Manual for Bryophytes

Morphotaxonomy, Diversity, Spore Germination, Conservation

Samit Ray, Shewli Bhattacharya

• Publisher: CRC Press
• Year: 2021
• Print ISBN:
  9781032076942
• eBook ISBN:
  9781000441451
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

This book will enrich the readers theoretical knowledge about the fundamental aspects of bryology. There is a great significance of bryophytes in land plant evolution, water retention, prevention of soil erosion, nutrient cycling, nitrogen fixation and pollution monitoring. Considering this, the authors found it necessary to provide a basic guideline to the students to study the bryoflora.
The present manual for bryophytes will provide practical guidelines for collection and recording of bryophytes, methodologies for studying it's morphology, morphometry and internal structure, modern approach to bryophytes systematics and characterization of genera, taxonomic importance of spore ornamentation as revealed by the Scanning Electron Microscopy, significance of bryophyte conservation and basic methods of in vitro study.

Please note: Taylor & Francis does not sell or distribute the Hardback in India, Pakistan, Nepal, Bhutan, Bangladesh and Sri Lanka.

Chapter 1
Introduction

The bryophytes, commonly referred to as the amphibians of the plant kingdom, are the most diverse of the land plants after Magnoliophyta (3,50,000 species). There are approximately 19,000 species (6000 - 8000 species of liverworts, 100 - 200 species of hornworts and 10,000 - 15,000 species of mosses). However, opinions vary on the number of species. According to Gradstein et al. (2001) there are 15,000 species while Cram (2001) put the number as 25,000. These organisms share a fundamentally similar life-cycle with a perennial and free-living, photosynthetic gametophyte alternating with a short-lived sporophyte that completes its entire development attached to the maternal gametophyte. All the three groups comprise the earliest lineages of land plants derived from green algal ancestors (the charophycean green algae belonging to Streptophyta). It is well established from the structural and morphological variability and molecular phylogenetic study that the bryophytes do not constitute a single monophyletic lineage and divided into three divisions - Marchantiophyta, Anthocerotophyta and Bryophyta. The bryophytes, according to morphological cladistics analyses are not monophyletic (Mishler and Churchill 1984, 1985; Mishler et al. 1994; Kenrick and Crane, 1997). Recent, molecular cladistics analyses also confirm non-monophyly. The most accepted view of interrelationship between three major bryophyte lineages is that of Qiu et al. (2006). Combining an extensive set of DNA sequences of representatives of the major lineages, it was proposed that hornworts share a common ancestor with vascular plants, whereas liverworts are a sister lineage to all other extant embryophytes. Mosses bridge the gap between liverworts and hornworts.

Fig. 1.1. Phylogenetic relationship based on gene sequence data and genomic structural characters (Kelch et al. 2004, Qiu et al. 2006).

Bryophytes are characterized by the following features - (i) they are referred to as amphibians of the plant kingdom because of their dependence on water for luxuriant growth and also sexual reproduction, (ii) bryophytes are unique among land plants in that they possess an alternation of generations which involves a dominant free-living, haploid gametophyte alternating with reduced dependent diploid sporophyte, (iii) the gametophyte may be thallose or foliose and exhibit wide range of developmental, structural and morphological variations, (iv) lignified vascular tissue, meristematic tissue and secondary growth are absent, (v) most of the bryophytes are poikilohydric (poikilohydrous organisms maintain equilibrium with atmospheric humidity, gaining or loosing water readily), (vi) some bryophytes do contain vascular tissues including highly specialized water conducting cells. These cells do not form lignified walls. Among the liverworts an internal strand of specialized water conducting cells occurs in gametophytes of Calobryales and in some members of Metzgeriales. These are dead cells without cytoplasmic content at maturity. In the sub-class Bryidae water conducting system consists of elongate cells, lacking cytoplasmic content at maturity called hydroids. The hydroids associated with stereides (thick walled living cells) form a central strand in the leafy stem of the gametophyte sometimes referred to as hydrom. Hydroids do not contain lignin and lack secondary wall pattern. In the members of polytrichales, besides hydro ids. specialized cells with marked morphological similarity to the protophloem sieve cells intrachaeophytes are found called leptoids. They occur both in gametophyte and sporophytic seta. Leptoids have been found to be preferential route for the translocation of organic nutrients, (vii) the male sex organ antheridium producing biflagellate antherozoids and female sex organ archegonium containing egg are multicellular structures where fertile cells are surrounded by sterile jacket (viii) delayed meiosis of zygote and interpolating mitotic cell divisions result in the production of an embryo, (ix) the embryo differentiates within the venter of the archegonium and ultimately develops into a sporophyte, (x) there are distinct embryonic regions determined to develop into the three organotrophic zone of the mature sporophyte - foot, seta and capsule, (xi) within the capsule of sporophyte meiotic cell divisions take place to produce spores; spores are homosporous. (xii) this sporophyte is monosporangiate and matrotrophic in growth and parasitic on gametophyte, (xiii) spores germinate to form a protonemal stage; the protonemal stage in the liverworts is globose while in the mosses the protonema is filamentous and more conspicuous in the life-cycle.

In the Devonian period, a diversity of plants adapted to the terrestrial environment and were able to absorb water, nutrients. Absorbed water and nutrients were transported and distributed throughout their aerial shoots. The ability to exist on land is the result of numerous complex interactions that involved several structural and physiological adaptations - (i) anchorage and water uptake, (ii) structural support and water transport, (iii) protection against desiccation and radiation, (iv) gaseous exchange, (v) reproduction on land.

On the basis of a variety of ultrastructural. biochemical and molecular data Duckett and Renzaglia (1988) suggested that the principal bryophyte groups had separate origins and that hornworts, liverworts and mosses represent the earliest divergent lineages of extant land plants. Phylogenetic evidence suggests that bryophytes in general and liverwort like plants in particular should have been important components of early terrestrial floras (Bateman et al. 1998; Renzaglia et al. 2007).

The earliest recognizable bryophytes in the macrofossil record include liverworts that appear to have their closest affinities with the Metzgeriales. The earliest liverwort in the fossil record is Metzgeriothallus sharonae from Givetian (upper middle Devonian) shales and siltstones from New York (Van Aller Hernick et al. 2008). There are some records of mosses as early as Carboniferous (Walton 1928). One of these is Muscites plumatus, an impression of a small leafy shoot in rocks of Mississipian age (Thomas, 1972). The earliest macrofossil that bears some resemblance to a modem hornwort is Dendroceros victoriensis from the lower Cretaceous Koonwarra fossil bed in Australia (Drinnan and Chambers, 1986).

Bryophytes inhabit a very wide range of ecosystems, habitats, microhabitats including substrates on which vascular plants cannot live. It must be noted that in most of the environments vascular plants are the dominant vegetation. However, in arctic, the antarctic, in alpine habitats, in mountains above tree line, in bogs, in fens and larger peatlands bryophytes are often the dominant plants in terms of both biomass and productivity. It is not only that they prefer some specific ecological conditions conducive for their growth but they also have important ecological functions beneficial for the living world. Some of their ecological significance are briefly mentioned here - (i) extensive carpets of bryophytes on soil help moisture conservation, absorb and retain water and this absorbed water can be released for a long period, (ii) they are good in trapping nutrients from air and also absorb nutrients carried by water. These nutrients are available after the bryophytes die and decay, (iii) nutrition is also supplemented by N₂-fixation where Nostoc colonies grow in symbiotic association with genera like Anihoceros, Blasia, (iv) some species are very good soil binders and maintain soil structure under the crust, (v) bryophyte colonies provide niches for numerous invertebrates which form a part of food chain for higher organisms, (vi) bryophytes are often the first plants to colonize barren surfaces like road cuttings, road out ropes and volcanic ash. (vii) moss carpets also help in seed germination for higher plants by providing a moist seed bed.

The world wide reduction, fragmentation and degradation of habitats important for bryophytes have led to loss of species richness and genetic diversity. Fortunately conservation of bryophytes has been emphasized in the United Nations Convention of Biological Diversity. One of the significant aspects of the action plan is taking initiative to create inventories to determine bryophyte richness in different regions and habitat types and to determine which species are locally common, rare or threatened. Another important aspect is comparing bryophyte floras from undisturbed and disturbed habitats to determine the impact of disturbance and to identify those species unable to survive in disturbed areas. Both these aspects need good knowledge of morphology of bryophytes.

Bryophytes show extensive phenotypic plasticity. Depending on the ecological parameters, taxonomically important morphological and structural features change. So there must be a thorough study of the correlation between ecological parameters of the habitat and the morphological features of a particular bryophyte species. There is another aspect of this kind of study. Bryophyte species tend to be highly specific for particular microenvironment and respond to factors as temperature, light, water availability, substrate chemistry etc. Thus these plants can be good ecological indicator species.

Chapter 2
Classification of Bryophytes

The names Bryophyta and Pteridophyta were introduced by Haeckel (1866) but he never gave them rank of a division. Schimper (1879) treated Bryophyta as a division of the plant kingdom. The system of classification in which cryptogamic portion of the plant kingdom is placed in three divisions - Thallophyta, Bryophyta and Pteridophyta first appeared in Eichler's "Syllabus... " (1883). Division Thallophyta included the classes Algae and Fungi and Division Bryophyta was divided into classes - Hepaticae and Musci. This system of classification helped the botanists of that era to distinguish between algae and bryophytes. Engler (1892) recognized Hepaticae and Musci as two classes and divided Hepaticae into three orders - Marchantiales, Jungermanniales and Anthocerotales and Musci also into three orders - Sphagnales, Andreaeales and Bryales. After detail study. Anthocerotales, usually listed as an order of Hepaticae. was given an isolated position by Underwood (1894). Howe (1899) raised the order Anthocerotales, containing Anthoceros and related genera, to the rank of a class and divided the division Bryophyta into three classes — Hepaticae. Anthocerotes and Musci. This system was followed by all leading workers like Smith (1938, 1955), Takhtajan (1953) and Schuster (1958). The name Anthocerotes was changed to Anthocerotae.

Cavers (1911) omitted the words Hepaticae and Musci and divided Bryophyta into ten orders - Sphaerocarpales, Marchantiales, Jungermanniales, Anthocerotales, Sphagnales, Andreaeales, Tetraphidales, Polytrichales, Buxbaumiales and Eubrayles. But this classification ignores the fundamental difference between the Hepaticae and the Musci and does not seem to be justified.

Rothmaler (1951) changed the nomenclature of the three classes of Bryophyta to Hepaticopsida, Anthoceropsida and Bryopsida, which are in accordance with the recommendations of International code of Botanical Nomenclature, Utrecht 1956. Proskauer (1957) was of opinion that the name Anthoceropsida be changed to Anthocerotopsida. The differences between the Hepaticae and Musci are so great that some taxonomists want to raise them to divisions. Bold (1956) gave the status of divisions to the classes Hepaticae and Musci and they were named Hepaticophyta and Bryophyta respectively.

The placement of liverworts, hornworts and mosses in a single division, Bryophyta, carries the message that these three groups of plants are closely related to one another phylogenetically. The bryophytes are very similar in lifecycle pattern, the plant bodies in each case a free living gametophyte on which a partially or completely parasitic sporophyte is borne (the relative balance between the two generations are similar). However, the morphological, anatomical and developmental differences among them are sufficiently significant to revise the classification of the liverworts, hornworts and mosses and elevate them as separate divisions of the plant kingdom - Hepatophyta, Anthocerotophyta and Bryophyta (Stotler and Crandall-Stotler, 1977 and Crandall-Stotler, 1980). Bold et al. (1987) adopted similar classification in their book "Morphology of Plants and Fungi".

The divergent conclusions regarding their origin and phylogenetic relationship also supported such a classification. Characteristics such as biflagellate sperms, multicellular sex-organs, sporic meiosis, aerial spore dissemination and the terrestrial habitat may seem significant and important common attributes between these three groups but all these characters also occur in the algae or in the vascular plants.

Stotler and Crandall-Stotler (1977) divided the Hepatophyta into three classes - Marchantiopsida, Jungermanniopsida and Haplomitriopsida. Division Anthocerotophyta had a single class Anthocerotopsida and the Division Bryophyta was divided into three classes — Sphagnopsida, Andreaeopsida and Bryopsida. Presently, the division Bryophyta is divided into classes — Takakiopsida, Sphagnopsida, Andreaeopsida, Andreaeobryopsida, Oediopsida. Tetraphidopsida, Polytrichopsida, Bryopsida (Goffinet, Buck and Shaw, 2009). Modern studies of cell structure and molecular biology confirm that bryophytes comprise three separate evolutionary lineages which are recognized as liverworts (Marchantiophyta — this divisional name based on type species Marchantia polymorpha), hornworts (Anthocerotophyta), and mosses (Bryophyta). Kenrick and Crane (1998) proposed that the three divisions (groups) of bryophytes represent a grade or structural level in plant evolution, identified by their monosporangiate life-cycle. Detail characteristic features of the three divisions are as follows:

Marchantiophyta - Dorsiventrally differentiated (sometimes radial and erect) gemetophytes are either simple thallose or with leaf-like appendages (foliose), leaves are lobed and without midvein, internal tissues are homogeneous or differentiated into photosynthetic and storage region oil bodies are present in the cell, rhizoids are hyaline and one-celled, sex organs are borne terminally (in leafy forms) or developed from superficial dorsal cells, sporophyte is simple or differentiated into foot, seta and capsule, it is always of limited growth, seta elongates prior to spore release, sporogenous cells of endothecial origin, capsules contain spirally thickened elaters but no columella, capsule dehisces into four valves.

Anthocerotophyta - Gametophyte dorsiventrally differentiated, simple thallose, not foliose, internal tissue homogeneous, oil bodies present and cells ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Preface
• Foreword
• Acknowledgements
• Contents
• List of Plates
• Chapter 1 Introduction
• Chapter 2 Classification of Bryophytes
• Chapter 3 Collection, Preservation, Working out the Morphology
• Chapter 4 General Morphology of Liverworts, Hornworts and Mosses
• Chapter 5 Life Forms and Growth Forms of Bryophytes
• Chapter 6 Moss Spore Germination
• Chapter 7 Conservation of Bryophytes
• Plates
• Glossary
• References