In this article we will discuss about Cycadales. After reading this article you will learn about: 1. Meaning of Cycadales 2. General Characters of Cycadales 3. Genera 4. Phylogency.
Meaning of Cycadales:
Cycadales, consisting of 11 living genera and more than 100 species, are distributed in both tropical and warm temperate regions of the world They appeared first in Upper Triassic times, and are present even today. This indicates clearly that they are extremely ancient plants.
Seward (1933) has mentioned that “The cycads of today may rightly be called “living fossils” because they come down from the remote past with so little change that if a man from Cretaceous is brought today, he would very easily recognize the modern living forms of today”.
Cycads are woody plants which resemble palms on casual observation. They have a stocky cylindrical stem possessing a crown of very large palm-like leaves. They possess naked seeds on modified leaves (sporophylls) and are thus true gymnosperms.
General Characters of Cycadales:
1. The plant body is sporophytic, and the sporophyte is differentiated into well-developed roots, columnar and generally un-branched stem and pinnately compound leaves.
2. Members exhibit xerophytic characters. The growth of cycads under xerophytic conditions is extremely slow. It has been estimated that a plant of Dioon grows only 2 to 2.5 metre in one thousand years under natural conditions.
3. Young leaves show circinate vernation.
4. The wood is manoxylic.
5. Mucilage canals are present in the pith as well as cortex.
6. The leaf trace is diploxylic
7. Plants are dioecious without exception, and the reproductive organs are generally in the form of cones.
8. The cones are generally terminal or lateral in position.
9. In male cones the microsporophyll’s are arranged on cone axis, and form a compact structure.
10. Microsporangia are arranged on the abaxial side of the microsporophyll’s.
11 The flagella on the sperms are arranged in spiral bands. The male gametes are very large. These measure 80 μm in length in Microcycas, 180-210 μm in Cycas revoluta and as much a 400 μm (largest) in Chigua.
12. Megasporophylls are foliage leaf-like structures, the tip portion of which is sterile. Several ovules (2-8) are arranged in the middle region of the megasporophylls.
13. The ovules are orthotropous.
14. The apical meristem in all Cycads is extremely massive.
Genera of Cycadales:
Under-mentioned is the list of all the eleven living genera of Cycadales arranged alphabetically: Bowenia (Fig. 8.1 A), Ceratozamia, Chigua, Cycas (Fig. 8.1B), Dioon (Fig. 8.1 C), Encephalartos, Lepidozamia, Macrozamia, Microcycas, Stangeria (Fig. 8.1 E) and Zamia (Fig. 8.1 D).
Johnson (1959) recognized Lepidozamia with three species. Pant (1973), however, opined that Lepidozamia should be treated only as a section or sub-genus of Macro-zamia. Chigua has only recently been reported by Stevenson (1990) from South America. Cycadales are thus now represented by 11 genera.
Phylogeny of Cycadales:
Origin of Cycadales from Pteridospermales:
The evolutionary history of an organism or of a taxonomic group of organisms is called phylogeny. In general, it is believed that Cycadales originated from some members of Pteridospermales, which are also called Cycadofilicales or popularly known as “seed ferns”.
Views of some workers, who support the origin of Cycadales from Pteridosperms, are under mentioned:
On the basis of the detailed anatomical studies of several cycads, Worsdell (1906) opined that Cycadales originated from some members of Medullosaceae, a family of Pteridospermales, which extended from the Upper Carboniferous to the Permian. According to him cycads descended from Medullosa stellata whose stem had a medullated vascular cylinder.
The centripetal parts of the stelar rings of M. stellata were lost progressively and this resulted ultimately into a complete loss of central star rings. There also existed a close similarity between the vascular organization of Cycas siamense and Medullosa porosa.
Secondary wood occurs in the form of successive rings in both Cycadales and Medullosaceae. Worsdell’s view of Medullosan ancestory of Cycadales has also been supported by Stewart and Delevoryas (1956) in a detailed article in Botanical Review.
The view that the eustelic condition of cycads has originated from the monostelic condition of some members of family Lyginopteridaceae of Pteridospermales was put forward by Scott (1909) and was later supported by Arnold (1953) in an article published in Phyto-morphology.
According to De-Fraine (1912) members such as Sutcliffa, gave rise to the polystelic forms like Medullosaceae on one hand and Cycadales and Lyginopteridaceae along another line of evolution. She opined that Sutcliffa stem possesses a large centrally located protostele surrounded by many small-sized steles, and this shows a definite resemblance with both polystelic as well as monostelic Pteridospermales.
According to Tailor (1969) Cycadales originated from some members of Palaeozoic Pteridospermales because of the existence of some definite similarities in both these groups. These similarities include the presence of well-developed persistent cortex, mucilage canals, manoxylic wood, centripetal xylem and structure of ovule in both these groups.
Palaeozoic origin of Cycadales has been advocated by Mamay (1969) because of his discovery of some Cycas-like megasporophylls from Lower Permian period of Palaeozoic era. Such megasporophylls had their distal end laminate and their proximal end bore two rows of ovules, and similar types of megasporophylls are found also in several species of Cycas these days.
In-spite of the close resemblances of Cycadales with Pteridospermales, these members also show similarities and differences with other gymnosperms, such as Bennettitales, Pentoxylales, Cordaitales and even Ginkgoales.
Some of these are discussed below:
Resemblances of Cycadales with Bennettitales:
Bennettitales, a group of fossil gymnosperms, have been treated by several earlier workers under Cycadophyta because of their close resemblances with cycads. The order Bennettitales was even named as Cycadeoideales.
Some of the close resemblances between Cycadales and Bennettitales are under mentioned:
1. Fronds in both the groups are pinnately compound and cycad-like.
2. Stem in both is short and remains covered with an armour of persistent leaf bases.
3. Cortex is well-developed and massive in both.
4. Pith is very large in both Cycadales and Bennettitales.
5. The secondary wood is manoxylic in both the groups.
6. The embryo is dicotyledonous.
7. Both Cycadales and Bennettitales (e.g. Williamsonia) are dioecious.
8. Fructifications are unisexual in both Cycadales and Bennettitales (e.g. Williamsonia).
9. Similar to Bennettitales some species of Cycas possess bisporangiate synangia.
In spite of these similarities Cycadales show definite differences from Bennettitales.
Resemblances and Differences in Cycadales and Pentoxylales:
Both Cycadales and Pentoxylales resemble each other in showing:
(i) tree-like habit;
(ii) presence of both syndetocheilic and haplocheilic stomata,
(iii) presence of both centrifugal and centripetal xylem in their leaf traces,
(iv) presence of circular bordered pits on the radial walls of their tracheids,
(v) unisexual reproductive organs, and
(vi) monolete type of microspores.
Some of the striking differences of Cycadales from Pentoxylales exist in their:
(i) leaf (simple in Pentoxylales and compound in Cycadales),
(ii) wood (pycnoxylic in Pentoxylales and manoxylic in Cycadales),
(iii) stele (polystelic in Pentoxylales and monostelic in Cycadales),
(iv) organization of their male and female strobili, and
(v) ovules (integuments remain free from nucellus in Pentoxylales while they remain fused with the nucellus in Cycadales).
Resemblances of Cycadales with Cordaitales:
Cycadales resemble with members of the fossil order Cordaitales in possessing:
(i) Xerophytic nature,
(ii) Large pith and cortex in the stem,
(iii) Abundant sclerenchyma and mesarch vascular bundles in leaves,
(iv) Single integument with three distinct layers in the ovule, and
(v) Similarity in the vasculature of the ovule and structure of the seed
Cordaitales, however, differ from Cycadales in the presence of pycnoxylic wood, simple leaves and compound strobili.
Resemblances of Cycadales with Ginkgoales:
Cycadales also resemble with Ginkgoales in:
(i) The structure of their apical meristem,
(ii) Multi-ciliate spermatozoids,
(iii) Haustorial nature of their pollen tube,
(iv) Well-developed female gametophyte,
(v) Presence of large venter and massive egg in the archegonium,
(vi) Presence of several free-nuclear divisions in the pro-embryo, and
(vii) Dicotyledonous nature of their embryos.