SEED SCALE COMPLEX

SEWART

STEWART hypotheses - P erhaps the Lycopsida form a gymnosperm because l.s. of Selaginela megasporangium it shown megaspore 3 abortive , ligule, sporophyll and megasporangia.

Sporophyll-bract scale

Ligules- ovuleferous scale

Megasporangiophyll-cone

They are analogous parts

As for as leaf is concerned –Lycopodium. clavatum –they are microsporophyll’s as that

of conifers

Lycopsida - It originate conifer

Filicopsida - It originated Cycas and Pteridosperm

FLORIN

Florin’s hypotheses (SWEDEN, 1894-1965)

Florin 1951 –ovuliferous scale is compound structure and not simple as proposed by stewart.

Lebachia - Ernetiodendron –Voltiza – Pseudovolzia – Olmannia

During carboniferous and permian these plant appeared and source of coal

FLORIN called is “samen schoppen komplex” which means Seed scale complex .

Voltziales and the origin of the ovuliferous scales

Although the evidence from organography,comparative anatony,and ontology supports the view that ovulate cones in the conifers is a compound strobilous,the phylogenetic origin of the ovuliferous scales remains to be explained. The scale is similar to sporophyll in that it bears ovules, but its axillary position with reference to a bract represents a puzziling situation from an evilutionary point of view, of all the conflicting and involved theories that have been proposed during the past century,the most plausible and best supported view was advanced by Florin:

Phylogenitically,the ovuliferous scales is a highly condensed and modified fertile shoot and hence is not a simple sporophyll. In other words, the scale evolved from a leafy, ovule-bearing,dwarf shoot and its present simple appearance is the result of the fusion and specialization of both the sterile and fertile components of such an ancestral structure.

The earliest precursors of the modern ovuliferous scales occurred in

Lebachia and ernestiodendron, two genera of voltziales from the upper carboniferous-permian. In lebachia piniformis, a eusporangiate or ovulate cone consisted of a series of helically arranged, bifid bracts,in the axils which developed short fertile shoots.usually all at one of the scale like leaves of the fertile shoot are sterile. The fertile scale (termed megasporophyll by some authors) consisted of a stalk and an act terminal,bilaterally symmetrical ovule that had the axis of cone.most of the

materials studied by florin(1951) were impression-compression fossils. Recent studies of permineralised cones have supported,in general, florin’s concept of the early origin of conifers reproductive structures. A fertile shoot of lebachia lockardii consisted of twenty five to thirty sterile scales and one or two fertile scales at the base of shoot . there was one terminal inverted ovule per fertile scale (mapes and rothwell,1984). Pollen grains have also been found in the pollen chamber. In contrast to lebachia,all scales were fertile in ernestiodedron filiciforme. Florin termed the fertile shoot in these and other conifers “seed scale complexes” and belived they support the view that the compound megasporangiate strobilus is the primary form in the coniferales, with the exception of the taxales (taxaceae). The great man regarded the taxales as having the separate evolutanory history because of they lack definable seed cones: this view has been questioned in recent years (Miller,1977).

Genera from the late permian and arly part of the mesozoic,illystrate additional evolutoinary steps towards the modern day type of ovuliferous scales in the coniferales . in pseudvoltzia liebeana, for example, the fertile shoot consist of five partially fused scales, forming a dorsiventral “ovuliferous scales.” The middle lobe and the two lateral lobes each had a reflexed ovule, adnate to its base.

The entire ovuliferous scale complex was fused with the subtending bracts to about its midpoint; vascular strands to the ovuliferous scale and bract,however,were not fused, an indication that the ovuliferous scales and bracts probably were not fused in ancestral type (schweitzer,1963). In the triassic genus voltzia, the ovuliferous scale also consisted of five partially fused scales and three inverted ovules whose stalks were adnate to the ovuliferous scale.

In summary, the main trends in seed cone & seed scale evolutoin have been towards:--

(1)the elimination of all but a few sterile scales, which became fused into the so-called ovuliferous scale,

(2)the recurvation of ovules and suppression of their stalks,

(3)their final incorporation with the lower adaxial side of the ovuliferous scales in derived conifers. On the basis of the ground plan-divergence method of cladistics,Miller(1982) have proposed a phylogeny of the voltziales and modern-day families of the coniferales based upon fourteen characters of the bract, seed scale complex.

In modern day conifers, evolutionary advancement in the ovulate cone is shown by the various degrees of fusion between the ovuliferous scales and its sub-tending bracts.throughout the pinaceae, the bract is only basally adnate to the scale, whereas these structure are more or less completely united in members of the taxodiaceae,cupressaceae,and araucariaceae. Comparative studies of the vascular anatomy of the ovulate cone in the pinaceae reveal that the traces which enter the ovuliferous scale diverge from the stele in fundamentally the same manner as the branch traces of an axillary vegetative shoot in pinus meritima,for example, Aase(1915) found that a single vascular strand, with the xylem oriented towards the adaxial surfaces, as in a vegetative leaf,extends as a separate trace into the bract. In contrast, the branch like vascular supply of the ovuliferous scale consist of three ar four traces that diverge at a higher level from the stele:these strands branch dichotomously in the scale

and form a series of veins with their xylem oriented towards the lower surface of the scale. According to Florin(1951), the so-called “inversion” of the vascular bundles in the ovuliferous scale in the pinaceae indicates its phylogenetically original radial symmetry. Each of the two ovules borne on the ovuliferous scales is vascularised by a single strand derived as a branch from an adjacent lateral bundles.

Lemoine-sebastian(1968,69) has studied the vasculature of the bracts and ovuliferous scales of the ovulate cones of various genera in the Taxodiaceae and cupressaceae. The bract is vascularised by a single trace that only rarely branches in its course through the appendage. But the pattern of vasculature of the ovuliferous scales is usually very complex and varies acording to the genus. In some genera, only the single system of inversely oriented strands is formed: in other genera, the radial branching of the strands forms two vascular arcs;an adaxial series of strands with inverted orientation of the xylem and an abaxial system comprising normally oriented bundles. These varied patterns of vasculature of the ovuliferous scales in the taxodiaceae and cupressaceae are of considerable morphological and systematic interest, but their phylogenetic significance is an open qustion; there are however, recommendations that the two families should be merged, based upon analysis of

numerous morphological characters.

Conifer cone possesing seed scale complex

Mature female Pinus coulteri cone

An Aleppo Pine cone

A cone (in formal botanical usage: strobilus, plural strobili) is an organ on plants in the division Pinophyta (conifers) that contains the reproductive structures. The familiar woody cone is the female cone, which produces seeds. The male cones, which produce pollen, are usually herbaceous and much less conspicuous even at full maturity. The name "cone" derives from the fact that the shape in some species resembles a geometric cone. The individual plates of a cone are known as scales.

The male cone (microstrobilus or pollen cone) is structurally similar across all conifers, differing only in small ways (mostly in scale arrangement) from species to species. Extending out from a central axis are microsporophylls (modified leaves). Under each microsporophyll is one or several microsporangia (pollen sacs).

The female cone (megastrobilus, seed cone, or ovulate cone) contains ovules which, when fertilized by pollen, become seeds. The female cone structure varies more markedly between the different conifer families, and is often crucial for the identification of many species of conifers.

Female cones of the conifer families

Pinaceae cones

Intact and disintegrated fir cones

Young cones of a Colorado Blue Spruce

The members of the pine family (pines, spruces, firs, cedars, larches, etc.) have cones that are imbricate (that is, with scales overlapping each other like fish scales). These are the "archetypal" cones. The scales are spirally arranged in fibonacci number ratios.

The female cone has two types of scale: the bract scales, derived from a modified leaf, and the seed scales (or ovuliferous scales), one subtended by each bract scale, derived from a highly modified branchlet. On the upper-side base of

each seed scale are two ovules that develop into seeds after fertilization by pollen grains. The bract scales develop first, and are conspicuous at the time of pollination; the seed scales develop later to enclose and protect the seeds, with the bract scales often not growing further. The scales open temporarily to receive gametophytes, then close during fertilization and maturation, and then re-open again at maturity to allow the seed to escape. Maturation takes 6–8 months from pollination in most Pinaceae genera, but 12 months in cedars and 18–24 months (rarely more) in most pines. The cones open either by the seed scales flexing back when they dry out, or (in firs, cedars and golden larch) by the cones disintegrating with the seed scales falling off. The cones are conic, cylindrical or ovoid (egg-shaped), and small to very large, from 2–60 cm long and 1–20 cm broad.

After ripening, the opening of non-serotinous pine cones is associated with their moisture content—cones are open when dry and closed when wet.[1This assures that the small, wind disseminated seeds will be dispersed during relatively dry weather, and thus, the distance traveled from the parent tree will be enhanced. A pine cone will go through many cycles of opening and closing during its life span, even after seed dispersal is complete.[2] This process occurs with older cones while attached to branches and even after the older cones have fallen to the forest floor. The condition of fallen pine cones is a crude indication of the forest floor's moisture content, which is an important indication of wildfire risk. Closed cones indicate damp conditions while open cones indicate the forest floor is dry.

Araucariaceae cones

Members of the Araucariaceae (Araucaria, Agathis, Wollemia) have the bract and seed scales fully fused, and have only one ovule on each scale. The cones are spherical or nearly so, and large to very large, 5–30 cm diameter, and mature in 18 months; at maturity, they disintegrate to release the seeds. In Agathis, the seeds are winged and separate readily from the seed scale, but in the other two genera, the seed is wingless and fused to the scale.

Podocarpaceae cones

Berry-like Podocarpus cone

The cones of the Podocarpaceae are similar in function, though not in development, to those of the Taxaceae (q.v. below), being berry-like with the scales highly modified, evolved to attract birds into dispersing the seeds. In most of the genera, two to ten or more scales are fused together into a usually swollen, brightly coloured, soft, edible fleshy aril. Usually only one or two scales at the apex of the cone are fertile, each bearing a single wingless seed, but in Saxegothaea several scales may be fertile. The fleshy scale complex is 0.5–3 cm long, and the seeds 4–10 mm long. In some genera (e.g. Prumnopitys), the scales are minute and not fleshy, but the seed coat develops a fleshy layer instead, the cone having the appearance of one to three small plums on a central stem. The seeds have a hard coat evolved to resist digestion in the bird's stomach, and are passed in the bird's droppings.

Cupressaceae cones

Juniper "berries", which are used to flavor gin, are actually modified cones.

Giant Sequoia cones

Members of the cypress family (cypresses, arborvitae, junipers, redwoods, etc.) differ in that the bract and seed scales are fully fused, with the bract visible as no more than a small lump or spine on the scale. The botanical term galbulus (plural galbuli; from the Latin for a cypress cone) is sometimes used instead of strobilus for members of this family. The female cones have one to 20 ovules on each scale. They often have peltate scales, as opposed to the imbricate cones described above, though some have imbricate scales. The cones are usually small, 0.3–6 cm long, and often spherical or nearly so, like those of Nootka Cypress, while others, such as Western Redcedar, are narrow. The scales are arranged either spirally, or in decussate whorls of two (opposite pairs) or three, rarely four. The genera with spiral scale arrangement were often treated in a separate family (Taxodiaceae) in the past. In most of the genera, the cones are woody and the seeds have two narrow wings (one along each side of the seed), but in three genera (Platycladus, Microbiota and Juniperus), the seeds are wingless, and in Juniperus, the cones are fleshy and berry-like.

Tetraclinis cones

Sciadopityaceae cones

The cones and seeds of Sciadopitys (the only member of the family) are similar to those of some Cupressaceae, but larger,

6–11 cm long; the scales are imbricate and spirally arranged, and have 5-9 ovules on each scale.

Taxaceae and Cephalotaxaceae cones

Berry-like yew cone

Members of the yew family and the closely related Cephalotaxaceae have the most highly modified cones of any conifer. There is only one scale in the female cone, with a single poisonous ovule. The scale develops into a soft, brightly coloured sweet, juicy, berry-like aril which partly encloses the deadly seed. The seed alone is poisonous. The whole 'berry' with the seed is eaten by birds, which digest the sugar-rich scale and pass the hard seed undamaged in their droppings, so dispersing the seed far from the parent plant.

Cycadaceae cones

This group of cone-bearing plants retains some types of 'primitive' characteristics. Its leaves unfurl, much like ferns. There are three extant families of Cycads of about 305 species. It reproduces with large cones, and is related to the other conifers in that regard, but it does not have a woody trunk like most cone-bearing families.

Welwitschiaceae cones

A unique cone-bearing plant in the Order Welwitschiales, Welwitschia mirabilis is often called a living fossil and is the only species in its genus, which is the only genus in its family, which is the only family in its Order. The male cones are on male plants, and female cones on female plants. After emergence of the two cotyledons, it sets only two more leaves. Those two leaves then continue to grow longer from their base, much like fingernails. This allows it great drought tolerance, which is likely why it has survived in the desert of Namibia, while all other representatives from its order are now extinct.

REFERENCES:-

Morphology and evolution of vascular plants

‗ernest m gifford

‗adriance s foster

---seed scale complex in conifers,430

Google –encyclopedia,seed scale complex