Evolution of sporophyte in bryotphytes
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The document discusses the evolution of sporophytes in bryophytes, detailing two contrasting theories: progressive evolution, which suggests that complex sporophytes evolved from the sterilization of simpler forms, and regressive evolution, which proposes that the simplest forms are more advanced due to a reduction of complexity. It outlines various stages of sporophyte development from simplest forms like Riccia to complex structures found in species such as Funaria and Polytrichum. The text emphasizes that the evolutionary trends in bryophyte sporophytes involve transformations in both the fertilization and sterilization of tissues.
Evolution of sporophyte in bryotphytes
- 1. Evolution of Sporophytes in Bryophytes Prepared By- Dr. Sangeeta Das Assistant Professor Department of Botany Bahona College, Jorhat, Assam, India.
- 2. What is a Sporogonium? It is the sporophyte of bryophytes. It consists of a single, terminal sporangium with a bulbous foot and with or without an unbranched stalk or seta. The sporogonium is very delicate, short-lived and nutritionally dependent on its gametophyte.
- 3. Theories of evolution of sporophytes: There are two contrasting views: 1. Evolution of sporophytes by progressive sterilization of potential sporogenous tissue called Theory of Progressive evolution. 2. Evolution of sporophyte due to progressive reduction or simplification called Theory of Regressive evolution.
- 4. Theory of Progressive evolution Theory of progressive evolution or Theory of sterilization, was put forward by Bower (1908-35) and supported by cavers (1910) and Campbell (1940). According to this theory, the sporophytes of complex forms (e.g., Funaria , Sphagnum, Pogonatum) have evolved due to progressive sterilization of the potential fertile tissue of the simpler forms (e.g., Riccia, Marchantia).
- 5. First Stage: The increasing sterilisation of sporogenous tissue from simple sporophyte of Riccia to the most complex type of Funaria can be arranged through the following stages: The simplest known sporophyte among Bryophtes is that of Riccia. In all species of Riccia, the sporophyte consists of only capsule ,there being no trace of seta and foot.
- 6. This stage is seen in Corsinia, belonging to Hepaticopsida, where a very small sterile foot develops. The capsule has a single-layered jacket, but inside the capsule some of the sporogenous cells, instead of forming spores, form sterile nutritive cells. Second Stage: Third Stage: Further sterilization is seen in Spharocarpus where the sporophyte has a sterile bulbous foot and a narrow seta, in addition to fertile capsule.Fig: Mature sporophyte in Sphaerocarpus.
- 7. This stage is represented by Targionia, where the sporophyte consists of a sterile bulbous foot, a sterile narrow seta and a fertile capsule. Thus in Targionia, more sterilisation of sporogenous tissue has been observed. Fourth Stage: This stage is illustrated by Marchantia, where further sterilisation of sporogenous tissue has been noted in comparison with Targionia. In Marchantia, the sterile tissue consists of a broad foot, a massive seta, a single-layered jacket of capsule, sterile apical cap at the apex of capsule and a large number of long elaters with spiral thickening. Fifth Stage: Fig: Mature sporophyte in Corsinia.
- 8. Fig: Evolution of Sporophyte in Bryophytes: A. Riccia sp.; B. Sphaerocarpos sp.; C. Targionia hypophylla; D. Marchantia polymorpha; E. Pellia epiphylla.
- 9. Sixth Stage: This stage is represented by some members of Jungermanniales like Pellia, Riccardia, etc. Here more sterilisation of sporogenous tissue has been observed. Sporophyte is differentiated into foot, seta and capsule having multilayered jacket. The sporogenous tissues produce mass of sterile elatophores and diffused elaters.
- 10. This stage is illustrated by members of Anthocerotophyta like Anthoceros. Here marked reduction in the sporogenous tissue has been noted. The multilayered capsule differentiates into epidermis with stomata and chlorophyllous cells. Seventh Stage: Eigth Stage (Final Stage): The members of Bryopsida like Funaria, Polytrichum, Pogonatum etc., show the highest degree of sterilisation. The sporophyte is differentiated into a foot, a long seta and a capsule. The sterile tissue of capsule consists of the apophysis, operculum, many- layered jacket, the columella, trabeculae, the wall of spore sac and the peristome. The sporogenous tissue is restricted to the spore sacs only.
- 11. Fig. : Sporophyte of Anthoceros sp.
- 12. Fig.: L.S. of mature capsule of Funaria.
- 13. Thus the sporophyte of Riccia is the simplest amongst the bryophytes, with a very high proportion of fertile tissue and the sterile tissue is very small. Whereas , the members of the class Bryopsida (e.g., Funaria, Polytrichum , Pogonatum) have the most complex sporophyte with a very high degree of sterility. The evolution of sporophyte in bryophytes as such is considered to have taken place by progressive sterilization of the fertile tissue .
- 14. Theory of Regressive evolution: This theory is known as regressive or retrogressive theory. It was supported by several scientists like Church (1919), Kashyap (1919), Goebel (1930) and Evans (1939). According to this theory, the simplest sporophyte of Riccia (comprised of a simple capsule) is the most advanced type which has been evolved by the simplification or progressive reduction of the complex sporophytes mosses (e.g. Funaria, Pogonatum, Polytrichum, etc.).
- 15. Stages of progressive reduction: 1. The semiparasitic foliose sporophyte gradually lost its leaves and became embedded within the gametophyte. 2. Gradual reduction of photosynthetic tissues in the sporophytes. 3. Restriction or absence of stomata (liverworts) only in the apophysis region (e.g. Funaria, Polytrichum). In Sphagnum, the stomata of apophysis are non-functional and become rudimentary. 4. The capsules of most mosses, hornwort and some Jungermanniales are multilayered which subsequently became single-layered (e.g., Marchantia, Riccia, etc.) by progressive reduction.
- 16. Along with these changes, there was gradual elimination of the seta and the foot. All these changes are accompanied by the progressive increase in the fertility of the sporogenous cells. Evidence from comparative morphology and experimental genetics support the view that the simple sporophyte of Riccia is not an advanced but a reduced structure.