EVOLUTION OF SPOROPHYTES IN BRYOPHYTES

            According to the complexity of structure, the sporophytes of bryophytes may be arranged in a series between the simples and the most elaborate. The series starts with the simple sporophyte of Riccia, runs through that of Sphaerocarpos, Targionia, Marchantia, Pellia, Anthoceros and finally ends in a highly complex sporophyte of Funaria and Pogonatum. However, the evolution of sporophytes has been explained with the help of two theories put forward by botanist – (a) Theory of Sterilization and (b) Reduction Theory.

            1. Theory of Sterilization:

            This theory was put forward by Bower and was supported by Cavers, Campbell and Smith. This theory illustrates that a natural advance in the progressive elaboration and complexity of the sporophyte. The fundamental principle upon which he formulated his argument is “the progressive sterilization of the potentially fertile cells (sporogenous tissue)”. Instead of forming spores and serving a propagative function they remain sterile. These sterile cells are put to other uses such as nutrition, support, dehiscence, dispersal, etc. This hypothesis of Bower is called theory of sterilization.

            The detailed process of sterilization of some of the important genera are discussed as follows –

            (a) Riccia Sporophyte. In Riccia, the zygote divides and redivides to form a mass of spherical mass of 20-30 undifferentiated cells. Periclinal segmentation forms an inner mass of cells called endothecium and outer single layer amphithecium. The amphithecium forms the single layered capsule wall. The endothecium forms the central mass of sporogenous tissue. Practically, all the sporogenous cells are fertile and develop into spores. However, few of them undergo degeneration to form the nurse cells.

            The sporophyte of Riccia is the simplest among all the bryophytes and has the least amount of sterile cells. The entire embryo forms the spore producing capsule. There is no foot and seta. It is just a spore producing organ without any distributing function.

            (b) Marchantia Sporophyte. Sterilization of the fertile cells is more advanced in this genus. Half of the embryo derived from the hypobasal region remains sterile. It forms the foot and the seta. The upper epibasal half is fertile and forms the spore producing capsule. The sterile cells elongate, develop spirally thickened walls and become the elaters. A few of the cells of sporogenous cells at the top may differentiate into sterile, apical cap.

            The capsule of Marchantia has both spore producing and spore distributing body. It illustrates a step further in the progressive sterilization of the sporogenous tissue.

            (c) Anthoceros Sporophyte. It illustrates a step further than Riccia and Marchantia in the progressive sterilization of the potentiality of fertile tissue. The endothecium cells become completely sterile and forms a group of cells known as columella. The sporogenous cells arise from the innermost layer of the amphithecium. It surrounds the columella. The sporogenous cells become differentiated into spore mother cells and pseudo-elaters. The archesporium of Anthoceros is extremely reduced. The outer amphithecium develops into several cells layer thick capsule wall. The capsule wall develops a well ventilated photosynthetic tissue protected by the epidermis.

             (d) Funaria Sporophyte. In Funaria, major portion of the sporophyte remains sterile to form the foot and the seta. The capsule is differentiated into central column of endothecium surrounded by many layered amphithecium. The inner layer of the endothecium forms the sterile columella and the superficial cells forms the sporogenous tissue. Thus the archesporium arises from the outermost layer of cells of the endothecium. It is thus extremely reduced and consists of single layer of fertile tissue. The amphithecium becomes differentiated into the epidermis, the photosynthetic tissue of the capsule wall and the outer spore sac.

            Thus Bower’s theory of sterilization gives a clear explanation of the evolution of the sporophyte into upward direction. This theory is more convincing and reliable.

            2. ReductionTheory:

            This theory was put forward by Kashyap, Church, Goebel and Evans. They hold that the evolution of sporophyte has been in downward direction. They hold the fact that the evolution of sporophyte is retrogressive evolution. They mainly based their theory on the reduction of different organs which results in the simplification of the structure of the sporophyte. On the basis of this view the simplest type of sporophyte of Riccia is considered as the most advanced one.

            The significant steps in the reduction series are –

            (a) Simplification of the dehiscence apparatus.

            (b) Reduction of the green photosynthetic tissue in the capsule wall.

            (c) Disappearance of stomata and intercellular spaces.

            (d) Increase in the thickness of capsule wall.

            (e) The gradual elimination of seta and foot.

            (d) All these changes are accompanied by the progressive increase in the fertility of the sporogenous cells. The change eliminates the presence of sterile cells and elaters in the capsule.

            Evidence from comparative morphology and experimental genetics support the view that the simple sporophyte of Riccia is an advanced but a reduced structure.