In this essay we will discuss about cycas which belongs to the family of class cycadopsida. After reading this essay you will learn about:- 1. Distribution Pattern of Cycas 2. Root of Cycas 3. Stem 4. Leaf 5. Affinities.
Essay # 1. Distribution Pattern of Cycas:
The distribution pattern with identifying characteristics of Indian species of Cycas are:
1. C. Circinalis:
It grows naturally in the deciduous forests of Western Ghats, in Western Peninsular India, and in dry hills of Orissa. It is about 2-4 m tall. The leaves are about 1.5-3m long with flat acuminate leaflets. The male cones are cylindric-ovoid. The mega- sporophylls are linear-ovate with sharply spinose teeth bearing 3-6 pairs of ovules.
2. C. Pectinata:
It grows naturally in the ‘Shorea robusta’ forests of Sikkim, Assam, Khasi hills and Manipur. It is about 2-3 m high. The leaves are about 1.5-2 m long with flat and linear leaflets. The male cones are cylindric-ovoid. The megasporophylls are large and pectinate with numerous serration bearing 2-3 pairs of ovules.
3. C. Beddomei:
It grows naturally in the hills of Cuddapah district of Tamil Nadu and Eastern Andhra Pradesh. It is a low shrub with dwarf trunk, only up to 40 cm high. The leaves are 1 m long with revolute leaflets and rachis is covered with tufted hairs. The male cones are oblong-ovoid. The megasporophylls are ovoid lanceolate, dentate bearing 2-3 pairs of ovules.
4. C. Rumphii:
It is found naturally on the beach forests of Andaman and Nicobar Islands. It is a tall tree, up to 6 m height. The leaves are 1 -2 m long, with flat and broad (12 mm) leaflets. The male cones are ellipsoidal. The megasporophylls are linear-ovate and dentate, bearing 3-5 pairs of ovules.
5. C. Revoluta:
It is a native of China and Southern Japan and is commonly cultivated in India as an ornamental plant in gardens, parks etc. It is about 3 m tall. The leaves are 1-2 m long with revolute leaflet .The male cones are cylindrical. The megasporophylls are large lobed (pectinate), tomentose, bearing 3-4 pairs of ovules.
6. C. Siamensis:
It is a native of Burma (Myanmar), China, Thailand and Yunnan and is grown in India as an ornamental plant. It is about 3 m tall. The leaves are 1-1.5 m long with flat and short leaflets. The male cones are ovoid-oblong and stalked. The megasporophylls are broad and pectinate bearing 2 pairs of ovules.
Essay # 2. Root of Cycas:
At the beginning, there is a primary root system in Cycas. This normal tap root system is short-lived and is replaced by large, fleshy, persistent, branched adventitious roots arising from the stem. Some of the lateral branches grow horizontally in the soil and become apogeotropic. They have numerous lenticels on their surface.
These roots frequently get inflected with bacteria (Pseudomonas, Azotobacter), fungi and blue green algae or Cyanobacteria (Anabaena cycadae, Nostoc punctiforme, Oscillatoria and Calothrix). The infection causes a distortion of their shape.
The infected roots repeatedly dichotomise to become a mass of tubercles apparently looks like a coral. These are called coralloid roots (Fig. 1.13B) which grow horizontally in soil and get swollen at the tip. The exact role of these endophytic Cyanobacteria is not known. However, it has been reported that these Cyanobacteria can fix nitrogen and promote the growth of Cycas plant.
Anatomically, a normal primary root is differentiated into epidermis, cortex and a central vascular system (Fig. 1.13A). Epidermis is comprised of single-celled parenchymatous cells with occasional hairs. The cortex is multi- layered, made up of parenchymatous cells filled with starch.
Some cells of cortex contain tannins and sphaeraphides. The single-layered endodermal cells have characteristic carparian bands and the multilayared pericycle is made up of parenchymatous cells. The stele is diarch with exarch xylem. A mature tap root shows secondary growth in thickness both instrastelar and extrastelar.
Anatomically, the coralloid roots are more or less identical with the primary tap root except for a well-defined ‘algal zone’ in the cortex having abundant intercellular spaces occupied by blue-green algae or Cyanobacteria (Fig. 1.13C). Besides, some fungi and bacteria are also reported from this zone. The stele is diarch to tetrarch. There is a little or no secondary growth in the coralloid roots.
Essay # 3. Stem of Cycas:
The cycas stem is columnar with a thick armour of persistent leaf bases. However, C. siamensis has underground short tuberous stem. The stem is usually unbranched, but sometimes it shows branches which are produced from the bulbil. The stem is irregular in outline because of a number of persistent leaf bases.
Internally, the stem shows a similarity with a dicot stem which is differentiated into epidermis, cortex and vascular cylinder (Fig. 1.14A). The single-layered epidermis in discontinuous due to the presence of leaf-bases. The epidermis is covered with a thick cuticle.
The cortex is massive, made up of parenchymatous cells rich in starch grains. The cortex is traversed by several mucilaginous canals and leaf bases. The endodermal and pericycle layers are not distinct.
In the young stem, the vascular cylinder is very small comprising of several vascular bundles arranged in a ring. Thus the Cycas stem shows ectophloic siphonostelic configuration. The each vascular bundle is conjoint, collateral and open which is separated by parenchymatous medullary rays. The xylem is endarch, comprising of tracheids and xylem parenchyma only.
Several leaf-traces are observed in a T.S. of stem. Each leaf receives four traces. Out of the four traces, two traces enter directly into the leaf after arising from the stelar cylinder on the same side are called direct traces.
The other two traces arise from the stelar cylinder on the opposite side of the leaf and enter the leaf after turning round a semicircle or girdle around the stelar cylinder. These traces are called girdle traces (Fig. 1.14C). The girdle of leaf traces is a characteristic of Cycas and allied members of Cycadaceae. The two girdle traces join with the direct traces and branches of other girdle traces finally enter the leaf as two traces.
The central part of the stem is occupied by a large parenchymatous pith rich in starch grains. Pith cells also contain tannins and mucilagenous substances.
The normal intrastelar secondary growth is observed in early stages due to the bifacial activity of cambium ring. This primary cambium ring is short-lived, thus remains functional for a short time. The secondary xylem thus produced centripetally shows tracheids with 4-5 rows of bordered pits and scalariform thickenings and one row of thin-walled cells in between (Fig. 1.14B).
The three types of vascular rays are produced:
(i) Uniseriate Rays,
(ii) Multiseriate Rays, and
(iii) Foliar multiseriate Rays.
The phloem is well-developed which may exceed the volume of xylem. The sieve elements contain numerous sieve areas on their radial wall. The companion cells are absent, instead some phloem parenchyma termed albuminous cells — are formed, closely associated with sieve cells. The stem does not produce any annual ring.
A peculiarity of Cycas stem is that the young stem which is monoxylic becomes polyxylic at maturity due to the formation of accessory cambium rings in the cortex (Fig. 1.14A). The second cambium ring forms similarly the secondary xylem towards the inner side and secondary phloen towards the outer side.
The second cambium ring remains functional for a short time and thereafter its activity ceases. In a similar fashion, successive cambium rings initiate in the cortical region. The first two vascular rings are thicker than the other vascular rings, of which the second vascular ring is wider than the first.
The subsequent rings are narrower than these two rings, gradually diminishing towards the periphery. Usually 3-4 rings of wood are produced in Cycas, but in C. pectinata as many as 20 rings of wood may be formed. The extrastelar secondary growth has also been observed in the stem of Cycas which forms the periderm.
Essay # 4. Leaf of Cycas:
Cycas exhibits leaf dimorphism in possessing green foliage leaves and scale leaves.
The scale leaves are small, dry, non-green and triangular in shape, covered with ramenta. They have small persistent leaf bases. Their only function is to provide protection to apical meristem and other aerial parts.
The foliage leaves are large, unipinnately compound (Fig. 1.15A). A single leaf bears 75- 100 pairs of leaflets which are arranged on either side of the rachis in opposite or alternate manner. The leaflets are sessile, elongated having revolute (e.g. C. revoluta, C beddomei) or flat (C. circinalis, C. pectinata, C. rumphii) margins.
Each leaflet is provided with a single unbranched midvein without having any lateral veins. The leaves possess a very long and strong rachis with a short petrole. The base of the petiole is provided with two rows of small stiff spines. The young rachis as well as leaflets are circinately coiled like those of ferns (Fig. 1.15B).
The T.S. of rachis shows circular outline. The outermost layer is the epidermis which consists of cuticularised thick- walled cells interrupted by sunken stomata. The epidermis is followed by hypodermis, which is composed of a variable mixture of chlorenchyma and sclerenchyma followed by a parenchymatous ground tissue with many mucilage canals. Initially two endarch bundles enter the leaf base and then they spite up into several bundles.
These bundles are then dispersed in the ground tissue and are arranged in an inverted Greek letter “omega” (Ω) like arc. The vascular bundles are conjoint, collateral and open, bearing both the centrifugal and centripetal xylem.
Thus, they are diploxylic or pseudomesarch in nature. As the bundles move up into pinnae, they become exarch without having any centrifugal xylem. The phloem lies on the side of the protoxylem. The bundles are covered with a bundle sheath.
The Cycas leaflet is dorsiventral showing strongly xerophytic characteristics. The T.S. of a leaflet shows a thick-walled, highly cuticularised epidermis. The upper epidermis is continuous, while the lower epidermis is interrupted by stomata (Fig. 1.16).
The stomata are haplocheilic, situated in pits with overarching rims. The two guard cells are bordered by 8-10 subsidiary cells arranged in a ring. The hypodermis is 1-2 layered thick except for the midrib or at the margins where it becomes 3-4 layered. The hypo- dermal cells are thick-walled, highly lignified.
The mesophylls are differentiated into palisade and spongy parenchyma (Fig. 1.16). The palisade cells are arranged vertically below the hypodermis. These cells have long fibrous thickening that make them resistant against the external radial and longitudinal pressures. The lower part of the mesophyll consists of spongy parenchymatous cells with intercellular spaces.
The vascular bundle is diploxylic in nature, made up of a broad triangular centripetal exarch metaxylem and two small patches of centrifugal endarch primary xylem (Fig. 1.16). The centripetal xylem is separated from contrifugal xylem by a few parenchymatous cells.
Some tracheidal cells situated on either side of the centripetal metaxylem are termed transfusion tissue. Additionally, there are 3-4 layers of transversly elongated thin-walled colourless tracheidal cells in-between upper palisade layer and lower spongy hyer.
These cells constitute the accessory transfusion tissue which serves for lateral conduction in the portion of leaflet that lacks veins. The phloem is situated on the abaxial side below the xylem which is made up of sieve cells and parenchyma only.
A small arc of cambium is present in-between the centripetal xylem and phloem. Many calcium oxalate crystals are found in the parenchymatous tissue below the vascular bundle. The vascular bundle is surrounded by a fibrous bundle sheath.
Essay # 5. Affinities of Cycas:
Cycads probably originated from the Pteridosperm ancestor (Palaeocycas megasporophyll with Taeniopteris leaf) in the Upper Carboniferous period. Among the order Cycadales, Cycas is the most primitive genus because of the absence of female cone. Cycas shows relationship with different groups of plants based on their morphological and anatomical features.
Relation to Ferns (Filicopsida):
Cycas resembles ferns in exhibiting the following characteristics:
i. The young rachis as well as leaflets are circinately coiled like those of ferns.
ii. Microsporangia are borne in sori on the abaxial surface of the microsporophylls like those of ferns.
iii. The sperms are very large and motile with spiral band, bearing countless cilia like those of ferns.
Relation to Pteridosperms:
i. Most of the pteridosperms have stout unbranched columnar stem like those of Cycas.
ii. Presence of manoxylic wood with large pith and extensive cortex in stem.
iii. The leaves are pinnately compound.
iv. The ovules have three-layered integument with two systems of vascular bundles.
v. The presence of nucellar beak with a distinct pollen chamber.
Relation to Bennettitales:
The points of similarity between Cycas and Bennettitales are:
i. The plants are small trees with a barrel- shaped trunk.
ii. The stem anatomy shows a very thick cortex, relatively thin vasculature and large pith. Stele is of siphonostelic with exarch xylem.
iii. Presence of manoxylic wood.
iv. Leaves are pinnately compound with striking similarities in form, size and venation.
v. The presence of dicotyledonous embryo.
vi. The presence of monocolpate (monosulcate) pollen grains.
Relation to Ginkgoales:
There are several common features shared by Cycas and Ginkgo.
i. The presence of broad pith, cortex and numerous mucilaginous ducts in stem. Mucilage ducts are also present in leaf, petiole, embryo or even in root.
ii. The presence of mesarch xylem in the cotyledon leaves and adult leaves.
iii. The presence of nucellar beak with a distinct pollen chamber.
iv. Both the micro- and megasporophylls are foliar in nature.
v. The sperms are motile with spiral band bearing numerous cilia.
vi. There are extensive free-nuclear divisions during the early stage of embryogeny.
vii. The mature seed consists of an endosporic embryo with two cotyledons.
Relation to Cordiatales:
The similarities between Cycas and Cordiatales include:
i. The presence of large pith and centripetal wood in stem
ii. The presence of extensive leaf-sclerenchyma.
iii. The seeds are comparatively simple and fairly large with a single integument.
iv. The presence of motile sperms.
Relation to Coniferales:
Cycas resembles the Coniferales in having the following characteristics:
i. The presence of large pith, broad cortex and centripetal wood in stem.
ii. The presence of leaf-sclerenchyma.
iii. The presence of haplocheilic sunken stomata in leaves.
iv. The seeds are with three-layered integument.