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.
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.
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