Unit 7 Origin of Land Plants PDF
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This document provides an overview of the origin of land plants, focusing on the different theories and hypotheses surrounding the evolution of bryophytes and vascular plants. It covers topics such as the alternation of generations, stelar evolution, and the origin of seeds, offering comparative morphological analysis and phylogenetic insights. The document also explores the different views and theories presented by various botanists.
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Unit 7 Origin of land plants – Terrestrial algae and Bryophytes; alternation of generations. Early vascular plants – Stelar evolution; Sporangium evolution; seed habit and evolution of seed. Angiosperms – Phylogeny of major groups. Origin of land plants Bryophytes are small, non-vascular land pla...
Unit 7 Origin of land plants – Terrestrial algae and Bryophytes; alternation of generations. Early vascular plants – Stelar evolution; Sporangium evolution; seed habit and evolution of seed. Angiosperms – Phylogeny of major groups. Origin of land plants Bryophytes are small, non-vascular land plants, which have multicellular jacketed sex organs. The defining features of bryophytes are that their life cycle features alternating haploid and diploid generations with a dominant, branched gametophyte stage. Bryophytes are quite soft, and therefore, they lack fossil records. Also, no intermediate forms are available. Hence, the question of the origin of bryophytes has been debated at length. All the views are based on the evidence under the following three heads: Evidence from comparative morphology of the living plants Evidence from ontogeny of the living plants Evidence-based on analogies with the living plants of other groups Bryologists are divided into two schools of thought on the origin of bryophytes. One school of thought supports the Algal hypothesis of the origin of bryophytes and the other supports the Pteridophytean hypothesis of the origin of bryophytes. Algal Hypothesis This view has been supported by Church (1919), Campbell (1940), Frye & Clark (1937-45), Fritsch (1945), and others. According to the Algal hypothesis, the Archegoniatae ( bryophytes and pteridophytes) were derived from algae. The archegoniatae in turn gave rise to Spermatophyta (seed plants). It is assumed that bryophytes sprang from such semi-terrestrial algal ancestors in which, due to the scarcity of an external aquatic environment, the fertilization was dependent on waters (rains or dew). As a result, the sporophyte assumed viable form zygote and adapted to the land habits gradually. The occurrence of cytological alternation of generations in algae and bryophytes also supports this view. Affinities with Algae Amphibian nature. Lack of vascular tissues in the plant body (thallus). Autotrophic mode of nutrition. Prominent gametophytic life cycle. Presence of flagellated spermatozoid. Water is essential for fertilization. Church’s View (1919) Church postulated his theory of the origin of bryophytes from algae on the following assumption: The primitive Hepatics, like algae, had morphologically similar sporophytes and gametophytes. A terrestrial plant had descended probably from submerged aquatic species of green algae in which cytological alternations were already attained and also remained effective through migration to land plants. Somatic equipment of Phaeophyta such as in the multiseptate organization, differentiation into massive main axis and lateral branches, dichotomy, conducting tissues, etc. Campbell’s View (1940) According to Campbell, bryophytes have descended directly from Chara. His view is based on the close similarity of the sex organs of Chara and Bryophytes. The sex organs of Chara have a sterile enveloping jacket like the archegonium of bryophytes. Frye & Clark’s View (1937-45) Frye and Clark held that both Chara and ancestral primitive hepatics arose on parallel lines from ancient algae, from such primitive hepatics present-day bryophytes (hepatics) sprung. The primitive ancient algae were supposed to have filamentous gametophytes with apical growth and hairy sperm cells constituting the male filament and enlarged egg cells in a branch that were surrounded by sterile branches. The primitive ancestral hepatics from which present-day hepatics were derived, had dorsiventral body coherent branches, dorsal sex organs, formation of an archegonial jacket, coherence of filaments, etc. Pteridophytean Hypothesis According to the Pteridophytean hypothesis, the bryophytes have been descended from pteridophytes by means of reduction. The scientists formulated their argument on the basis of the following two features: Close similarity between the sex organs of the two groups. The resemblance between sporogonium of Anthocerose, Sphagnum, and terminal sporangium of fossil pteridophytes: Sporogonites and Horneophyton. This second view, the Pteridophytean hypothesis of the origin of bryophytes has been strongly suggested by many workers such as Kashyap (1919), Christensen (1954), Andrews (1960), Proskauer (1 960), etc. In fact, this theory is a regressive theory that states that the evolution of simple forms has taken place from complex forms due to a reduction in histological complexity. Rather it is not always that complex forms have been derived from simplest forms by elaboration. Affinities with Pteridophytes Distinct and well-defined heteromorphic alternation of generations in the life history. Methods of reproduction. The origin and the formation of spores from spore mother cells. The similarity of their pigments. Structure of cell wall. Kashyap’s View (1919) According to Kashyap, the hepatics may have arisen from the pteridophytes as there are several features common to both pteridophytes and bryophytes like; Thallus structure (thalloid gametophyte). The similar structure of sex organs. Methods of spore formation. Christensen’s View (1954) Christensen also suggested the pteridophytean origin of bryophytes. According to him, bryophytes have been derived from any one of the following conditions: From pteridophytes bearing leaves in the stems of both the gametophyte and sporophyte. From leafless pteridophytes like the members of Rhyniaceae. Polyphyletically from different types of pteridophytes. Divergent evolutionary trajectories of bryophytes and tracheophytes from a complex common ancestor of land plants | Nature Ecology & Evolution Stelar evolution Stele (= Greek word meaning a column) can be defined as the unit of vascular system that is made up of xylem, phloem, interfascicular tissues, medullary rays, pericycle and pith (if present). The term stele refers to the central core of plant axis and is restricted only to primary tissues. The term is used in case of pteridophytes and is seldom applied in case of angiosperms and gymnosperms. Endodermis delimits a stele on the peripheral side. Types of stele There are two basic types of stele: 1. Protostele, and 2. Siphonostele. 1. Protostele: A protostele is composed of a solid core of xylem mass surrounded by phloem, which in turn remains encircled by pericycle. Endodermis delimits protostele on the peripheral side. In protostele pith is absent and the protoxylem is exarch. There is no leaf gap. Some dicotyledonous roots have radial stele where pith is completely absent. Such radial stele is also referred to as protostele. Three Types of protostele (A) Haplostele: Haplostele has a cylinder of phloem that surrounds a smooth core of xylem. The xylem mass appears circular or oval in outline as seen in cross-section. Protoxylem is exarch. Ex. Selaginella, Lygodium, the extinct psilophytes Rhynia and Homeophyton etc. Types (Contd.) (B) Actinostele (Figs. 15.1 & 15.3A): Actinostele has a cylinder of phloem that surrounds a star-like mass of xylem. As seen in cross-section, the xylem mass has radiating ribs of varying number. Protoxylem occurs at the tip of radiating ribs. Phloem also occupies the position between the xylem lobes and furrows. Ex. Psilotum and the extinct psilophyte Asteroxylon. Types (contd.) (C) Plectostele: Plectostele has masses of xylem that are in the form of plate-like lobes. As seen in cross-section the plates are of different sizes and some of the plates are united at one end. Cylinder of phloem surrounds xylem masses and phloem also occurs between xylem plates. Ex. Lycopodium clavatum. Siphonostele Siphonostele: Siphonostele, where xylem is in the form of a hollow cylinder, has parenchymatous pith at the central region of xylem. The xylem is surrounded by phloem that in turn remains encircled by pericycle. The whole stele is limited outside by a continuous endodermis. In siphonostele xylem and phloem are in the form of a continuous or split vascular cylinder. Types of siphonostele Ectophloic siphonostele: Ectophloic siphonostele has a continuous cylinder of phloem surrounding the peripheral side of xylem. Parenchymatous pith occurs at the central region of xylem. The whole stele is delimited outside by a continuous endodermis. Ex. ferns like Osmunda, Schizaea etc. and dicotyledonous angiosperm like Phlox, Lindenbergia etc. Types of siphonostele Amphiphloic siphonostele has cylinders of phloem on the peripheral and inner side of xylem. The peripheral phloem is termed as outer phloem and the other as inner phloem. Pericycle and endodermis appear both outside and inside of vascular tissues. i. Solenostele: Solenostele can be defined as a type of amphiphloic siphonostele with non-overlapping leaf gap. The leaf gaps are distantly spaced. Solenostele consists of two vascular strands- the small leaf trace and the large principal vascular strand as seen in a cross-section of stem at node. The principal vascular strand appears horse-shoe-shaped due to the presence of parenchymatous leaf gap. ii. Dictyostele: Dictyostele can be defined as a type of amphiphloic siphonostele with overlapping leaf gaps. The upper part of a leaf gap overlaps the lower part of the upper adjacent leaf gap. The gaps are not distantly spaced from each other and occur in parallel manner. Eustele Eustele can be defined as a type of ectophloic siphonostele with overlapping leaf gaps. The leaf gaps occur parallel to each other and are not distantly spaced. The upper part of a gap overlaps the basal part of the upper adjacent gap. iv. Atactostele: Atactostele can be defined as a type of eustele where collateral vascular bundles are arranged in an irregular manner (Fig. 15.2). It is the characteristic of monocotyledonous stem where there is no distinction between pith and cortex. v. Polystele (Fig. 15.3D): Polystele can be defined as having more than one protostele as observed in the cross-section of a stem. In Selaginella willdenowii (Fig. 15.1) three protosteles occur. Each protostele has xylem surrounded by phloem, the whole being bounded by endodermis. vi. Polycyclic stele: Polycyclic stele can be defined as having two or more coaxial cylinders of vascular strands as observed in the cross-section of a stem. The individual cylinders are interconnected at the base of inner stele. Polycyclic steles are also referred to as polycyclic siphonostele where the innermost vascular cylinder is amphiphloic siphonostele (Fig. 15.2). The other cylinders remain separated by parenchyma. Ex. Matonia pectinata. Stelar Evolution in plants The well known genus Rhynia represents the simplest kind of vascular plant. It is rootless, leafless and the axis is dichotomously branched. A stele runs through the centre of stem. The cells in the very centre of stele consist of tracheids only and this xylem is among the simplest kind of wood cells known. A zone of phloem tissue surrounds the xylem core. Phloem is conspicuous among living plants, but not especially evident in extinct plants. This very simple kind of cylindrical stele where phloem surrounds a solid core of xylem is termed as protostele. Protostele is found not only in ancient plants like Rhynia and Psilophyton, but also in stems of Lycopodium, some ferns and in many kinds of dicotyledonous roots. With the increase in complexity of sporophyte of vascular plants the vascular system elaborates. Both phyolgenetically and ontogenetically protostele is regarded as most primitive (Fig. 15.4). During evolution protostela undergoes certain modifications that result in the formation of different forms of protostele and siphonostele. Haplostele is regarded as most primitive among protostele. Actinostele is somewhat more advanced kind of stele, e.g. extinct Asteroxylon, extant Lycopodium serratum etc. Plectostele is most advanced kind of stele in protostele, e.g. Lycopodium clavatum. It is thought that the line of evolution among protostele proceeds from haplostele to actinostele and then to plectostele. In the course of evolutionary specialization the circular vascular strand of haplostele, as seen in cross-section, becomes stellate thus forming actinostele. This happens when the smooth core of vascular cylinder folds up at different places. Plectostele results when the folding becomes complete followed by separation of xylem masses. It is always observed that non-living conducting cells like tracheids etc. are in close association with living cells of some kind. There is no known physiological basis of this association. Anatomists believe that ‘because of the very close association between non-living conducting cells and living cells, there must exist some unexplained relationship between them. In haplostele, when the stele is small, each tracheid is not very far from a living cell, that is, phloem that surrounds the xylem. With the growth of sporophyte the stele also increases by the formation of living cells. Many tracheids may be of considerable distance from living cells. So to have a close association between non-living and living cells the xylem mass of haplostele becomes ridged and furrowed to form actinostele. During the course of evolutionary specialization plectostele arises where the living cells surround the xylem plates and intersperse between xylem masses. Theories – Expansion and Invasion theory Expansion Theory: Proponents of this theory believe that pith has originated from stelar tissues. During differentiation of xylem certain living cells at the centre never modified into non-living xylem. In protostele the solid central core of xylem mainly consists of tracheids. The siphonostele (of pteridophyta) possesses parenchyma cells at the centre that is pith, surrounded by tracheids In the transitional forms some amount of parenchyma remains mingled with tracheids- termed mixed pith. So the evolution occurred in the following way: stele without pith-stele with mixed pith-stele with pith (siphonostele). Invasion theory According to this view pith is regarded as extrastelar in origin. Jeffrey (1917), who proposed the invasion theory, is of opinion that the cortical parenchyma cells invaded the central core of xylem of protostele where they established as pith. According to Jeffrey the invasion occurred through the leaf-and branch gaps. As evidence Jeffrey cited the occurrence of inner endodermis between pith and vascular cylinder as endodermis is regarded as the integral part of cortex. Jeffrey opined that endodermis together with cortical parenchyma penetrated through the leaf-and branch gaps towards the centre of protostele. This view has been a subject of much debate because in Selaginella Ptendium etc. the endodermis is stelar in origin. This view gained little support. Sporangium evolution Diversity in sporangial function is evident in plants that exhibit heterospory. In such plants, two distinct types of sporangia are present: microsporangia and megasporangia, producing microspores and megaspores respectively. These spores are analogous to male (micro-) and female (mega-) gametes. This heterosporous trait is observed in certain bryophytes, lycophytes, select ferns, and spermatophytes, including angiosperms and cycads. Interestingly, in some instances, both spore types are encapsulated within a single sporangium. Conversely, equisetophytes and a majority of bryophytes exhibit homospory, producing a singular spore type within their sporangia. These spores give rise to monoecious gametophytes, which house both male and female reproductive organs. The location of sporangia varies across species. They may be positioned at stem or leaf apices, or even along their sides. In fungi, sporangia predominantly arise at hyphal tips. A notable feature in many sporangia is the sporangiophore, a stalk that elevates the spore-containing sac. Additionally, a structure known as the ‘columella’ often extends into the sporangium, providing support. This can either be a fungal derivative or be formed from host materials in parasitic fungi. In ferns, sporangia are typically aggregated into ‘sori’, which manifest as conspicuous dots on the frond’s underside. In contrast, lycophytes bear sporangia on the upper leaf surface or along stems. Angiosperms, or flowering plants, house their microsporangia within the stamen’s anther, where microspores mature into pollen grains