General accounts and phylogenetic postion of Cycadales

General accounts 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.
> 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”.
General Characters of Cycadales:-
i. The plant body is sporophytic, and the sporophyte is differentiated into well-developed roots, columnar and generally un-branched stem and pinnately compound leaves.
ii. 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.
iii. Young leaves show circinate vernation.
iv. The wood is manoxylic.
v. Mucilage canals are present in the pith as well as cortex.
vi. The leaf trace is diploxylic
vii. Plants are dioecious without exception, and the reproductive organs are generally in the form of cones.
viii. The cones are generally terminal or lateral in position.
ix. In male cones the microsporophyll’s are arranged on cone axis, and form a compact structure.
x. Microsporangia are arranged on the abaxial side of the microsporophyll’s.
xi. 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.
xii. 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.
xiii. The ovules are orthotropous.
xiv. The apical meristem in all Cycads is extremely massive.
Genera of Cycadales:- Bowenia, Ceratozamia, Chigua, Cycas, Dioon, Encephalartos, Lepidozamia, Macrozamia, Microcycas, Stangeria and Zamia.

Phylogenetic postion 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”.
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:
i. Fronds in both the groups are pinnately compound and cycad-like.
ii. Stem in both is short and remains covered with an armour of persistent leaf bases.
iii. Cortex is well-developed and massive in both.
iv. Pith is very large in both Cycadales and Bennettitales.
v. The secondary wood is manoxylic in both the groups.
vi. The embryo is dicotyledonous.
vii. Both Cycadales and Bennettitales (e.g. Williamsonia) are dioecious.
viii. Fructifications are unisexual in both Cycadales and Bennettitales (e.g. Williamsonia).
ix. Similar to Bennettitales some species of Cycas possess bisporangiate synangia.
Resemblances in Cycadales and Pentoxylales:-
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.
vi. monolete type of microspores.
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.
v. Similarity in the vasculature of the ovule and structure of the seed.
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.

Cycas:-
Classification:-
Division - Cycadophyta
Class - Cycadopsida
Order - Cycadales
Family - Cycadaceae
Genus - Cycas
Important Species:-
i. C. revoluta (Sago Palm)
ii. C. circinalis
iii. pectinata
General Morphology:- The plant body is a sporophyte. It is differentiated into root, stem and leaves.
1. Stem:- 
> Stem is unbranched. It is covered by thick, woody, persistent leaf bases. It makes the stem rough. 
> The apex of the stem is ensheathed by a group of brown scales. 
> The growth of the stem is very slow. 
> It produces a cluster of leaves each year.
> Older leaves fall of after two years.
2. Leaves and scales:- 
> The leaves are produced in the axils of the scales near the apex. 
> Each leaf is composed of a petiole, rachis and lateral pinnae. 
> The young leaves show circinate vernation.
> Scales are also produced each year. Therefore, the clusters of green leaves and scales alternate with each other. 
> Scales are also persistent. 
> Scales and leaf bases cover the surface of the old stem.
3. Roots:- 
> The primary root persists in Cycas. It becomes tuberous.
> Cycas produces coralloid roots. 
- Coralloid roots are short tufts and dichotomously branched roots. 
- These roots contain an endophytic algae in the inner part of their cortex. 
- Sometimes, bacteria are also present in the cortex. Bacteria fix nitrogen.
Anatomy of Root, Stem and Leaf:-
Anatomy of Root:- 
> Resembles that of a dicot root.
> Xylem is diarch or triarch.
> In the coralloid root, the algal zone is in the cortex, which is termed as middle cortex.
Anatomy of Stem:-
> Show irregular outline due to the presence of leaf bases, therefore epidermis is not a continuous layer.
> Broad cortex is traversed by simple and girdle leaf traces.
> Numerous mucilage canals, starch grains also present.
> The vascular bundles in stem are conjoint, collateral open and arranged in a ring.
> The xylem of the vascular bundles consists of tracheids only.
> The pholem comprises of sieve tube and phloem parechyma, they lack companion cells.
> The secondary growth in a young stem is normal, but as it becomes older, new rings are formed giving rise concentric rings of secondary vascular tissues.
Anatomy of Leaf:-
> Tough and leathery texture and other structures such as strongly cutinized thickend epidermis, sunken stomata restricted only to the lower surface, presence of transfusion tissue; all these point to a
xerophytic adaption of Cycas.
> Transfusion tissue is present in the leaves which help in conduction.
> Mesophyll is differentiated into palisade and spongy tissue.
> Vascular bundle of are conjoint, collateral.
> Xylem is diploxylic, the larger patch is the centripetal xylem and the two lateral patches are centrifugal xylem.
> Phloem is present below the centrifugal xylem.
Reproduction:-
> Cycas is dioecious. The male and female plants are separate.
> Sometimes, Cycas plants also reproduce vegetatively. It produces buds on the stem. These buds grow to form new plant.
Male Cone:-
- Male cones are produced on the male plants.
- Number of cones produced each year varies from one to many.
- Each male cone is fusiform in shape. 
- Each cone has a central axis. It bears a number of spirally arranged microsporophyll.
- The microsporophylls are woody in texture. They are wedge shaped.
- The microsporangia (Pollen sacs) or sori cover the lower surface of the microsporophylls. 
- Each sorus contains 2-6 sporangia or anthers which are filled up with pollen grains or microspores. 
- Microspore represents the first cell of male gametophyte stage. 
- The upper part of microsporophyll is sterile and pointed called apophysis.
- A large number of spores are produced in Cycas.
Female Cone:-
- There is no true female cone is Cycas.
- The megasporophyll arise at the apex of the female plant and are loosely arranged in rosette manner.
- On either side of the broader part of sporophyll, 4-6 red coloured ovules or megasporangia are arranged.
- Each ovule is covered by a single massive integument. It has a narrow micropyle at the tip. Integument projects around the micropyle to form a small beak. Nucellus projects into the micropyle. But later the nucellar cells in this region disorganize to form a small cavity called pollen chamber. It plays an important part in early stages of fertilization.
- One of the nucellar cells increases in size and becomes megaspore mother cell. It undergoes meiosis to form four megaspores. Three megaspores degenerate. Only one becomes functional megaspore.
Fertilization:-
> The pollen grains reach the ovule through the medium of air.
> The method of fertilization in said to be siphano-zoidogamy (because of multiciliate antherozoids and tube).
> In Cycas polyembryony is often seen as egg of almost all the archegonia are fertilized and produce embryo.
> In Cycas, alternation of generation in quite prominent.
Economic Importance of Cycas:- Cycas are used for decorative purposes, horticulture and in ceremonies. They are also used as food and medicine.
i. Many species of Cycas are used as ornamental plants.
ii. Starch is obtained from Cycas revoluta (sago palm) and some other species.
iii. Some parts of the Cycas species plants are also used to prepare alcoholic drinks.
iv. Some of the Cycas species produce edible seeds.
v. Leaves are used for decorative purposes and to prepare baskets, hats, etc.
vi. Seeds and raisins obtained from some species are used to treat ulcers, sores and swellings.
vii. Juice of young leaves is used to treat vomiting and flatulence.
​​viii. Young succulent leaves of various species are often cooked as vegetables.
ix. Cycas seeds are also used to treat hypertension and musculoskeletal disorders.