Evolution of Sporophyte in Bryophytes

 

The sporophyte of Bryophytes is a solid object, radially constructed and represents the diploid asexual generation in the life history of a plant. A product of diploid zygote, the sporophyte has its chief function, the production of spores which through a process of meiosis always possess the haploid chromosome number.

The sporophyte is incapable of self-nutrition and is wholly or partially dependent upon the parent gametophyte to which it is originally attached throughout its life. In form, it varies from only a spherical spore producing structure as in Riccia to an elaborate object differentiated into foot and capsule as in Corsinia to more elaborate structure consisting of foot, seta and capsule as in Polytrichum or Funaria

The foot functions as an anchoring and absorbing organ. The seta helps in conduction and aids in spore dispersal. Chlorenchyma tissue, stomata and air spaces for efficient food synthesis. Elaters, Operculum, Peristome for dehiscence of capsule and spore dispersal. Columella for storage of water and soluble food (in mosses) and for mechanical strength (as in Anthoceros)

Two theories have been put forth to explain the process of evolution of the sporophyte.

Theory of Progressive Sterilization              

This theory is was proposed by Bower and supported by Cavers, Campbell and Smith. According to this theory, the most primitive sporophytes in Hepaticae occurs in the genus Riccia. Such sporophyte, according to some bryologists, are the nearest hypothetical ancestors or represents the actual ancestors of more highly evolved group of plants.

The sporophyte of Bryophytes, according to their complexity of structure may be arranged in a series between the simplest and the most elaborate. This series starts with simple sporophyte of Riccia, runs through that of Marchantia, Pellia, Anthoceros and finally culminates in the highly complex sporophyte of Funaria and Polytrichum.

According to Bower, this series runs in the upward direction. And illustrates a natural advance in the progressive elaboration and complexity of the sporophyte. It is based on the fundamental principle of ‘Progressive sterilisation of the potentially fertile cells (sporogenous tissue)’. Instead of forming spores, the sterile cells develop into elaters, nurse cells and elatophore in different genera. These sterile cells are put to other uses such as nutrition, support, dehiscence, dispersal, etc.

The progressive sterilisation from Riccia to Polytrichum occurred through following stages:

Step – I : The simplest known sporophyte among bryophytes is that of Riccia. It consists of a spherical capsule and seta and foot are absent. The zygote divides by a transverse and then by a vertical division to form a 4- celled embryo. This becomes 20-30 celled by further division.

Periclinal division at this stage differentiates the single layered amphithecium from an inner multicellular endothecium. The amphithecium forms the   layered capsule wall. The endothecium transforms into archesporium, which divides and redivides to form the  fertile sporogenous tissue.

The entire sporogenous tissue is fertile and is forms the spores. There is large output of sores and no or very little sterilisation of the fertile cells. The entire embryo forms the spore producing capsule.

In Riccia crystalline some of the sporogenous cells fail to form spores, but instead they form sterile nutritive cells.

Step – II: In this step, further sterilisation of the embryonic tissue led to the formation of a basal sterile foot of few cells. The Oospore divides into a hypobasal and an epibasal cell. The hypobasal cell gives rise to the foot. The epibasal cell form an outer amphithecium and inner endothecium. The amphithecium gives rise to a single layered jacket and the endothecium differentiates into fertile sporogenous tissue and sterile nurse cells. The nurse cells are long, elater-like, but lack characteristic thickening.

Thus, in Corsinia, the sterilisation has gone a step further and resulted in nurse-cells and foot.

Step – III: Further sterilisation is seen in Sphaerocarpos, where the sporophyte has a sterile bulbous foot and a narrow seta, in addition to fertile capsule. The amphithecium forms the single layered jacket of the capsule and the endothecium forms the sporogenous tissue and the sterile nurse cells.

Step – IV: Targionia exhibits a further step in progressive sterilisation of the potentially sporogenous tissue. The sterile region in this genus consists of a broad foot, a well-developed seta and a large number of elaters with 2 or 3 spiral thickenings.

The amphithecium give rise to a single layered jacket of the capsule. About half of the endothecial cells gives rise to fertile sporogenous tissue and the remaining half form the sterile elaters.

Step – V: This step is exemplified by the sporophyte of Marchantia. The lower half or hybobasal cells of the embryo forms the foot and seta. While, the upper half or epibasal cells forms the capsule. The amphithecium gives rise to a single-layered capsule wall or jacket.

The endothecium forms the sporogenous tissue. Half of the  sporogenous tissue forms spores and the remaining half of the tissue elongate, develop spirally thickened bands on their walls and becomes elaters. The elaters are hygroscopic and help in the dispersal of spores.

Thus, the capsule of Marchantia has specialised both as a spore  producing and spore dispersal body. It illustrates a step further in the progressive sterilisation of the sporogenous tissue and consequent elaboration of its sporophyte.

Draw Marchantia Sporophyte from the Record

Step – VI: Further sterilisation of the sporophytic tissue can be seen in Jungermaniales (e.g., Pellia, Riccardia). The hypobasal half of the zygote takes no part in the development of the sporophyte. The entire sporophyte including the foot and seta is developed from the epibasal half.

Epibasal half differentiates into an outer amphithecium surrounding the inner endothecium. The amphithecium forms the  2or 3 layered capsule wall. The endothecium in whole develop into sporogenous tissue.

The sporogenous cells at the base of the capsule remain sterile. These sterile cells elongate considerably and develop spiral thickenings on their walls to become elater-like. This elater-like cells attached at their lower ends to the cavity of the ca psule is called the basal elaterophore.

In addition, some other dispersed cells in the rest of the sporogenous tissue form elaters which remain unattached.

Only small percent (10%) of sporogenous tissue forms the spores.

Step – VII : this step is illustrated by Anthoceros sporophyte. There is complete sterilisation at the centre. The entire endothecium remains sterile and from a central column called the columella, which is useful for storage of water and food and for mechanical support.

The amphithecium forms the capsule wall and sporogenous tissue. Thus sporogenous tissue arises from the innermost layer of the amphithecium. The wall of the capsule is multilayered consisting of epidermis, stomata , the photosynthetic chlorenchyma .

Even the sporogenous tissue differentiated into fertile spore mother cells and sterile pseudo elater mother cells. The pseudoelater mother cells forms the pseudoelaters as they do not possess spiral thickenings.

Thus, the spsore producing area is greatly reduced and the sporophyte shows a great degree of nutritional independence due to photosynthetic tissue.

Draw Anthoceros sporophyte from the Record

Step – VIII : The highest degree of sterilisatio of the sporogenous tissue is found in the class Bryopsida (Mosses) e.g., Funaria and Polytrichum. Major portion of the embryo remains sterile to form seta and foot. The amphithecium becomes differentiated into the epidermis, the photosynthetic tissue and air space or lacunae.

Except the superficial layer there is complete sterilisation of the endothecium to form the central columella which is continuous right up to the top of the capsule. The archesporium arises from the outermost layer of the endothecium. It is thus extremely reduced and consists of a single layer of cells and confined to the theca region only.

The sterilisation of archesporium towards the base results in the increase in size of the photosynthetic tissue in the apophysis region. The fertility is arrested towards the top due to specialisation of this region for spore distribution because of the formation of peristome, operculum and annulus.

Thus, moss capsule is mechanically more efficient in dispersal of spores, consequently the archesporium is much more reduced.

Bower’s theory of sterilisation offers a plausible explanation of the evolution of the sporophyte in the upward direction.

Theory of Regressive Evolution or Reduction Theory

This is also known as theory of progressive simplification or reduction theory
This theory was proposed Kashyap and supported by Church, Goebel and Evans.

According to this theory Riccia sporophyte is the most evolved or the advance one formed due  to reduction as a result of progressive simplification . They think that the primitive sporophytes is  like that of Mosses (e.g. Funeria and Polytrichum)from which the sporophyte of  Marchantia, Jungermanniales and Anthocerotales have been evolved by  reduction of the tissues.
Church suggested that the hypothetical ancestors of bryophytes had foliose sporophytes with complex assimilatory tissue and  epidermis with functional stomata.  From such a plant,  evolution progressed  regressively as follows:
1. The erect leafy sporophyte became permanently attached to the gametophyte and gradually lost its leaves.
2. Reduction of the green photosynthetic tissue in the capsule wall.
3. Associated with the above is the disappearance of stomata and intercellular spaces.
4. The multilayered sterile jacket of the capsule ( Funeria, Anthoceros)became single layered by (Riccia, Marchantia)by reduction
5. Gradual elimination of the anchoring tissues like foot, seta, apophysis and the conducting tissues like columella
6. Archesporium formation shifted from amphithecium to endothecium.
7. Progressive increase in the fertility of the sporogenous cells these changes eliminated the presence of sterile cells and relatives in the capsule.