Classification of Confieropsida: 2 Orders

The following points highlight the two orders under which confieropsida has been classified. The orders are:- 1. Ginkgoales 2. Czekanowskilaes.

Order # 1. Ginkgoales:

The Ginkgoales are included within the Coniferopsida because of its coniferous type tree, wood and gametophytes. But, its distinctiveness has given rise to the argu­ment as to whether it should better be placed in a separate class Ginkgopsida or even a separate division Ginkgophyta.

The present-day monotypic Ginkgo biloba is a relic of a much more extensive group of plants which probably evolved in the Devo­nian, began to expand in the Triassic and attained its climax in the Jurassic after which it began to wane. Even the single species present today probably escaped ex­tinction because of its cultivation in Chinese and Japanese temples.

Fossil Ginkgoales:

Fossils which have been definitely ascribed to the Ginkgoales are known since the Permian. Besides these, there are several forms leading to the Upper Devonian which have been suspected to belong to this group. Ginkgophyllum and Platyphyllum leaves from the Upper Devonian look Ginkgoan but it is not definite whether they are not fern leaves.

Psygmophyllum is a similar leaf fossil widely known from the Upper Devo­nian and Permian.

In India, it is known from the Lower Gondwana Kashmir Ganga- moptenes beds. (Lower Permian). Rhipidiopsis from the Indian Lower Gondwana (Raniganj—Upper Permian and Barakar—Lower Permian) also has a similar position. Dichophyllum from Kansas Upper Carboniferous shows finely dissected branching (without clear leaves) and, with its seed, has been suspected to have Ginkgoan affinity.

Among the more definite early members ascribed to the Ginkgoales is Trichopitys (Fig. 738A) from French Lower Permian. In this the spirally arranged leaves are finely dissected dichotomously and in the axils of the leaves are borne branched systems bearing 4 to 6 (sometimes even 20) ovules.

Florin (1949) suggests that such a structure could have given rise to the modern Ginkgo by evolving perennial dwarf shoots and reducing the number of ovules. Sphenonbiera is another genus spreading from Lower Permian to Lower Cretaceous showing finely dichotomously dissected leaves but these were in short shoots as well as long shoots.

The male shoot were also dwarf in which the central axis branched and then bifurcated, each bunch terminating in a cluster of 3 to 5 sporangia. There are also a few other fossil genera which probably belonged to the Ginkgoales,viz., Saportaea (Permian), Phoenicopsis (Jurassic-Cretaceous), Ginkgodium (Jurassic), Windwardia (Cretaceous).

Fossil genera which are considered to belong to the Ginkgoaceae itself are Baiera Ginkgoites and those assigned to the genus Ginkgo. Besides the leaf shape they also show similar cuticle and stomata. Baiera (Fig. 738B), found from Lower Permian to Creta­ceous, has deeply incised leaves (almost segmented) without petioles and showing distinctly dichotomous venation.

Several seeds were borne on branched axes of the female shoot while the microsporangia were on male shoots consisting of several sporo­phylls each with six or more sporangia. The name Ginkgoites (Triassic to Tertiary) has been loosely applied to all Ginkgo leaf fossils but this should better be applied to highly incised leaves with petioles (Fig. 738C) only.

Typical Ginkgo leaves showing different degrees of incision has been found from the Middle Jurassic onwards (Fig 738D).

In the Indian Upper Gondwana Ginkgo crassipes is known in the Rajmahal series (Lover Jurassic) while Jabalpur rocks (Upper Jurassic) show Ginkgoites lobata as well as Phoenicopsis sp.

Family Ginkgoaceae: Ginkgobilobal:

At present the order Ginkgoales is reduced to a single family Ginkgoaceae con­taining the single genus Ginkgo with the solitary species biloba called the ‘maiden-hair tree’ for its leaves resembling the ‘maiden-hair fern’ (Adiantum). The name Ginkgo originated as a wrong form of the Chinese name Ginkyo.

It was sometimes known as Salisburia adiantifolia Sm. and, until Hirase discovered multiciliated sperms in it, it was considered as an exceptional member of the Taxaceae.

The tree genus is one of the oldest known as its representatives (mostly leaf fossils) can be traced definitely to the Lower Jurassic (Liassic) and possibly even to Upper Triassic when it had world­wide distribution. It has now shrunk to the single species even which has probably escaped extinction by cultivation in Chinese and Japanese temples.

Its presence in the wild state has been disputed although Li (1956) claims its presence in the forests of South-East China. Because of such a history Ginkgo is called a ‘living fossil’. It is now being cultivated throughout the world in temperate and subtropical regions. One or two plants are known at Darjeeling, Dehradun and Mussoorie and more are now being grown elsewhere in India.

Sporophyte Plant:

Young Ginkgo plants show the characteristic ex-current habit of the conifers but branching becomes irregular with age, the crown becoming spreading and broad. The trunk is of slow growth but, ultimately attains a height of 30 metres and a girth of 1 metre. It bears two kinds of branches the long blanches of indefinite growth bearing short branches (duiarf shoots or brachyplasts) of limited growth (Fig. 739).

Leaves are deciduous and spirally borne on the long shoot with spurs in axillary positions. Each spur (dwarf shoot) bears leaf scars, some scale leaves and a crown of leaves (internodes being suppressed) resembling a miniature Cycas tree. Each spur shoot remains covered by bud scales when young.

As stated above, the beautiful leaves resemble those of Adiantum. The venation is symmetrical and dichotomous. On the long shoots the leaves are lobed while those on spurs have merely wavy margins.

The anatomy of the stem is different in the long shoot (Fig. 740A) and in the spurs (Fig. 740B). The long shoot has a comparatively smaller pith and cortex, its wood is harder and there are not as many mucilage canals. Mucilage canals are found in the pith, primary cortex, root, petiole, leaf blade, seed with its peduncle and even the sporangium.

The stem anatomy conforms to the coniferous type with its small pith and cortex, persistent cambium, large secondary wood and primary endarch solenostele of collateral bundles. Protoxylem tracheides are spiral. The tracheid’s of the secondary wood are with bordered pits occurring irregularly in one or two rows and showing ‘bars of Sanio’.

Medullary rays are characteristically short in vertical extent, many of them being only one cell in width and almost always one cell in height.

There are no parenchyma cells in the wood excepting the medullary rays. The wood shows feeble annual rings which are not as well developed as in Pinus. There is the development of a distinct periderm.

In the seedling, the cotyledon shows a mesarch bundle, a cycadean character. This character is lost in the leaves where two exarch, collateral vascular bundles covered by a sheath enter each petiole and ultimately dichotomises in the veins. The spur leaves do not contain palisade cells but those on the long shoots (Fig. 741) have them.

The upper epidermis is conti­nuous while stomata are present in the lower. The lower mesophyll parenchyma cells are placed parallel to the leaf surface forming a transfusion tissue. The vas­cular bundles (veins) are closed collateral with the .The root is diarch with radial xylem and phloem. The steel is surrounded by a pericycle, an endodermis and large cortex. Secondary growth takes place here also.

Ginkgo is dioecious, there being strictly male and female trees. Male and female strobili (Figs. 742A & 743A) are borne in the axils of the scale leaves in the crowns of the dwarf shoots forming clusters there.

The male strobilus (Fig. 742A) consists of an axis bearing spirally a large number of elaborate microsporophyll’s. The slender microsporophyll (Fig. 742B) is surmounted by a hump containing a large mucilage cavity and bears two pendent microsporangia

The hump is considered by some as a sterile third microsporangium capable of deve­loping a fertile one. The microsporophyll, in that case, is peltate as in Taxus. Each microsporangium develops in the eusporangiate way, an archesporial cell giving rise to a primary wall cell and a primary sporogenous cell, the latter giving rise to a sporogenous tissue.

The spore mother cells give rise to the microspores or pollens on reduction division. The n chromosome number is 8. Each pollen is covered by are intine and an exine but the latter is incomplete so that two lateial ears are seen protruding from the pollen (Fig. 744A). The microsporangium ruptures by a longitudinal split and the pollens are discharged.

In the female dwarf shoot the peduncles bearing the ovules are borne in good numbers on such a spur shoot as in the case of the male strobili (Fig. 743A). In the young stage a peduncle bears two megasporangia or ovules (Fig. 743A & B) but, usually one aborts and only one matures (Fig. 739).

The young ovules become mature while still covered by the bud scales and then break through them. Each ovule is surrounded by a collar-like growth which is supposed by some to represent the sporophyll a structure otherwise missing in Ginkgo. As the peduncle bears four vascular traces, it is probable that it represents the axis of a female strobilus bearing two megasporophylls.

The ovule (Fig. 743C) generally resembles Cycas. The prominent nucellus has a large pollen chamber surmounted by a beak which becomes hard. The nucellus is free from the integument at top. The single integumer.t has an outer green fleshy layer, a thin hard middle part and a thin fleshy inner part which later becomes papery.

The two vascular strands sup­plying the base of the integument do not branch. There is a. micropylar opening on top of the nucellus. One or two megaspore mother cells are formed but only one of them undergoes reduction division forming a linear tetrad of megaspores of which only one matures and becomes functional.

Gametophytes:

Male Gametophyte:

The pollen begins to germinate while still within the micros­porangium. It divides to form a small first prothallial cell and a large antheridial initial cell (Fig. 744A). The antheridial initial cuts off a second prothallial cell which persists while the first prothallial cell disintegrates early (Fig. 744B).

The antheridial cell now divides to form a generative cell (Fig. 744C). Usually the microsporangium dehisces and the pollens are shed at this 4-celled stage. The wind-carried pollens are deposited on the micropyle of the ovule whence they are sucked down along with the drying up of a drop of liquid (pollination drop) and ultimately deposited in the pollen chamber.

Here the bulging intine (which part was never covered by the exine) on the pollen protrudes as a pollen tube and becomes anchored in the tissue of the nucellus where, as in Cycas, it acts as a haustorium, the original function of a pollen tube.

The spermatogenous end of the pollen tube now advances towards the female gametophyte breaking down and absorbing the tissue that comes on the way. Ultimately the pol­len chamber is enlarged so that nothing remains between the pollen tubes and the female gametophyte.

Then the generative cell divides into a stalk cell and a body cell (Fig. 744D). The body cell divides to form two multiciliated sperms just as in Cycas and the blepharoplasts behave in the same way (Fig. 744E). The sperms are slightly more elongated than in Cycas and the spirally arranged cilia are more restricted to the apical region.

Female Gametophyte:

The functional megaspore (Fig. 743B) enlarges and the nucleus divides by rapid free nuclear division. Cell wall formation then begins at the periphery and gradually proceeds towards the centre. A spongy nutritive tissue is formed round the growing gametophyte as in Cycas. The gametophyte becomes covered by a thin membrane in addition to the megaspore membrane.

The gameto­phyte is soon filled with an endosperm tissue which develops chlorophyll at a later stage as the integument is more or less transparent. Archegonium initials appear long before wall formation in the gametophyte has reached the centre. Usually two (rarely three) archegonia are formed in the top part of the gametophyte above the general endosperm.

At first a two-celled neck and a rapidly increasing central cell are formed. The central cell nucleus divides to form a ventral canal nucleus and an egg nucleus between which a definite wall is seen (Fig. 745A).

In this respect Ginkgo seems to be more primitive than Cycas where no such partition wall is formed. In Pinus there is an ephemeral partition wall. Further, instead of the cup-shaped archegonial chamber of the Cycads. Ginkgo has a somewhat cylindrical chamber surrounding a mass of solid gametophytic tissue upon which the nucellus wall rests like a tent on a pole (Fig. 745B).

Pollination and. fertilisation take place as in Cycas. The process is slow although not as slow as in Pinus. The strobili appear in February-March, pollination takes place about May and fertilisation about the end of September.

Embryo Sporophyte and Seed:

Fertilisation is followed by free nuclear division which usually takes place eight times giving rise to 256 free nuclei evenly placed in the egg cytoplasm when cell walls are formed simultaneously all through.

The embryonic tissue now faintly shows three zones- the micropylar cells are somewhat elongated, the middle cells remain large, the basal cells become smaller and more compact by rapid division.

Subse­quent growth takes place only at the base which rapidly forms two cotyledons, the radicle and the plumule containing rudiments of five leaves (Fig. 746). Nor­mally there is only one embryo in a seed. The seeds (Fig. 739) are quite large (about 1 inch long) and the outer fleshy layer of the seed coat (mature outer layer of integument) contains lot of mucilage.

The seed germinates quickly. Germination is hypogeal. The terminal parts of the cotyledons remain inside the seed until all the endosperm (gametophytic tissue) is absorbed. The cotyledons come above surface only after the radicle has developed the tap root.

Economic Importance:

The seed is edible and yields an oil. The timber is of commercial value. In tem­perate America it has been recommended as a hardy roadside tree.

Affinities of the Ginkgoales:

The Ginkgoales are included within the Coniferopsida because of the coniferous habit of the tree, the coniferous wood and the gametophytic structures. But, it does not seem to be as close to the Coniferales as the Cordaitales although, possibly, the Cordaitales had motile spermatozoids in common with the Ginkgoales.

Nevertheless, the Ginkgoales have been placed within the Cycadopsida in Engler’s Syllabus (1954) as the ovule and the manner of fertilization resembles Cycas and because, if the collar of the ovule be regarded as a modified sporophyll, the ovules becomes phyllospermous.

Coulter and Chamberlain (1910) were impressed by the cordaitean affinity of the Ginkgoales and suggested that either the Ginkgoales originated out of the Cordaitales or both the orders evolved from a common ancestor.

Arnold (1947) suggested a Pteridospermous origin because of the phyllospermous ovule and the fern-like leaf, seeds, motile sperms and double leaf traces. But it is quite possible that the origin of the Ginkgoales lies even beyond them and still undiscovered, may be near the Pteridospermic group of Progymnosperms.

Because of this uncertainty, although the majority opinion is in favour of a coniferous affinity, there are quite a number who think that Ginkgo should be in an independent division or class (Ginkgophyta or Ginkgopsida) representing a separate line of evolution of seed plants in parallel with the Cycads and the Conifers as Ginkgo shows some characters common to each of the two groups.

Order # 2. Czekanowskiales:

Czekanowskia is a fossil of leafy shoots from the Jurassic and was supposed to belong to the Ginkgoales. It is a short shoot with scales on the lower parts and a cluster of about 10 leaves at the tip. The leaves are about 15 cm long, dichotomise 3 or 4 times and the ultimate segments are very narrow (only 1 mm broad) bearing single reins.

While Czekanowskia was considered as a member of the Ginkgoales, Harris (1951) descri­bed Leptostrobus (Fig. 747), the seed bearing structure found in associa­tion with Czekanowskia leaves.

This seems to be a fertile shoot with some scale leaves at the base and fruiting capsules arranged on the axis at an interval of 5 mm. Each such fruiting capsule shows a pair of lobed valves up to 5 mm long and 5 mm broad with 3 to 5 seeds borne on the inner face of each valve.

Such a peculiar seed structure is so different from the Ginkgoales that these fossils seem to represent a quite different group of Gymnosperms and have been placed at least in a separate order, Czekanowskiales (with the single family Czeka- nowskiaceae) which some suppose to have the same relationship with the Ginkgoales at the Taxales has with the Coniferales.