Diversity of seed plants and their systematics

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CYCAS CYCADOPSIDA CYCADALES CYCADACEAE CYCAS The plants are short or medium sized trees and look like palms. Cycas is found both as cultivated or in wild state in tropical and subtropical regions of the world. It normally grows in well drained soil exposed to sun, such as sunny slopes of the hills, and thus exhibits many xerophyic characters. The genus is distributed in the Eastern Hemisphere, has several species spread over Madagarcar to Japan including Australia. Cycas is the only genus of family Cycadaceae that occurs in India. It grows in Tamil Nadu, Karnataka, Kerala. Andaman and Nicobar Islands, Bengal, Bihar, Orissa, Assam and Sikkim (Fig.1) It also grows in neighbouring countries such as Nepal, Myanmar (Burma) and Srilanka. Four species of Cycas grow wild in India. 1. C. beddomei Dyer: This is found only in the hills of Cuddapah district of Tamil Nadu and eastern Andhra Pradesh. 2. C. pectinata GriffL It Grows in the sal forests of Sikkim and Assam, Khasi hills and Manipur. 3. C circinalis linn: This is the most abundant, naturally occurring species. It grows in the deciduous forests of Western Ghats and on eastern side as for as Orissa 4. C rumphil. Miq : It is distributed on the beach forests of Andaman and Nicobar Islands. Apart from the above mentioned wild species. 5. C revoluta Thumb. and 6. C siamensis Miq. Are commonly cultivated in the gardens. MORPHOLOGY: The plants soprophyte possess columnar aerial trunk and a crown of pinnately compound leaves(Fig. 2A). The stem is usually unbranched but branched old stems are not rare. The stem in the young plants, is stout, almost tuberous and subterranean, the top bearing a crown of spirally arranged leaves. The growth is very slow and in the course of time the stem builds up into an aerial, thick, columnar unbranched trunk from the summit of which arises a crown of large fern-like leaves. (Fig.2A,B; 4C) A thick armour of alternating bands at large and small rhomboidal leaf bases covers the aerial stem in a spiral manner. The large leaf bases belong to the foliage leaves and smaller ones to the scale leaves (in male plants) and sporophylls. (in female plants). The armour is persistent and the age of Cycas Plants can be determined by counting the number of leaf bases on the trunk and dividing their number by half the number of leaves in the crown. Two crowns of leaves are produced annually one in the spring and other during monsoon, coinciding with the favourable seasons of growth. The stem bears adventitious buds at the base. These are large and are covered with scales and called the bulbils. (Fig. 4E) There serve as means for vegetative propagation of the plant both artificially and in nature. Cycas produces two types of leaves: Large green foliage leaves and smaller scale leaves. Crown of foliage and scale leaves alternate regularly. The scale leaves are much smaller with a highly reduced lamina and a well developed base. The scale leaves brown due to a heavy coating of ramental hairs. The foliage leaves are unipinnately compound with numerous thick and leathery, sessile, ovate-lanecolate, spine-tipped leaflets (Pinnae) which may be alternately or oppositely placed. (Fig. 2C; 4D) The leaves posses a long rachis and a short petiole. The base of the petiole shows two rows of small stiff spines. The number of pinnae per rachis increases with age and in an adult plant it may be as high as one hundred or even more on either side of the rachis. The leaflets are mostly narrow and linear. There is a single midrib running the entire length of pinnae. The margin of leaflets may be revolute or they may be flat. The margin of the pinnae is of taxonomic value. There are recolute in C revolute and C. beddomei, and flat in C. circinalis, C. rumphii, C. Pectinata and C. Siamensis. Like the ferns the rachis is straight or incurled and the leaflets are inrolled in the bud conditions. ie. show eircinate vernation. (Fig.2C; 3A, B) 1
The Primary root system is a tap root which dies early and is released by larg, fleshy, persistend adventions, root. Some of the lateral branches, growing near the ground surface, frequently get infected with bacteria, fungi and the blue green algae which enter through lenticels. Some of the lateral roots give out branches which become apo-geotropic, grow vertically upward just below the ground level. They branch repeatedly to form dwarfed dichotomously branched coral – like masses (Fig. 4A, B). They are called the coralloid roots or corallorhiza. They become inhabited by a blue green alga. A large number of lenticels are present on the coralloid as well as normal roots. The presence of numberous lenticels, large air spacer in the cortex and their peeping out of the ground points to the role as aerating organs. ANATOMY ROOT: Primary Root:- There is a single layered cpidermis surrounding a large thin walled parenehymatous cortex. Tannin-filled dark brown cells are interspersed all over. The cortex is delimited by single layered endodermis. (Fig. 5A) Next is the single layered pericycle. The stele is diarch at the tip but becomes tetrarch or even more (up to 8 protoxylem groups) higher up at the point of attachment. The primary root is short lived and is soon replaced by adventitious roots which are mostly diarch. The xylem elements alternate with phloem. The xylem consists of exarch protoxylem elements with spiral and scalariform thickenings. The mataxylem shows bordered pitted tracheids. There is a small paranehymatous pith in the centre. The metaxylem elements may fuse or meet in the centre. In this case there is not pith. Secondary growth takes place in the older roots. Several concentric rings of vascular bundles as wells as the periderm develop in old roots (Fig. 5B-D) The secondary xylem is manoxylic with many multiseriate rays. The tracheids show scalariform and bordered pitted thickenings. Coralloid Root:- A transverse section of the coralloid root shows anatomical features similar to that of the normal root but differs in possessing a much wider cortex and in showing a characteristic greenish algal zone almost in the middle of the cortex. The outermost zone consists of a few layers of cork cells frequently interrupted by lenticels. The cortex is divided into three zone-the outer cortex, middle cortex and he inner cortex. (Fig. 6A, C) The outer and inner cortical regions comprise parenchyma and the middle cortex the algal zone consists of radially elongate thin walled cells with largeintercellular spaces occupied by blue-greeen algae or cyanobacteria such as Anabena, Nostoc and Calothirix. Rarely these may be present iultracellularly (Kubitzki 1990), initially, these cells are more richly cytoplasmic than the adjacent cells. The stele is exarch, diarch or triarch or tetrarch and surrounded by pericyele followed by endodermis (Fig. 6A, C) The development of vascular tissue is poor as compared to normal root and there is little or no secondary growth. There is a difference at opinion, regarding nitrogen fixing ability of coralloid roots. However there are reports to show that cyanobacteria fix nitrogen and promote the growth of host plant (see Kukitzki, 1990) Aerial Roots:- Non- coralloid aerial roots have been reported in Cycas cireinalis, C. revoluta and C. rumphil (Pant and Das 1990). There roots are positively geotrophic, adventitious and grow out from the lower sides of young and old bulbils or leaf bases while still attached to the plant. Sometimes these roots may be branched. The roots are di-to polyarch, and show a wide cortex and a mixed parenchymatons pith with scattered xylem element. The older roots exhibit fair amount of secondary growth, showing unusually branched xylem elements, some with pores like those of vessels. The characteristic algal zone of coralloid roots is absent in these roots. STEM: The stem anatomy shows certain primitive features. There is a large cortex and pith and contains surprisingly small amount of wood (Manoxylic) A young stem of Cycas has an irregular outline on account of persistent leaf bases (Fig. 7A, B). The epidermis is discontinuous and ruptured owing to the presence of the persistent leaf bases which completely ensheath the stem. Cortex forms the greater bulk even in old stems and consists of thin walled parenchymatos cells densely filled with starch grains (In some species, starch is commercially exploited as “Sago”) Numerous mucilage canals are found in this region. They are connected with the mucilage canals of the pith by medullary rays. Peculiar, curving (girdle) leaf traces occur in the cortex. Endodermis and pericycle are not distinet. The vascular Cylinder comists of a primary ring of numerous, closely set vascular bundles. The bundles are endarch, conjoint, collateral and open. The xylem consists of tracheids and xylem paranehymma. The protoxylem has tracheids with spiral thichening and the metaxylem with scalariform thicknings and bordered 2
Page 1: Diversity of seed plants and their
Page 5 and 6: layers of radially seriated seconda
Page 7 and 8: small as compared to tube cell and
Page 9 and 10: As many as a dozen pollen tubes may

Diversity of seed plants and their systematics

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