Q.8. Discuss the anatomical characters of root of cycas.
Ans. Transverse section of cycas root shows following parts:
Epiblema: -
Outermost layer made up of thin walled cells. It has root hairs.
Cortex: -
Multilayered made up of thin walled parenchymatous cells with starch. Numerous mucilage canals with air space and tannin filled cells are frequent.
Endodermis: -
It is single layered with casparian bands and passage cells.
Pericylce: -
It is multilayered rich in starch grains.
Vascular Cylinder: -
Stele is diarch but protoxylem points are 3-8 vascular bundles are radial i.e. xylem alternates with phloem patches. Protoxylem is exarch with spiral thickenings. Metaxylem is made up of scalariform tracheids. Phloem is made up of sieve tubes and phloem parenchyma and fibres also in old roots. Pith is small and composed of parenchymatous cells, or absent.
Q.9. Give a well illustrated account of the anatomy of the leaf of cycas and comment upon the features of special interest found there in.
Related Questions -
Q. Describe the internal structure of Rachis of cycas. (2012)
Q. With the help of the labelled diagram, describe anatomy of cycas leaflet. (2010, 11)
Q. Describe anatomy of leaf let of cycas. (2005)
Q. Draw diagram of internal structure of leaflet of cycas. (2003)
Ans.
Cycas leaves are dimorphic, i.e., of two types-scale leaf and foliage leaf. The scale leaves are very much reduced and do not have any special features in its anatomy.
The foliage leaf, as we know, is a pinnately compound leaf and consists of a (i) Rachis or petiole (ii) Leaflet.
(i) Rachis: -
The petiole or rachis of the pinnately compound leaf of Cycas is very stout, woody and biconvex in outline. It bears two rows of leaflets. The transverse section of rachis shows a outermost layer, epidermis. It is heavily cutinized and possess stomata, it is followed by a thick walled sclerenchymatous hypodermis. Below is the parenchymatous ground tissue remain present. These tissue on its peripheral region consists of several mucilage canals. A large number of vascular bundles remain arranged in omega shaped manner, in the ground tissue. Each vascular bundle is open and collateral. which is surrounded by thick walled bundles sheath and followed by an endodermis. The xylem in each vascular bundle is diploxylic in that it is differentiated into centrifugal (endarch) and centripetal xylem (exarch). The cambium lies below the centrifugal xylem. The vascular bundles different in structure at the base centre and apex of the rachis, At the base of rachis the bundles are endarch, in centre they are mesarch while at the apex they are exarch.
Cycas Leaflet: -
Leaflets of Cycas are dorsiventral and hypostomaticic, stomata on lower side only. Each leaflet is made up of a lentral swollen mid-rib and two side arms.
Transverse section shows :
Epidermis - Outermost layer, made up of thick walled pitted cells. It. is broken on lower side due to stomata. It is covered by cuticle which is protective and check transpiration.
Stomata: - Stomata opens into “epistomatal cavity and inside into a hypostomatal chamber.” It is a heplocheilic type. Each stomata consists of two guard cells surrounded by two subsidiary or buffer cells to regulate the mechanism of stomatal movement. The guard cells communicate with outside atmosphere through epistomatal cavity bounded by modified epidermal cells the stomata are amphicyclic surrounded by two rings of subsidiary cells.
Hypodermis - Below upper epidermis a single layer of thick walled sclerenchymatous hypodermis remains present. In the midrib region the hyodermis is two layered and lies below upper and lower epidermis both. It serves as a heat screen and protects the plant from overheating and excessive transpiration. It also provides the mechanical support.
Mesophyll - After hypodermis is a zone of chlorophyllous mesophyll cells. The mesophyll is act as assimilatory tissue, differentiated into outer palisade layer and inner spongy parenchyma. Palisade cells are elongated rich in chloroplast longitudinal fibres and lignin are reported in the walls of these cells.
Palisade layer may be continous over midrib as in C. revoluta or may not be in C. circinalis palisade. Spongy parenchyma consists of small oval or irrigular and loosely arranged cell, containing chloroplasts and enclosing air spaces. It serves as assimilating and aerating tissue.
Transfusion Tissue - It is present between palisade and spongy parenchyma. It is made up of horizontally elongated hyaline porous parenchymatous cells connected with stele. This is considered as lateral extension, of centripetal xylem or derrivatives of leaf parenchyma or modified pericycle or mesophyll. Bordered pited thickenings have also been observed in the cell walls of transfusion cells. A few lignifled tracheids with bordered pits are also scattered among transfusion paranchyma. The transfusion tracheids continute primary transfusion tissue while the radial parenchyme constitutes the secondary transfusion tissue. Some of the parenchyma with calcium oxalate crystals called sphaeroraphides are present around the stele.
Vascular Bundle - It is surrounded by a sclerenchymatous sheath. The protoxylem is diploxylic, the centripetal xylem lies towards the sides of upper epidermis. Protoxylem is mesearch phloem lies towards lower epidermis. Cambium is present between xylem and phloem.
Xerophytic Characters
Following xerophytic characters are important:
1. presence of thick cuticle.
2. Epidermis is thick walled
3. Leaves with stomata on lower side.
4. Hypodermis sclerenchymatous lignified and present below and above epidermis.
5. Palisade cells lignified with fibres.
6. Sunken stomata present only on the ventral side.
7. Presence of transfusion tracheids and transfusion parenchyma.
8. Lateral veins are absent.
9. Unbranched midrib.
10. Diploxylic vescular bundles.
11. Crystals of calcium oxalate and numerous mucilage canals are present.