Gymnosperms Lecture Notes PDF

Gymnosperms The gymnosperms are a group of seed-producing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek composite word (gymnos, "naked" and sperma, "seed"), meaning "naked seeds", after the unenclosed condition of their seeds (c...

Gymnosperms The gymnosperms are a group of seed-producing plants that includes conifers, cycads, Ginkgo, and Gnetales. The term "gymnosperm" comes from the Greek composite word (gymnos, "naked" and sperma, "seed"), meaning "naked seeds", after the unenclosed condition of their seeds (called ovules in their unfertilized state). Their naked condition stands in contrast to the seeds and ovules of flowering plants (angiosperms), which are enclosed within an ovary.  Gymnosperms Gymnosperm seeds develop either on the surface of scales or leaves, often modified to form cones, or at the end of short stalks as in Ginkgo. The gymnosperms and angiosperms together compose the spermatophytes or seed plants. By far the largest group of living gymnosperms are the conifers (pines, cypresses, and relatives), followed by cycads, Gnetophytes (Gnetum, Ephedra and Welwitschia), and Ginkgo (a single living species).  Classification of Gymnosperms Class: Class: Class: Gnetopsida Coniferopsida Cycadopsida Order: b) Order: a) Order: Order: Gnetales Taxales Coniferales Cycadales Cupressus Ephedra & Thuja Pinus Cycas  Characteristics of Cycads Cycads resemble palms or tree-ferns in overall appearance but differ greatly in detailed structure and reproductive behavior. Cycads are dioecious (i.e. male and female reproductive structures are borne on separate plants), and reproduction is by seeds produced on open carpophylls or seed-bearing leaves. Although technically woody plants, unlike other woody plants, cycads have a thick, soft stem or trunk made up of mostly storage tissue with very little true wood. The living cycads include about 250 species, with 11 genera in 3 families.  Cycas( description) This very symmetrical plant supports a crown of shiny, dark green leaves on a thick shaggy trunk that is typically about 20 cm (7.9 in) in diameter, sometimes wider. The trunk is very low to subterranean in young plants, but lengthens above ground with age. It can grow into very old specimens with 6–7 m (over 20 feet) of trunk; however, the plant is very slow- growing and requires about 50–100 years to achieve this height. Trunks can branch multiple times, thus producing multiple heads of leaves.  Cycas The leaves are a deep semi glossy green and about 50–150 cm (20–59 in) long when the plants are of a reproductive age. They grow out into a feather-like rosette to 1 m (3.3 ft) in diameter. The crowded, stiff, narrow leaflets are 8–18 cm (3.1– 7.1 in) long and have strongly recurved or revolute edges. The basal leaflets become more like spines. The petiole or stems of the sago cycad are 6–10 cm (2.4–3.9 in) long and have small protective barbs that must be avoided.  Cycas Roots are called coralloid with an Anabaena symbiosis allowing nitrogen fixation. Tannins-rich cells are found on either side of the algal layer to resist the algal invasion. As with other cycads, it is dioecious, with the males bearing pollen cones (strobilus) and the females bearing groups of megasporophylls. Pollination can be done naturally by insects or artificially.  Study of external features of the plant Cycas 1. Plant body is differentiated into an underground root system, that is distinguished into an erect stem and a crown of leaves. 2. Roots are of two types: (i) primary or normal root and (ii) secondary of coralloid root. 3. Normal root is a tap root, growing deep into the soil (positively geotropic). It is sparsely branched and sometimes grows as thick as aerial stem. 4. Secondary roots are negatively geotropic projecting above the soil surface, repeatedly dichotomously branched and appear as coralloid clusters. 5. The young stem is almost tuberous but when grows old, it becomes thick, colummar and unbranched (Branching is rare and is caused due to injury, etc.). The trunk is covered by persistent leaf bases.  Study of external features of the plant Cycas 6. Leaves. The stem bears a terminal group of leaves which are dimorphic (i.e. of two types) (i) foliage leaves (green assimilatory fronds) and (ii) scale leaves (brown and hairy). These leaves alternate with one another. 7. Young foliage leaves are circinately coiled and are covered with ramenta (hairs). 8. Mature leaves are spirally arranged and pinnately compound. Each leaf has about 80- 100 pairs of pinnae that are closely arranged, opposite one another on the rachis with a decurreht base. Each pinna is tough, leathery and entire with a definite midrib but no lateral veins. 9. Scale leaves are small, simple , brown with aborted lamina and covered with hairs. These leaves cover the apex and young developing foliage leaves. Scales are also persistent, like leaf bases. Young foliage leaves are circinately coiled and are covered with ramenta (hairs)  Study of external features of the plant Cycas 10. Reproductive organs. Cycas is dioecious and, as such, bears terminally, either male cone or female reproductive structures. 11. The male cone is borne terminally at the apex of the stem and the further growth of the stem continues by axillary bud (developed at the base of the cone) which pushes the male cone on one side. The branching in Cycas stem is thus referred to as sympodial. 12. The female reproductive structures are the sporophylls developing in place of foliage leaves. The vegetative apex continues to grow as usual. 13. The sporophylls are smaller than the foliage leaves. They are brown or light brown in colour and are densely covered with wooly hairs. Male cone Different types of Megasporophyll Cycas- Megasporophyll with young ovule Cycas- Young ovule  Study of anatomy of normal young root 1. The section is circular in outline. It shows an outer layer or epiblema, cortex and centrally located stele. 2. Epiblema is made of single layer of thin walled cells. Some of these cells bear unicellular root hairs. 3. Cortex is multilayered with starch filled parenchymatous cells. A few tannin filed cells are also scattered in this region. 4. Endodermis is single layered and indistinguishable. Many-layered pericycle separates the cortex from vascular tissues. 5. The central stele is made of radial and exarch vascular bundles. There are two protoxylem groups and thus condition is diarch. Anatomy of normal young root  Study of anatomy of older part of normal root. 1-It shows secondary growth, rest of the structures being similar to that of a young root. 2. The epiblema is ruptured due to the thick walled cork cells fonned below it. Cork cells are a few layered deep and are arranged in brick-like fashion. 3. Cortex is large, parenchymatous and multilayered. It is present below the cork. A few tannin filled cells occur scattered in the cortex. 4. Endodermis is single layered. It is followed by many layered pericycle. 5. Primary phloem is the outennost (near the pericycle) and is crushed during secondary growth. Secondary phloem follows this layer, the cells of which are intact.  Study of anatomy of older part of normal root. 6. Cambium arcs are found along the inner edges of phloem in the vascular region. 7. Secondary xylem is situated towards pith. The primary xylem is situated in the same region as it was before the secondary growth. 8. Medullary rays are found. 9. In the centre is a small parenchymatous pith. Anatomy of older part of normal root  study of anatomy of coralloid root. 1. The structure is almost similar to that of a normal root. It consists of epiblema, differentiated cortex and vascular tissues. 2-Epiblema is outermost and single layered. 3-The cortex is divisible into three regions -outer, middle and inner. These are similar in size. Cortex parenchymatous. 4-The middle cortex is also called algal zone. The cells are radially elongated. A blue-green alga Anabaena cycadae occurs endophytically in these cells. It is believed to be symbiotc and helps in nitrogen fixation.  study of anatomy of coralloid root. 5-Endodermis separates cortex and vascular tissues. It is single layered and followed by many layered pericycle. 6-Vascular bundles are radial and xylem is triarch and exarch. 7. Secondary growth is generally absent; if present, it is verry less. T.S of Coralloid root Cycas-Coralloid root  Study the anatomy of young part of stem. 1. Outline of the section is irregular due to the presence of numerous persistent leaf bases. 2. The structure is divisible into cortex, vascular tissue and pith. 3. Cortex. Greater part of the stem is made of starch filled parenchymatous cortex. It is traversed by many cut, girdle- shaped leaf traces, supplying the leaves. Many mucilage ducts are irregularly scattered in this region. (In Cycas, a leaf is supplied by two large girdle traces, two direct traces and numerous smaller radial traces. The two girdle traces arise from the side of the stele, opposite the leaf. These unite, bifurcate and take a circular route through the cortex before entering the leaf. The radial traces arise from other points of vascular ring but contrary to girdle traces, they adopt a straight radial course in the cortex. They bifurcate producing anastomosing branches which get attached to the girdle traces. In a trancverse section large number of girding leaf traces are cut. This is one of the most conspicuous features of the stem anatomy).  Study the anatomy of young part of stem. 4.Stele is an ectophloic siphonostele. 5.Endoermis surrounds the stele. It is single layered while underlying pericycle is few celled thick 6.Vascular cylinder is composed of many vascular bundles arranged in a ring. Ring of vascular bundles lies near the centre and is very small in comparison to the massive cortex. 7. The vascular bundles are conjoint, collateral, endarch and open. 8. Xylem is made of tracheids only and xylem parenchyma. Vessels are absent. 9. Phloem consists of sieve tubes, phloem parenchyma and phloem fibres. 10. The young stem is monoxylic (Le. with one ring of vascular bundles only). 11. Pith. There is parenchymatous pith in the centre, with scattered mucilage canals. Anatomy of young part of stem  Study of anatomy of the old stem. 1. It shows almost the same structures as those in young stem, except those formed after secondary growth. 2. A periderm is present on the outer side. It is composed of thick walled cells. 3. Cortex is large and parenchymatous. It forms most part of the section. A few mucilage canals are also present. 4. Vascular bundles are formed, due to successive development of cambium rings. Thus, the old stem is polyxylic (with more than one ring of vascular bundles). The number of vascular bundles as well as the thickness of the successive vascular rings, thus formed, is lesser than the first formed ring. Anatomy of the old stem  Study of anatomy of the old stem. 5. Secondary vascular tissues. The successive rings of secondary vascular tissue are separated by parenchymatous zone. This loose, soft and scanty wood is called manoxylic. 6. Medulllary rays are present. 7. Pith. A large pith lies in the centre.. Cells are parenchymatous and starch filled. Many mucilage canals are also present.  Study of anatomy of rachis. 1. Outline. It is cylindrical. It shows insertion of pinnae on the adaxial side (upper side). 2. The rachis is differentiated into epidermis, hypodermis, ground tissue and a ring of vascular bundles. 3. Epidennis is single layered, thickly cuticularized and is interrupted by stomata throughout its surface. The condition is known as amphistomatic. 4. Hypodennis is mainly composed of thick-walled cells (sclerenchyma). Intermixed with these cells are a few cells having chloroplast schlorenchyma. 5. This sclerenchymatous hypodermis is 2-3 layered toward adaxial side and many layered toward abaxial side.  Study of anatomy of rachis. 6. Ground tissue. The rest of the tissue that forms most part of the section is called ground tissue. It is parenchymatous. 7. Mucilage ducts are scattered throughout the ground tissue. Mucilage ducts are double layered, the inner layer being composed of epithelial cells and the outer of tangentially elongated sc1erenchymatous cells. 8. The vascular bundles are arranged in an inverted omega (Q) shaped arc. Each vascular bundle is surrounded by a thick walled, singlelayered bundle sheath. It is conjoint, collateral land open.  Study of anatomy of rachis. 9. The arrangement of xylem and phloem differs in vascular bundles at the base, middle and upper region of the rachis. (i) Higher up and for most part of the rachis, bundles are diploxylic i.e. two types of xylem elements are present - centripetal and centrifugal xylem. The centrifugal xylem occurs in two small groups, present on both the sides of large triangular and centrally located centripetal xylem. The phloem is situated on the abaxial side of the rachis. (ii) At the very base of the rachis, vascular bundles show only centrifugal xylem which is endarch. Phloem occupies the abaxial side of the rachis. (iii) Little higher up the base of rachis, vascular bundles show centrifugal xylem on abaxial side and centripetal xylem on adaxial side. In the centre of these two xylem groups, lies the protoxylem. This conditionis is said to be mesarch. ‫التركيب الداخلى للحامل النصلى ‪Internal structure of rachis‬‬  Study of anatomy of leaflet (pinna). 1. The leaflet shows a distinct midrib and the wings. 2. The midrib is swollen, while wings on the lateral sides are narrower and flattened. (i) In C. revoluta midrib is less projected than in C. circinalis, where it is much projected on the upper side. (ii) Margins of wings are revolute in C. revoluta, and C. beddomei while they are straight in C. circinalis, C. rumphii, C. pectinata and C. siamensis. 3. Upper epidermis is present on the upper side. It is thickly cuticularized and single-layered. 4. Hypodermis is present below the epidermis. It is sclerenchymatous. (i) In C. revoluta, hypodermis is present in the midrib (near both upper and lower epidermis) and wings (below the upper epidermis). (ii) In C. circinalis, hypodermis in the midrib region is present on both the sides (upper and lower) while in the wings, it occupies only the comers, being absent from rest of wings. Internal structure of leaflet  Study of anatomy of leaflet (pinna). 5. Mesophyll lies below the hypodermis and is well developed. It is differentiated into upper palisade layers and lower of spongy parenchyma. (i) In C. revoluta, palisade is present beneath the hypodermis, both in the midrib and the wings. (ii) In C. circinalis palisade is absent from the midrib region 6. Spongy parenchyma with many intercellular spaces lies immediately above the lower epidermis. 7. Transfusion tissue. On either side of the centripetal metaxylem of mid rib bundle and somewhat connected with it, are present two tracheid-like cells-transfusion tissue. 8. Accessory transfusion tissue. Between the palisade and spongy parenchyma cells, there are 3 or 4 layers of tracheid-like, long colourless cells which run transversely from the midrib to near the margin of the lamina. This is known as accessory transfusion tissue. It is connected with the xylem of the vascular bundle of midrib through the transfusion tissue.  Study of anatomy of leaflet (pinna). 9. Lower epidermis bounds the leaflet from lower side. It is thickly cuticularized and single layered. Sunken stomata are found in the lower epidermis in the midrib region. 10. Stomata are very much sunken in the lower epidermis in C. revoluta, while they are not so much sunken in C. circinalis. 11. Midrib bundle. In middle of the swollen portion representing the midrib lies a single vascular bundle surrounded by parenchymatous tissue (with calcium oxalate crystals). Vascular bundle has a definite and thickened, parenchymatous bundle sheath. 12. The vascular bundle is similar in all respects to that found in the upper region of the rachis. It is conjoint, collateral, open and diploxylic. 13. Phloem lies towards the abaxial (lower) side. In between xylem and phloem, cambium is present. 14. Xylem. It shows a large, triangular patch of centripetal xylem and two small groups of centripetal protoxylem.  Life Cycle of the Cycads Male reproductive structure, gynostrobilus grows a pollen is dispersed by wind or pollen tube, sperm cell insects to female reproductive swims through pollen tube structure. (haploid) using its tail, or flagella. (haploid )This fertilizes the egg and it becomes a zygote. (Diploid) Zygote develops into an embryo then a Development of the seed is developed in microspores and a coat that originates megaspores through from the ovary. meiosis (haploid) (diploid) Then the tree grows! (diploid)