In this article we will discuss about Cycadaceae. After reading this article you will learn about: 1. Distribution of Cycadaceae 2. External Features of Cycadaceae 3. Stem Anatomy 4. Leaf and Root Anatomy 5. Reproductive Structure 6. Embryogeny 7. Seed Formation and Germination 8. Economic Importance.
- Distribution of Cycadaceae
- External Features of Cycadaceae
- Stem Anatomy of Cycadaceae
- Leaf and Root Anatomy of Cycadaceae
- Reproductive Structure of Cycadaceae
- Embryogeny of Cycadaceae
- Seed Formation and Germination in Cycadaceae
- Economic Importance of Cycadaceae
1. Distribution of Cycadaceae:
Cycadaceae were almost worldwide in distribution in the past ages, and the members of this family have been in existence for at least past 200 million years. The members are, however, now restricted to four main regions in the world, namely Central America, South Africa, Eastern Asia and Australia.
Cycas occurs in Japan, Queensland and India, while Bowenia and Macrozamia in north-west Australia and Queensland and Encephalartos in South Africa. Stangeria is restricted to Natal (South Africa), while Zamia is found commonly in Chile, and Ceratozamia and Dioon in Mexico. Microcycas occurs only in Cuba, while Lepidozamia is found only in Australia.
A summary of the distribution of all the 11 genera of Cycadaceae is mentioned below:
(a) Eastern Hemisphere:
Encephalartos and Stangeria: South Africa
Bowenia, Lepidozamia and Macrozamia: Australia
Cycas: Indian sub-continent, China, Japan, Australia, Madagascar
(b) Western Hemisphere:
Ceratozamia and Dioon: Mexico
Zamia: Mexico, West-Indies, North-western South America and Florida.
Chigua: Reported only recently by Stevenson (1990) from Colombia.
2. External Features of Cycadaceae:
Plants of this family are slow-growing and provide a general look like that of a palm with thick, stout, cylindrical and generally un-branched stem, reaching up to 10-15 metres in some species of Macrozamia and Dioon (Fig. 8.1C).
The stem is spherical or tuberous in Bowenia and Zamia. The stem apices are massive and quite large in cycads, and in them the tunica is absent. A crown of pinnate leaves is present at the apex. The leaves are large, pinnate or bi-pinnate and spirally arranged.
In some genera, the rachis is incurved and the leaflets are enrolled in the bud condition showing circinate vernation as in Cycas. The rachis of the young fronds show sub-circinate vernation in Stangeria, Bowenia and Ceratozamia. The leaves reach only up to 4-5 cm in length in Zamia pygmaea while up to 3 metres in Cycas thouarsii and C. circinalis.
A distinct midrib is present in leaflets of Stangeria and Cycas. The midrib is absent in several other genera, and open dichotomous venation is seen in such cases. The leaves are long-lived and persist on the stem for several years. On being shed, they leave a scar on the stem.
3. Stem Anatomy of Cycadaceae:
Owing to the presence of persistent leaf bases, the cycadaceous stems are roughly circular in outline. Both centrally located pith and the peripheral cortex are large and well-developed, and contain mucilage canals. The endodermis and pericycle are not clearly demarcated.
The primary vascular bundles are conjoint, collateral, open and endarch. The leaf traces are cauline i.e. develop singly from the vascular cylinder of the stem. They completely girdle the stem cortex, and the presence of these ‘girdling bundles’ is a characteristic feature of the family (Fig. 8.2A).
The leaf traces, when enter the petiole, form an omega-shaped (Ω) or horse-shoe-shaped pattern in several cycads. Both centripetal and centrifugal xylem are present in the leaf trace bundles, i.e. they are diploxylic (Fig. 8.2B).
A leaf trace usually passes halfway round the stem almost horizontally, and thus enters the leaf almost opposite to its point of origin. At intervals it is being joined by other traces, and thus each leaf receives a number of bundles.
In many members of Cycadaceae the stem or trunk is very strong, stout, large and well- developed, but the amount of secondary wood is surprisingly small. The mechanical strength to such a strong trunk is provided by persistent leaf bases.
In majority of Cycadaceae, only a single persistent cambium is present. But in most of the species of Cycas and some species of Encephalartos and Macrozamia a succession of cambial layers in seen, and these cambial layers form co-axial cylinders of secondary xylem and phloem.
Presence of scalariform tracheids in the secondary wood, a primitive characteristic feature, is present in the stems of Stangeria and Zamia. The wood in Cycadaceae is very diffuse and contains 1-7 cells wide medullary rays.
4. Leaf and Root Anatomy of Cycadaceae:
Anatomically, the cycads leaves remain covered by a thick cuticle. Haplocheilic type of sunken stomata are present. The vascular bundles, in general, are diploxylic, i.e. two types of xylem (centripetal and centrifugal) are present. Usually at the centripetal xylem is triangular with a single protoxylem group. A parenchymatous region usually separates the two types of xylem.
Cycadaceous roots are usually polyarch. Several xylem bundles alternate with the equal number of phloem bundles. Towards the apex, the number of bundles decreases regularly resulting ultimately into a diarch condition at the tip of the root.
Concentric cylinders of secondary wood are produced by the accessory cambial rings in the older roots. The primary xylem is exarch. An algal zone is present in the cortex of the coralloid roots of majority of the living genera of Cycadaceae.
5. Reproductive Structures of Cycadaceae:
All Cycadaceae are strictly dioecious, i.e. possess male and female reproductive structures on different individuals of the same plant species. The sex of the individual plant, at least in some genera (e.g. Cycas), is known to be determined by X and Y chromosomes.
Megasporophylls and Female Cones:
The reproductive organs are borne in the form of compact cones in all living genera, except Cycas The female cones terminate the female plant. But in Cycas, loose and leaf-like megasporophylls (Fig 8.3 A) are spirally arranged and alternate with the cataphylls and vegetative leaves. Sub-opposite pairs of ovules are usually present in the lower part of each megasporophyll in Cycas.
The megasporophylls in the female cone remain spirally arranged around the cone axis. In Ceratozamia, Macrozamia and Zamia, the megasporophylls are peltate (Fig. 8.3B,C) structures. Only one female cone is usually borne at the apex but sometimes two cones may develop due to the development of another meristem.
The smallest female cones (up to 2-3 cm in length) of Cycadaceae develop in Zamiapygmea, while in Macrozamia denisonii they reach up to 60-75cm in length. In Dioon spinulosum also they reach up to 50-60 cm in length.
Because of its leaf-like nature, the megasporophyll of Cycas is regarded to be the most primitive by some botanists. These botanists believe that dunng the course of evolution among the Cycadales, the distal foliar part of their megasporophyll underwent gradual reduction. This was accompanied also by the reduction in the size and number of the ovules on the megasporophyll.
The megasporophylls of Cycas revoluta with leafy distal part and several pairs of ovules are, therefore, most primitive. Next in the series towards advance nature are the megasporophylls of Cycas circinalis, in which the distal part is laminate but does not appear leaf-life.
The megasporophylls of Macrozamia and Zamia are peltate, bear only two ovules, and their distal part is not leaf-like. These are, therefore, regarded as most advanced. This also suggests that ovules are foliar in origin, and Cycadales are. therefore, phyllospermous.
According to the another view, the Cycadales are stachyospermous and not phyllospermous. Botanists supporting this second view believe that peltate megasporophylls of Zamia and Macrozamia with two ovules are most primitive, and the entire series has progressed in an opposite direction.
Leaf-like megasporophylls of Cycas revoluta with several ovules represent the most advanced stage. According to this second view, the ovules are cauline in origin, and Cycadales are, therefore, stachyospermous.
Microsporophyll’s and Male Cones:
The male cones are compact structures consisting of a central cone axis covered by several spirally arranged microsporophyll’s. Except Macrozamia moorei, the male cones are terminal in position. The microsporophyll’s, in general, are triangular or conical structures possessing definite sterile and fertile portions.
The sterile portion is usually distal in position and produced into a single spinous projection as in Macrozamia (Fig. 8 .3D), Dioon (Fig. 8.3E) and Cycas. But in Ceratozamia mexicana it is produced into two such projections (Fig. 8.3F). In Zamia (Fig. 8.3G), the sterile distal part is least developed and does not taper into a fine projection.
The lower or abaxial surface of the microsporophyll contains thousands of microsporangia in most of the genera. In Zamia, however, only 20-50 microsporangia develop on a microsporophyll. Their number is only 5 or 6 in Zamia furfuracea. Microsporangia oil the microsporophyll remain usually arranged in definite groups called sori. Each sorus contains two to four or more sporangia.
The development of microsporangium in all Cycadaceae is eusporangiate. The mature microsporangium (Fig. 8.3H) remains surrounded by a massive sporangial wail consisting of several layers. The cells of the outermost layer become thick.
It functions as the epidermis. The sporangial wall encloses the sporogenous tissue, the outermost cells of which function as single-layered tapetum. The sporogenous tissue metamorphoses into diploid microspore mother cells. These spore mother cells undergo meiosis and each forms four haploid microspores or pollen grains.
The haploid chromosome number varies from 8-13 in Cycadaceae. It is 8 in Stangeria, 9 in Dioon, 11 in Cycas and 13 in Microcycas.
Ovule and Female Gametophyte:
Cycadaceous ovule (Fig. 8.3I) is generally a sessile structure. It consists of a nuceiu-5 surrounded by a single integument. A distinct micropyle is present at the distal end. The integument consists of three layers, of which outer and inner layers are generally fleshy, while the middle one is a stony layer.
The nucellus consists of parenchymatous cells, when young. A megaspore mother cell is soon distinguished in the nucellus. This mother cell divides meiotically into 4 haploid megaspores, of which three degenerate and only one remains functional. Several free-nuclear divisions in the functional megaspore followed by wall formation in the later stages result into the development of cellular female gametophyte.
As many as 1000 free nuclei may be seen, and the wall formation usually starts at the peripheral region. A vascular strand generally enters through the basal part of the ovule and constitutes its vascular supply. In some cycadaceous ovules, the vascular strand may divide even before entering the basal part of the ovule.
In most of the cases, however, two concentric vascular systems (outer and inner) are established at the base of the ovule. The outer vascular system consists of about twelve vascular strands and traverse upwards through the outer fleshy layer from chalazal end to micropylar end of the ovule. The strands of the inner vascular system move upwards through that part of the ovule where the inner fleshy layer remains in close contact with the nucellus.
Interestingly, stomata-like structures, possessing cells resembling guard cells and subsidiary cells, have also been reported by some botanists on the outer surface of the nucellus in the ovules of Ceratozamia, Cycas, Encephalartos and Zamia.
A lysigenous cavity develops within the nucellar beak in the cellular female gametophyte This cavity represents the pollen chamber. One or more archegonial initials appear at the micropylar end of the female gametophyte. About a dozen or even more such initials appear in some genera (e.g. Microcycas). Each of these initials develops into an archegonium (Fig. 8.3I).
Pollen Grain and Male Gametophyte:
Microspores or pollen grains are haploid structures, and each develops into a male gametophyte Each microspore remains surrounded by an outer thick exine and inner thin intine (Fig. 8.4A). The exine is more thick at the bottom and comparatively thinner at the top.
The generally thin intine is slightly thicker along the sides. The single prominent nucleus of the microspore is centrally located and remains surrounded by dense cytoplasm which contains some reserve food.
Germination of the microspore is precocious, i.e. starts within the microsporangium itself. They are released from the microsporangium when they are usually at 3-celled stage (a persistent prothallial cell, a generative cell and a tube nucleus; Fig. 8.4B). Their further germination takes place on the nucellus after the completion of the pollination process.
Pollination is affected by wind. Under this process, some of the pollen grains at 3-celled stage, reach up to the micropyle of the ovule and get entangled in the pollination drop. Through the micropyle such young pollen grains reach up to the pollen chamber of the ovule. Each pollen grain germinates by producing a pollen tube that penetrates through the nucellar tissue.
In majority of the genera the tube nucleus moves into the pollen tube. After a few weeks, the generative cell divides into a stalk cell and a body cell. The stalk cell usually remains in close contact with the prothallial cell The body cell divides into two multi-flagellate spermatozoids.
The spermatozoids are well-developed, and in Dioon they reach up to 275 μ in diameter, which is probably the largest size of the male gametes in the plant kingdom.
6. Embryogeny of Cycadaceae:
After fertilization, the diploid oospore enlarges in size inside the venter of the archegonium. The zygotic nucleus undergoes repeated free-nuclear divisions (Fig. 8.5A,B) and as many as 64 (Bowenia) to 1000 (Dioon) free nuclei are formed. Wall formation starts at the base (Fig. 8.5C) and proceeds towards the archegonial neck.
In Bowenia, Stangeria and Zamia the cell formation is restricted only up to the basal region, but in genera such as Cycas, Encephalartos and Macrozamia the entire embryo becomes cellular. The cells of the upper region elongate and develop into suspensor (Fig. 8.5D). Various parts of the young embryo are developed from the basal cells.
Soon, two cotyledons develop (Fig. 8.5E). The root apex develops in the young embryo towards the neck of the archegonium. Between the root apex and the suspensor develops the coleorrhiza. The suspensor becomes coiled, and in some cycads it reaches up to 7-8 cm in length Cycads show polyembryony, but only one out of the several developing embryos attains maturity. Usually, the embryos are dicotyledonous.
7. Seed Formation and Germination in Cycadaceae:
Variously coloured and beautiful seed coat of the seed is formed by the outer fleshy layer of the integument of the ovule. Middle stony layer forms its hard testa. The inner fleshy layer, if at all persists at maturity, changes into a papery layer called tegmen of the seed.
The nucellus may form a nucellar cap at the micropylar end. The micropyle persists as such in the seed. The coild suspensor of the embryo is finally pressed against the micropylar end. The embryo extends the exine length of the seed.
Cycadaceous seeds require almost no resting period for germination. Upon falling from the plant, a seed starts germination immediately if conditions are favourable The germination of the seed is hypogeal.
8. Economic Importance of Cycadaceae:
Cycadaceae, in general, are not of much use. However, some of their uses are under-mentioned. Economic importance of Cycas is discussed elsewhere in this chapter.
1. Beverage – Yielding Cycad:
A beverage is prepared from the pith of Encephalartos by some tribal people of Central Africa.
2. Edible Cycads:
Underground tuberous stems of Bowenia spectabilis are cooked and eaten by some native people of Queensland. Bread is prepared from the starch of Encephalartos, Macrozamia and Zamia. Boiled seeds of Dioon mejiae are also used for making bread.
The kernels of Dioon edule are roasted and eaten with taste by certain tribal people. The seeds of Encephalartos yield a gum which is eaten by the people where this plant grows.
3. Hats and Mats From Cycads:
Hats, mats, baskets and several other similar articles are prepared from the leaves of Encephalartos in some African countries. Ramental hairs obtained from the leaf bases of Macrozamia are used as stuffing fibres.
4. Medicinal Value:
A decoction prepared from the seeds of Dioon edule is used by the Mexicans for treating neuralgia.
5. Oil From Cycads:
Seeds of Macrozamia yield an oil used as palm oil by some native people.
6. Poisonous Cycad:
Some poisonous substances have been reported in the leaves of Macrozamia. Animals eating leaves of this cycad many show some symptoms similar to rickets.
7. Starch-yielding Cycads:
Sago starch or “arrowroot” is obtained from the seeds and stems of Macrozamia, Zamia and Encephalartos. This is used in laundering and several other purposes.