International Journal of Research on Social and Natural Sciences Vol. I Issue 1 June 2016 ISSN (Online) 2455-5916

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Exploring diverse conducting elements with habit preference in some acrocarpous and pleurocarpous : A comparative analysis

Shelly Sinha, Botany, Rabindra Mahavidyalaya, Hooghly, India

Article Record: Received March 30 2016, Revised paper received May 21 2016, Final Acceptance June 3 2016 Available Online June 7 2016

Abstract

Mosses are very successful flora with cosmopolitan distribution and harbour certain needs in respect to their niche constraints such as varied growth forms, diverse leaf cell configuration and systematic placement. Existence of a specific taxon reveals the detailed habit of an explicit area. This is mainly due to compact growth form, ability to retain moisture, endohydric or ectohydric type. The present study mainly focuses on fabrication of different conducting elements within two major orders of pleurocarpic mosses and acrocarpous mosses respectively, each showing wide occurrence in that specific selected area. Comparative studies elucidate that development and organisation of unspecialised water conducting cells manifest unique epiphytic habit dominance within the particular taxa whether being acrocarpic or pleurocarpic. Acrocarpous mosses with stable conducting components mostly confer either as terrestrial or lithophytes with turf or dendroid growth patterns. Paradoxically, one with less conducting elements shows epiphytic type being the pendants. Whereas, pleurocarpic mosses are mostly prostrate or creeping , dwell on wood with epiphytic selection. And those on rock and ground add new intertwining or overlaid growth each year to form mats. Therefore the habit preference is significantly correlated with wide distribution or formation of conducting elements both in leaves as well as in central axis.

Key words: Habitat, epiphytic, leaf cell pattern, central strand

1. Introduction

Incessant need for water manifest the drift of plants gradually from aquatic to terrestrial platform (Glime, 2015). Evolutionary lineage clarified that the movement onto land is conceded with their gametophytic dominance still acquired within some group members. Plants with advanced vascular tissues eventually been developed and gametophytes in these highly vascularised tracheophytes unravel water complications by being confined within the protection of sporophytic tissues. Parallely, diminution of the gametophyte might necessarily foster reduction in conducting tissues as the contiguous sporophytic tissue reduced available space and made gametophytic vascularisation less relevant (Raven, 2002). Sequentially, the gametophyte-dominant mosses inhabited on land for retrieving water and essential nutrients. Ironically, growth potential in this group is not as highly polarized as vascular plants; yet they are remarkably successful colonizers and significant in ecosystem dynamics (Vanderpoorten & Goffinet, 2009). In spite of being foremost terrestrial flora, a "top-down" view of land plants, seemingly expecting the bryophytes to be their simpler version and termed precisely as desiccation-tolerant tracheophytes (Raven, 2002).They obtain nutrients directly from substances dissolved in ambient moisture. However, they play a significant role in nutrient cycling, soil formation, providing microhabitat for other plants and animals, promote seed germination and fill gaps in the habitat (Glime, 2015). They exhibit extensive cover on the substrate and provide microenvironment for the activity of numerous microorganisms, constituting a kind of specific bryo-system.The division Bryophyta comprise of 13,700 species and form second largest group among terrestrial plants (Govindapyariet al., 2012). Interestingly, on one side, vascular plants signify the strategy of adaptation to irregular and inconsistent source of water on land. While, desiccation tolerant group of bryophytes represent an alternative, photosynthesising and growing when water is freely available and suspending metabolism when it is not (Proctor, 2000a). Mosses are different from stereotype tracheophytes by being ectohydric, (carrying external capillary water which can vary widely in quantity without affecting the water status of the cells). In spite of this, typical terrestrial plants use two major adapting approaches towards water economy, termed as homoiohydry and

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International Journal of Research on Social and Natural Sciences Vol. I Issue 1 June 2016 ISSN (Online) 2455-5916 poikilohydry (Proctor, 2000b). The former one endeavours to maintain a high water potential under water limiting conditions, whereas, the latter may be defined as the incompetent to control water loss to the environment with the result that cellular water content fluctuates in concert with external water availability (Wood, 2007). Progressively, mosses have evolved a unique physiological adaptation which allow suspension of metabolism during periods of drought and its resumption when water is once available, well known as poikilohydry (Oliver, 2009). Implementing the alternative strategy of evolving desiccation tolerance, growing during moist period and suspending metabolism during drought revealed the uniqueness of this group.Despite the typical relegation of bryophytes to the category of non-vascular, conduction has played a major role in the phylogenetic history of acrocarpous mosses (Sinha et al., 2009; Govindapyariet al., 2012). Whereas, evolutionary path elucidated that they survived both during gradual water loss and prolonged periods of drought (Proctor, 2000a). Studies explored that they have perfected physiological mechanisms that outdistance those of their tracheophyte counterparts (Oliver et al. 2000). This ability has led plant physiologists to use bryophytes as model systems for the study of desiccation tolerance physiology, even to the extent of attempting to introduce those genes to crop plants (Comis 1992; Oliver et al., 2000), and this use has made it into the agricultural literature with articles such as "Miracle Moss" (Comis, 1992). Mosses have undergone repeated morphological reduction and simplification often as a result of colonizing specialized and particularly xeric or ephemeral habitats (Vitt, 1981). Hedenäs (1999) examined the importance of various character states on the phylogenetic history of pleurocarpous mosses (typically the ones that grow horizontally) and determined that, based on redundancy analysis, gametophyte variance relates to characters associated with water conduction. Furthermore, one of the most important environmental variables in this phylogeny was the non-wetland to wetland gradient. Adaptation and structural modification caused regular speciation and resulted high species richness. However, the water conducting cell study was mainly focussed on acrocarpous mosses that exhibit turf pattern. The families Polytrichaceae and Mniaceae represent the well-developed endohydric groups (Proctor, 2000b) which has stereome, leptome and a central strand. On the other hand, some acrocarpous mosses also show ectohydric pathway, as they absorb water rapidly into the cells by small capillary systems (Buch, 1947). Hébant (1973) describes the living parenchyma cells around the central strand in the Polytrichaceaeto be a hydrome sheath, a term originated by Tansley and Chick (1901). Among different mosses, Sphagnum has the most unusual water system in its leaves of any bryophyte. Its leaves have two types of cells, and rarely a border in addition. These two types are the water-holding, colourless, dead hyaline cells and the green chlorophyllose(photosynthetic) cells. The hyaline cells serve as water reservoirs for the photosynthetic cells. Their walls have true perforations and are strengthened by spiral thickenings, suggesting the structure of tracheophyte vessels (Hébant 1977; Sinha et al., 2009).Further studies done by Ron & Kawai (1991) described the moss stem as having a basic structure much like that of tracheophytes with an epidermis surrounding the cortex. This basic structure describes most of the pleurocarpous mosses that move internal substances mostly horizontally. Eventually, the water conduction studies steadily moved to desiccation tolerance capacity of individual members. Unfortunately, comprehensive survey of mosses of a particular area in respect to their conducting elements with their habit constitution needs to be much more explored. Therefore, the present study mainly focuses on fabrication of different conducting elements within two major orders of pleurocarpic mosses and acrocarpous mosses respectively, each showing wide occurrence in that specific selected area.

2. Materials and methods

Mosses from Uttarakhand (mainly Mukteshwar, Uttarakhandand its adjoining areas) were collected in polythene bags during the month of November. They were air dried by spreading out on the blotting sheets for 3 - 4 days and stored in labelled standard sized (10x15 cm) paper packets. Date of collection, locality and habitat along with the substratum type etc. were recorded in the field note-book and also on the packets containing the material. For the study the dried material is soaked in water with 0.1% safranine solution for a few minutes in Petridish. The different parts of the samples were taken out with the help of needles, brush and forceps and were carefully observed under the binocular microscope and compound microscope. Transverse and longitudinal sections of the central axis were also cut down serially from basal to apical portion to study the comparative analysis of development of central strand. Whole mount of leaves were

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International Journal of Research on Social and Natural Sciences Vol. I Issue 1 June 2016 ISSN (Online) 2455-5916 made in Gum Chloral mounting Medium. Apical, middle and basal portions of the leaves were studied separately to observe cell patterns. The list of pleurocarpous taxa of selected mosses is given in Table-1 and those of selected acrocarpous mosses were enlisted in Table-2.

3. Results and Discussion

Collected mosses were identified with the help of literature; Flora of Uttarakhand (Asthana and Sahu, 2013; Alam, 2013). The taxa are arranged according to the classification of Goffinet et al. (2009). Among different types of both acrocarpous and pleurocarpous mosses collected, only those which share a common type of habitat were selected in this present study in Table-1. Whereas, the leaf cell pattern of some acrocarpous mosses were also considered for complete comparative studies and enlisted in Table-2. The two orders selected in the present study show dominance in our selected locations as well the genera selected from same order exhibit diversity in their habit, apart from other parameters.

Table 1. List of the pleurocarpous mosses genera collected from the selected area and with conducting elements in respect to growth form, habitat and leaf cell pattern

Order Family Genus Habitat Growth Leaf cell pattern form Amblystegium Bark Mat Quadrate rectangular cells with short costa Hypnales Amblystegiaceae Hygroamblystegium Bark Mat Rhomboidal cells with broad base Hypnales Leskiaceae Lindbergia Bark Mat Cells short and rectangular Hookeriales Hypopterygiaceae Cyathophorella Bark Mat Costa short, single or forked. Cells smooth and rhombic. Base is broader and apex is linear Hypnales Fabroniaceae Fabronia Bark Mat Cells rhomboid at tip and at base becomes quadrate – rectangular, costa short, single Hypnales Hypnaceae Hypnum Bark Mat Cells short, quadrangular, base somewhat broader with linear apex Hypnales Brachythecium Clay Mat Cells narrow rhomboid at soil apex, sub rectangular to rectangular at base Hypnales Brachytheciaceae Cirriphyllum Bark Mat Cells rectangular with broader base Hypnales Entodontaceae Entodon Bark Mat Costa two, short and covering 1/4th the leaf surface. Linear cells at upper region and alar cells at the basal region Hypnales Entodontaceae Erythrodontium Bark Mat Cells smooth, narrow, elliptical to linear at tip and at base triangular patches of transverse-ovate rectangular cells are present

Table 2. List of the some acrocarpous mosses genera collected from selected area with conducting elements in respect to growth form, habitat and leaf cell pattern Order Family Genus Habitat Growth Leaf cell pattern form Dicranales Acidic Cushion Lamina thin, lower leaf tissue condition possesses irregular, elongated and rectangular cells.

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International Journal of Research on Social and Natural Sciences Vol. I Issue 1 June 2016 ISSN (Online) 2455-5916

Dicranales Dicranaceae Arid areas Turf Costate, cells small and incrassate Dicranales Soil and bark Cushion Costate, scabrous cell pattern also exhibit Isobryales Erpodium Bark Mat Cells oval hexagonal, large Isobryales Leucodontaceae Leucodon Bark Turf Cells linear and thick walled Isobryales Meteoriaceae Meteorium Branch Pendant Costate, cells narrow and linear

Significant divergence is noted in the distribution and the type of the conducting elements in different members of taxa. In Dicranales order of acrocarpous mosses, flexibility is revealed both in habitat as well as leaf cell pattern. Interestingly, they also unveil different types and pattern of conducting elements within stem (Figure- 1A,B and C). Among them, Dicranum sp. show an elaborate and well established central core conducting tissue system (Figure-1E, with arrow) emphasizing its growth and habit pattern. Erpodium sp., a member of Isobryaleshave same habitat like most of pleurocarpic mosses yet there is moderate prevalence of thick walled cells (Figure 1 H and I). Apart from that, the leaf cells also demarcate thick walled laminar cells. Comparative analyses revealed that there is either less fabrication or no manifestation of thick walled cells with conducting elements in the selected taxa of pleurocarpous mosses (as in Figure-2 A to M). Govidapyariet al., (2014) also revealed that Hypnaceae is found to be most frequent as epiphytic growing moss members on tree bark and branches in their studies on pleurocarpous mosses in Imphal district. It seems that these species have high tolerance and adaptability to survive in the dry bark. Most of the pleurocarpous mosses in the present collection display mat form.

Figure 1. A-T.S. of stem of Dicranella sp.(x 200) showing developed thick walled conducting cells, B- T.S stem of Dicranum sp. (x400) showing central core of conducting cells with steroidal cells in outer cortex, C-T.S stem of Leucobryum sp. (x 400) showing thick walled cells, D- Leaf middle portion cells of Dicranum sp. (x50), E- L.S stem of Dicranella sp.(x 400) with arrow indicate well central strand of conducting tissue, F-T.S stem of Leucodon sp. (x200), G- Leaf cells of Leucobryum sp.(x50), H- Laminal cells in Erpodium sp.(x100) with arrow indicating papillae, I- T.S.stem of Erpodium sp. (x200), J-L.S stem of Erpodium sp. (x400), K and L-L.S stem of Leucobryumsp.andMeteorium sp. (x 400)

Mosses with varied leaves show different linear or narrow laminar cell patterns, absence of costa or rudimentary costa represent the poor conducting system (noted in all the selected pleurocarpic mosses, Figure -2 B,D and I). Whereas, the leaf cells of acrocarpous mosses show small, rounded and papillate laminal cells (Figure 1 D, G and H). Some correlation is also revealed between the developments of conducting tissues with regard to their taxonomic position. Such as mosses belonging to Polytrichales, Funariales and Eubryales show a prominent conducting central strand (Sinha et al., 2009). Dicranales with turf habit mosses show well developed axis. The present study also showed the wide distribution of conducting tissues in respect to their habitat conditions in the specialised group Dicranales. Moreover the order Isobryales show development of poor conducting elements but is widely distinguished from the Hypnales and Hookeriales due to the presence of broad leaf cells, costate and papillate (form channels for moisture retention) and indicate a well-defined tissue for water conduction. Thus, plasticity is found among the taxonomic group members of Hypnales and Hookeriales.

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International Journal of Research on Social and Natural Sciences Vol. I Issue 1 June 2016 ISSN (Online) 2455-5916

Overviews and well depicted reviews of central strand developmental studies revealed that well conducting axis often possess hydroids, surrounded by stereids that conduct water and together comprise the hydrome. Hydroids lack lignin and spiral thickenings, distinguishing them from tracheids and vessels of tracheophytes (Hébant, 1977; Glime, 2015). Figure-1 and Figure-2 in the present study also indicates the same type of variations in the cells of central strand or axis. In Figure 1 (A), (B) central hydrome is depicted in transverse section (T.S). Figure-1(E) and (K) also show thick walled stereidal cells which aids in conduction in longitudinal section (L.S). They are all acrocarpous mosses with erect, turf like growth pattern. But, no or very less amount of conducting thick walled steroidal cells are observed in the central axis of pleurocarpic mosses. Figure-2 G and H (T.S) as well as J and M (L.S) signify the absence of central stand. Interestingly, the deeply stained cells on further detailed analysis of pleurocarpic mosses specified that they possess gaps which acquired stain. These cells also help in conduction, but not as hydroids or stereids. They function as small capillary tubes and helps in the conduction from the surface of the axis. This favours their habit pattern. Moreover, the plants with narrow laminal cells possess poorly developed internal conducting system but well developed external conducting system (ectohydric). Ectohydric mosses depend predominantly on external transport of water and can absorb water over the entire plant surface (Glime, 2015). These taxa mostly possess either no water repellent layers or are constrained to specific locations as the apices of papillae, where are easily wetted (Proctor 1982; Proctor 1984). Movement is due to capillarity and the relationships are complex. As the moss becomes hydrated, its capillarity changes due to expansion of leaves, untwisting, and other forms of movement and gyration (Deloireet al. 1979). So, acrocarpous species more commonly have a central conducting strand, whereas pleurocarpous mosses remain close to the substrate and a central strand may not be useful. Typically, mosses function like sponges in the ecosystem by holding water and maintaining moisture in the soil below. But they also absorb water like a sponge, using capillary spaces. At times when water is limiting, the bryophytes are able to survive through their exceptional desiccation tolerance.

Figure 2. A- T.S stem of Amblystegiumsp. (x400), B- Leaf of Cyathophorella sp. (x100), C- T.S. basal stem portion of Fabronia sp. (x400), D -Leaf of Fabronia sp.(x100), E – T.S. stem of Hypnumsp. (x 400), F- L.S stem of Cirriphyllum sp. (x400), G- T.S. stem portion of Lindbergia sp. (x400), H – T.S stem portion of Brachythecium sp. (x400), I- Leaf of Entodon sp. (x100), J- L.S stem portion of Erythrodontium sp., K- T.S. apical stem portion of Hygroamblystegium sp. (x400), L and M show L.S stem portions of Cyathophorella sp.(x400) and Entodon sp.(x400) respectively. Each portion in the figure as indicated with relevant arrows indicateno or less developed thick walled cells or fabrication of conducting cells.

A B C D

E F G

H I J

K L M

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International Journal of Research on Social and Natural Sciences Vol. I Issue 1 June 2016 ISSN (Online) 2455-5916

4. Conclusion and future challenges

The present study explores firstly, an intimate relation between the fabrication of wide conducting elements in the axis and diverse leaf cell patterns. Secondly, a broad relation of earlier two parameters with regard to different growth forms in mosses of a particular area is also well recognized. Further, comparative studies explained that development and organisation of unspecialised water conducting cells manifest unique epiphytic (aerial) habit dominance within the particular moss taxa.Similarly, acrocarpous mosses with stable conducting components mostly confer either as true terrestrial or lithophytes. Paradoxically, the acrocarpous taxon with less conducting elements depicts epiphytic type similar to the pleurocarpous mosses. This signifies that the ectohydric conduction is adaptive mostly with the pleurocarpous in comparison to acrocarpous mosses. Finally, the study concludes with the promising correlation between developments of conducting element in poikilohydric, ectohydric, epiphytic habit preference in pleurocarpic mosses as compared to the well promised acrocarpous mosses of that particular study area. Comparingand correlating concerned parameters are elucidated in Figure-3 for better understanding the relationship among them.

Although, the work is a miniature to study this wide group, yet it would certainly help future researchers to explore more about moss anatomy together with its ecological dimension. Additionally, it will facilitate researchers to understand the eco-physiological divergence within these less explored non-vascular desiccation tolerant plants highlighting their evolutionary significance.

References

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International Journal of Research on Social and Natural Sciences Vol. I Issue 1 June 2016 ISSN (Online) 2455-5916

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