonias are not well preserved and their vegetative parts are unknown, e.g., W. indica, W. microps, W, blanfordi. Microsporangiate Williamsonias are W. sahni, W. santalensis, while W. sewardiana are unisexual. W. santalensis might be unisexual or bisexual.

Bucklandia has a compact secondary wood, uniseriate meduallary rays and tracheids have multiseriate bordered pits.

Five genera based on foliage are known from India, viz.,

a. Ptilophyllum,

b. Pterophyllum,

c. Zamites,

d. Otozamites and

e. Dictyozamites.

Ptilophyllum have linear leaves with pinnae attached to the upper face of the rachis, covering it almost completely. Base of pinna is rounded. Veins are parallel.

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8. Affinity of Bennettitales:

Bennettitales have some resemblances with Ferns, Cycads and Angiosperms.

(a) Affinities with Ferns:

1. Lateral branching.

2. Ramentum present.

3. Direct leaf trace.

4. Presence of synangia.

5. Scalariform thickening.

(b) Affinities with Cycads:

1. Fronds of cycadeoidea ingens similar to enfolding of Encephalartos, macrozamia and Bowenia, otherwise frond character is different.

2. In general habit, to some extent, and anatomy Cycadeoids resemble Cycads, but no girdling of leaf traces and absence ofmedullary rays.

3. Probable swim­ming sperms.

4. Similar seeds.

(c) Affinities with Angiosperms:

Parkin’s theory is that, in both the groups flowers are alike, because:

1. Connection with sympetaly owing to com­mon endarch siphonostele, etc.

2. Connection with Magno­lias, because of floral analogies, but not true relations.

(i) Cycadeoidean bracts beneath stamen ring, homologous to the perianth.

(ii) Larger number of stamens on the staminate disc.

(iii) Cone-shaped ovuliferous receptacle bearing cluster of separate fruit-like structures.

(iv) Tracheids scalariform in all Cycadeoidaceae and most Williamsoniaceae, which is characteristic of Magnolia.  

In certain respects the Cycadeoidales shows affinities with both the Pteridosperms and the Cycadales, particularly regarding the general habit, structure of wood, mesarch leaf trace, foliage and structure of the seed. Angiospermous affinities can also be considered by comparing their strobili with the flowers of Magnolia, but the similarity is only due to parallel evolution.

The relationship is drawn with the ferns in having the ramentum, synangia, and direct leaf traces. Hence, this order represents a combination of characters of Cycadofilicales, Cycadales, and Filicales along with some characters of its own.

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