12

Sporophyte of
Bryophytes

WHAT IS A SPOROPHYTE AND A SPORE? 12.1

•
From the technical viewpoint, the sporophyte is the diploid generation in an alternation of generations.
•
The sporophyte is the generation producing spores, which are haploid. In angiosperms, gymnosperms
and pteridophytes, the sporophyte is the main vegetative stage. In bryophytes, on the other hand,
the sporophyte grows directly from the archegonium of the gametophyte, and thus depends on the
gametophyte for its nutrition.
Spore is a small round cell from which a whole new plant is produced. In bryophytes, the spores
are haploid and are produced by the sporophyte. They are produced as a result of meiosis in the spore
mother cells
In bryophytes, the sporophyte is usually made up of three parts, viz. foot, seta and capsule. Foot
is the basal part which remains embedded within the gametophyte. It is basically an absorbing and
anchoring structure. Seta is an elongated, stalk-like structure and bears the third most important part of
the sporophyte, the capsule. Foot and seta are sterile structures while the capsule is the fertile structure
of the sporophyte which produces the spores.

HOW IS THE SPOROPHYTE DIFFERENT

FROM THE SPOROGONIUM? 12.2
•
Capsule is the spore-bearing structure of the sporophyte in bryophytes, and it has also been described as
•
sporogonium. The terms like "sporogonium" and "sporangium" are used as synonyms of the capsule.
Since, the sporangium is the spore-bearing structure in pteridophytes, its use in bryophytes should be
abandoned or stopped. The term "sporogonium" is usually used as an alternative for a sporophyte in
bryophytes. As mentioned in the Chambers Biology Dictionary, the term "sporogonium" is the "same
as the sporophyte in bryophytes". In Longman 's Illustrated Dictionary of Botany, sporogonium is "the
sporophyte of a moss and a liverwort consisting of a foot, seta and capsule."
206 ♦ Bryophyta

BRVOPHYTIC SPOROPHYTE: A BODY DEPENDENT ON

GAMETOPHYTE 12.3
•
In bryophytes, the sporophyte is always dependent on the gametophyte, and due to this some bryologists
•
even describe it as "a parasite on the gametophyte". However, since some cells of the wall of the capsule
contain chloroplasts, the sporophyte is not fully dependent for nourishment on the gametophyte, and
due to this it should not be described as a parasite on the gametophyte. In some bryophytes, the cells
of the seta also contain chloroplasts. A few cells of the foot also contain some chloroplasts in a few
bryophytes, such as Sphaerocarpos. In comparison to mosses, the chlorophyll content in the cells
of seta and capsule is very low in liverworts. Due to the presence of chlorophyll in some cells, the
sporophyte is not fully dependent on the gametophyte. It is partially dependent.
In Anthoceros, the sporophyte is highly organised, green throughout its life and even some of its
epidermal cells contain functional stomata, helpful even in gaseous exchange. A well-organised region
of mechanical support, also helpful in conduction, is present in the form of columella in the sporophyte
of Anthoceros. Some intercalary meristematic cells are also present in the basal part of the sporophyte.
All these make the Anthoceros sporophyte a highly organised and highly evolved body.

STRUCTURE OF SPOROPHYTE 12.4

•
As mentioned earlier, the sporophyte in most liverworts (e.g. Marchantia; Fig. 12.lA) and mosses is
•
made up of foot, seta and capsule. A brief discussion of all these parts is mentioned below.

f/;Q-v-,;J~~~- Foot

1;--+\+-1.+--Capsule wall
0 •
,,o O i'J t, ,:;c:,
}
0 /r"l o0 ~~c Spores
/ :· ~ ,),:; r, 1··
() ,r. 0 p
~ Leafy shoot
;,_/

~ 9,,.
~ -

o ,..
C, i!
Elaters
O ~j

.
(.I

'
¢;
'
t>

11------ Calyptra

"
Marsupium

1r A
B

Fig. 12.1 A, A mature sporophyte of Marchantia showing foot, seta and capsule;
B, A leafy shoot with sporophyte and enlarged marsupium
 Sporophyte of Bryophytes ♦ 207

12.4.1 Foot
The base of the sporophyte of a bryophyte, which is the part that attaches it to the gametophyte, is
known as foot. It functions for absorption and anchorage. Its size and shape are variable in different
bryophytes. In some bryophytes, the foot is absent, e.g. Riccia. In most bryophytes, the foot is globose
(e.g. Conocephalum, Corsinia, Marchantia, Anthoceros) to anchor-shaped (e.g. Pellia, Porella). In a
majority of thalloid liverworts, the foot is large and massive, but in a few liverworts it is very small
(e.g. Trichocolea). The foot is nearly spherical and bulbous in a few mosses (e.g. Micromitrium). In
Schistochila, a foliose liverwort, some rhizoid-like extensions develop from the foot. Marsupium, a
tuber-like outgrowth of the foot, is seen in Calypogeia (Fig. 12.1 B).

12.4.2 Seta
Seta is the stalk of the sporophyte of a bryophyte. When young, the seta is made up of elongate cells
meant for conduction and support. Seta is absent in some bryophytes, e.g. Riccia, Anthoceros and
Notothylas.
The seta usually elongates by elongation of its cells and not by division of its cells. Usually, the
elongation of the seta is as fast as 1 mm per hour, when young. It attains a length of over 5 cm
or more in some liverworts, e.g. Pellia, Monoclea, etc. In Marchantia (Fig. 12.lA), Corsinia and
Targionia, the seta is not a very long structure. In Pohlia natans, seta reaches up to 7 cm in length,
and in Drepanocladus jluitans, seta may attain a length up to 10 cm or even more. In genera such as
Phascum and Ephemerum seta is a very minute body.
Seta is long but a delicate and ephemeral structure, made up of thin-walled cells. Transverse section
of the seta of Jubula reveals that in circumference, it is only made up of a few cells (Fig. 12.2A). But
it is made up of many cells in Mylia (Fig. 12.2 B). In Caphaloziella, seta consists of four small central
cells surrounded by four larger cells (Fig. 12.2 C).

A B C

Fig. 12.2 Transverse sections of the seta of Jubula (A), Mylia (B) and Cepha/oziel/a (C)

Some of the striking differences between seta of liverworts and mosses are listed in Table 12.1.
In several species of Brachythecium and Dicranella, the seta contains some specialised papillae-like
structures. In Brachythecium rutabulum, papillae on the seta are quite large and can be seen even with
the naked eye.
208 ♦ Bryophyta

Table 12.1 Differences between the seta of liverworts and mosses

SNo. LIVERWORTS MOSSES

1. Seta is quite an elongate body much before the Seta is a short structure prior to the maturity of
maturity of capsule. capsule.
2. Seta is not highly specialised for conduction. Seta is a highly specialised structure for conduc-
tion.
3. It does not provide that much support to the capsule It provides more support to the capsule than in
as in mosses. liverworts.
4. It is more significant for dehiscence in liverworts. It is not so significant for dehiscence in mosses as
it is in liverworts.

12.4.3 Capsule
Capsule is the spore-producing organ of the sporophyte of a bryophyte, borne at the top of the seta. In
liverworts, it varies in its form in different members. It may be cylindrical, ovoid, subspherical or even
spherical. In Riccia (Fig. 7.10 J), Sphaerocarpos (Fig. 5.2 M), Frullania, Porella (Fig. 4.7), Pellia (Fig.
4.16 B), Fossombronia, Lejeunea and some other bryophytes, the capsule is nearly spherical in shape.
Capsules are almost ovoid or ovoid-cylindrical in Blasia and Riccardia while they are elongated in
Haplomitrium and Monoclea.
As far as the size is concerned, the capsules of mosses are usually larger than the capsules of
liverworts. In Marchantiales, they attain a diameter of 1 to 1.25 mm but in Jungermanniales, the
capsules are comparatively narrower and reach up to 0.6 to 1 mm in diameter. In Riccardia and Pellia,
the capsule reaches up to 1.5 mm or more in diameter.
In mosses, the capsule is usually cylindrical and erect (Funaria hygrometrica), subglobose (e.g.
Bartramia) or pyriform and pendulous (Bryum, Pohlia).

DEVELOPMENT AND DIFFERENTIATION OF

DIFFERENT ORGANS OF SPOROPHYTE 12.5
•
Development and differentiation of different organs of the sporophyte in bryophytes may be studied
•
in the form of five different stages, viz. embryogeny stage, protected stage, green stage, differentiation
stage and, dehiscence and dispersal stage.

12.5.1 Embryogeny Stage

A plant at an early stage of development is known as embryo, and the processes leading to the formation
of the embryo is known as embryogeny. So, early developments of the sporophyte of a bryophyte are
included under the embryogeny stage.
Two main findings of the embryogeny stage in liverworts are (i) early embryo passed through
the formation of an 8-celled octant stage (Marchantiales) or linear stage (Jungermanniales), and (ii)
origin of sporogenous tissue. The young multicellular embryo divides periclinally to form the outer
amphithecium and the inner endothecium, either of which form the sporogenous tsissue in different
members. Endothecium is, however, responsible for the formation of sporogenous tissue in most
liverworts.
 Sporophyte of Bryophytes ♦ 209

In mosses, on the other hand, the early embryogeny (Fig. 12.3 A-H) is more uniform. Polarity is
established in the initial stages, and elongation of the young sporophyte takes place by the activity of
an apical cell. In the lower part of the young sporophyte, a second apical cell starts functioning. Soon,
the ovoid embryo of the moss transforms into an ellipsoidal body and finally a narrow cylindrical body
tapering at both ends is resulted. The central endothecium is soon demarcated from the peripheral
amphithecium in this young multicellular cylindrical embryo. The archesporium originates from
the endothecium. The external wall layers and central columella are differentiated when the young
embryo is about 20 to 24 cells thick. Various mosses require different periods for the differentiation
and development of various parts (i.e. foot, seta and capsule) of the sporophyte. For example, the
processes from the time of fertilization up to the discharge of spores are completed within 2-3 weeks
in Phascum cuspidatum, but in Polytrichum the period required for completion of all these processes
1s over one year.
Amphithecium
Two-cell stage Quadrant stage

A B C D

Columella

Sporogenous
layer

E F G H

Fig. 12.3 A-H Diagrammatic representation of the early stages of embryogeny in a moss capsule.
A, Two-celled stage; B, Quadrant stage; C-D, Differentiaion of amphithecium and
endothecium; E-H, Differentiation of wall layers, sporogenous layer and columella

12.5.2 Protected Stage

Calyptra and positioning of foot are the two major ways which help in the protection of young
developing sporophyte. Besides calyptra and foot, some additional protective structures are also
present in some bryophytes. Involucre, green curtain of tissues and perianth are some such additional
protective structures. Well-developed pear-shaped involucres are present as a protective body in
Sphaerocarpos. Some green curtain of tissues develop amongst the archegonia in Corsinia. Some
bryologists consider these tissues as forerunners of carpocephalum. Infoldings of scale-like structures
develop as a protective tissue in the sporophytes of Targionia. A green involucre is present in Pellia. In
Marchantia, the protective structures of sporophyte include calyptra, perianth and involucre. A Chinese-
lantem-type additional protective covering is present in the sporophyte of Fimbriaria. Riccardia lacks
any additional protective structure.
210 ♦ Bryophyta

1. Calyptra
The calyptra is actually a hood of tissue produced from the wall of the archegonium, especially in
mosses. It is also formed in liverworts. Calyptra protects the young sporophyte. The size and shape of
the calyptra affect the shape and orientation of the capsule in mosses. The calyptra is highly variable
in different bryophytes in its extent of covering the capsule. In bryophytes, in general, and mosses in
particular, the shape of the calyptra serves as a useful tool of taxonomic importance. In Eucalypta,
the calyptra is an elongate cone-like structure which covers the capsule all over. In a majority of other
bryophytes, the calyptra is a small cap-like covering surrounding the basal part of the developing capsule.
Due to very fine hairy calyptra in Polytrichum, the name hair-cap-moss is given to this genus.

2.Foot
The foot of the sporophyte of bryophytes remains housed in the green gametophytic tissue. It provides
adequate nutrition to the sporophyte. In some leafy liverworts, the sporophyte is housed in a special
pouch-like structure of gametophytic origin, known as marsupium. It is a multilayered pouch-like
or tube-like structure made up of gametophytic tissue. In Geobelobryum, the marsupium is a well-
developed, elongated, tube-like structure bearing rhizoids, which are sometimes seen even buried into
the substratum like that of a root of higher plants.
In thalloid liverworts, the foot of the sporophyte is a globose mass of undifferentiated cells. In the
foot of mosses, some differentiation may be observed in the form of outer haustorial cells, intermediate
unspecialised cells and central cells. The central cells function like that of conducting cells. Intense
enzymatic activity can be observed in the cells of the outer region of the foot of mosses. Chauhan
(1988) reported cytochemical reactions for respiratory enzymes and phosphotases in the cells of the
foot region of Physcomitrium cyathicarpum. The haustorial cells of the foot of the sporophyte function
as the organs of absorption and transfer of nutrients, and due to these, they are named transfer cells. In
Physcomitrium cyathicarpum and Dendroceros, the peripheral cells of the foot show some infoldings
or invaginations, which form wall labyrinths. The characteristic feature of transfer cells is the presence
of these wall labyrinths in this genus. The transfer cells of the foot, therefore, form the junction between
sporophyte and gametophyte. Besides mosses and liverworts, transfer cells have also been reported in
homworts (e.g. Anthoceros punctatus) by Ligrone and Gamberdella (1988). Transfer cells are absent
in the foot region of Pellia and Sphagnum.

12.5.3 Green Stage

Photosynthetic efficiency of the sporophyte represents its green stage. It is an important stage because
it provides some independence to the sporophyte from the gametophyte. In almost all liverworts,
the young sporophyte remains buried in the gametophytic tissue. In developing and nearly mature
sporophytes, the capsule is pushed out of the gametophytic tissue due to the meristematic activity of
seta and absorbing nature of foot. However, due to low chlorophyll content in seta and capsule, the
sporophyte depends on the gametophyte for its requirements of inorganic and organic contents. In
mosses, on the other hand, more amount of chlorophyll is present in different tissues and parts of the
sporophyte. The sporophyte is, therefore, more photosynthetically efficient than liverworts. The long
seta of the sporophyte of mosses is quite green, especially when young, and hence more efficient in terms
of its photosynthetic activity. The seta, however, is more specialised for support and conduction.
In mosses (e.g. Bartramia, Bryum, Funaria), a well-developed green tissue is present in the wall
of the capsule of the sporophyte. More photosynthetic activity is possible in the sporophytes of these
 Sporophyte of Bryophytes ♦ 211

mosses. In Torula and some more mosses, the green tissue in the capsule region is less extensive, hence
there is less amount of photosynthetic activity. In Splachnum ampullaceum and many other species of
this genus, the apophysis region is most extensive amongst mosses.
Stomata are not found on the capsules of liverworts. In most mosses, they are present on the capsule,
specially in the apophysis region. In Pleuridium, the number of stomata are only 3-5 on a capsule. But
in some mosses (e.g. Philonotis), each capsule has as many as 200 or more stomata. Some mosses have
no stomata on their capsules, e.g. Fontinalis and Atrichum.

12.5.4 Differentiation Stage

Differentiation stage of the sporophyte is the demarcation of tissues which results into the formation of
sporogenous cells and spores in the sporophyte. The entire sporogenous tissue is endothecial in origin
in Marchantiales (e.g. Riccia) and results in the formation of spores. In Jungermanniales, a part of
the sporogenous tissue remains sterilised and forms the elaters. And, as we proceed further in mosses
the capsule elaborates and only very little of its tissue is meant for production of spores. Moreover, the
sterile structures like elaters are also not formed in capsules of mosses. A sterile region in the capsule
of Funaria and other mosses is present in the form of columella, which provides mechanical support
to the capsule and also helps in conduction. Columella is absent in liverworts. In Anthoceros and other
hornworts, a well-developed columella is also present.
The sporogenous tissue develops into spores, and the number of spores per capsule per plant have
been estimated in some bryophytic genera by bryologists. Some such findings are listed in Table 12.2.

Table 12.2 Approximate number of spores per plant in some bryophytes

SNo. GENUS NUMBER OF SPOREsfSPORE TETRADS PER PLANT PER CAPSULE (APPROX.)
1. Sphaerocarpos 200 spore tetrads per capsule
2. Pellia 4500 spores per capsule
3. Lophocolea cuspidata 24,000 spores per capsule
4. Diplophyllum albicans 40,00,00 spores per capsule
5. Eurhynchium 700,000 spores per capsule
6. Scapania undulata 10,00,000 spores per capsule
7. Marchantia polymorpha 7000,000 spores per plant

12.5.5 Dehiscence and Dispersal Stage

Structure of the capsule wall is of utmost importance in the dehiscence of sporophyte in bryophytes.
Marchantiales possess a unistratose capsule wall whereas in Jungermanniales the capsule wall is
multistratose. The unistratose capsule wall of Marchantiales sometimes has transverse thickenings
(e.g. Peltolepis quadrata, Fig. 12.4A). But in Plagiochasma intermedium (Fig. 12.4B), the cells of the
jacket are thin-walled and do not bear any ornamentation. In Asterella, the capsule jacket is without an
apparent thickening (Fig. 12.4 C). But in Conocephalum, the entire transverse wall is thickened (Fig.
12.4 D). The wall of Norwallia shows ornamentation in the longitudinal wall (Fig. 12.4 E) while the
longitudinal wall shows thickenings (Fig. 12.4F). The epidermal cells shows ornamentation in case of
Frullania (Fig. 12.4 G) but in Radula (Fig. 12.4H), the epidermal cells of the wall of the capsule show
212 ♦ Bryophyta

Transverse thickenings

A B C D

;f~
I~1"' \
(.,,

E
- I

F
'
~/

G H
Fig 12.4 A-H Wall of capsule of Pe/to/epis quadrata (A) showing transverse thickenings; capsule walls of
Plagiochasma (B), Asterella (C), Conocephalum (D), Norwallia (E), Arnellia (F), Frullania (G),
and Radula (H)

thickenings. It has been observed in different members that the jacket of the capsule ruptures along
four longitudinal lines resulting in the formation of four valves or flaps. Intact capsules of Cephalozia,
before (Fig 12.5A) and after dispersal Fig. 12.5B) and in Frullania before (Fig. 12.5 C,D) and after
dispersal (Fig. 12.5 E) are shown in Fig. 12.5.

~\.' '.>i
. \:,\' \ ~ ~ ;< / ~
L)I . _,

~ ~-~ - ··;.,·:?:-~
.. ((;-! ~s
-1/. ' , ,,
'--,. ., . ~
.I' .\ -;;y,r.1·~
•: ·.••
, , \ - --~V/ /, ·.-· ,
\ \fj//1·1 I 1/

111-0{

E
D
Fig. 12.5 A-B, Intact capsules before and after dispersal in Cephalozia; C-E, Intact capsules
before and after dispersal in Frullania
 Sporophyte of Bryophytes ♦ 213

In mosses, the jacket of the sporophyte is multistratose. The stomata are also present in its epidermal
wall in most mosses. The stomata when present, are either exposed (e.g. Funaria) or immersed (e.g.
Orthotrichum). Operculum, a cap-like structure, gets differentiated in the apical region of the jacket of
the capsule. The operculum ruptures and this triggers the dehiscence of the capsule. The operculum is
released by the annulus, which is made up of a ring of enlarged elastic cells. The shape and structure of
the operculum help in the dehiscence and dispersal of spores.
1. Categories of Dehiscence of Capsule and Dispersal of Spores
Two categories of dehiscence of capsule and dispersal of spores may be as under:

(a) Passive Dehiscence and Passive Dispersal This takes place in those bryophytic genera(e.g. Corsinia,
Riccia) in which the seta is absent in the sporophyte and dehiscence takes place by disintegration of
jacket of the sporogonium. Because of the absence of elaters or any specialised structures, the dispersal
is also passive.

(b) Active Dehiscence and Active or Passive Dispersal This takes place in those genera in which the
sporogonium wall ruptures along four lines of dehiscence and elaters are present. The elaters help in
the dispersal of spores.

2. Dehiscence and Dispersal in Liverworts

Dehiscence of capsule and dispersal of spores in liverworts mainly take place by three different
mechanisms, described below:
(a) hygroscopic mechanism (e.g. Pellia, Marchantia, etc.),
(b) water-rupture mechanism (e.g. most of the foliose liverworts), and
(c) spiral-ring mechanism (e.g. Frullania).
Spores are dispersed slowly in hygroscopic mechanism but very fast and violently in water-rupture
mechanism and spiral-ring mechanism.
In members showing hygroscopic mechanism, the spiral bands of elaters are weaker than those
showing the other two mechanisms. On the other hand, in members showing water-rupture mechanism
and spiral-ring mechanism, spiral bands of their elaters show great strength of bispiral thickenings, as
in Cephalozia and Lophocolea. In dry conditions, the coils of bispiral thickenings contract sharply,
resulting in extreme tension. This loosens the coils abruptly, untwists the elaters instantaneously and
results in dehiscence and violent dispersal of spores.
In the liverworts showing hygroscopic mechanism resulting into slow dispersal of spores, the elaters
bear weaker spiral bands. The spirals of bands of these elaters, of course, contract but this contraction
is not sufficient to induce water rupture. The elaters simply twist in the available atmospheric humidity,
and there is seen only gradual dispersal of spores.
The spiral-ring mechanism is characteristic of Frullaniaceae and Lejeuneaceae, and this is linked
with unique internal organisation of the capsule. A series of elaters are present from roof to floor in
the globose capsule of these members. Dehiscence mechanism in these members is also unique. It
takes place by four valves of its jacket, which curve outwards rapidly, resulting into the discharge of
spores.
3. Dehiscence and Dispersal in Mosses
Various types of dehiscence of capsules and dispersal of spores are observed in mosses. In Andreaea
(Fig. 12.6A,B), the dehiscence of capsule is of great taxonomic significance. The jacket of the capsule
214 ♦ Bryophyta

of this moss is thick-walled and possesses four or more lines of weakness which extend from the base
towards the apex. The mature sporogonium starts drying out, shrinks, and the shrinkage leads to lines
of weakness and finally yields into the dispersal of spores.
Columella

Jacket
Calyptra

Spores

Archegonium

Pseudopodium
A
B

Fig. 12.6 Andreaea rupestris showing entire sporogonium (A) and LS (B) of the same

In Ephemerum and some species of Physcomitrium, the capsules are closed or cleistocarpous. They
open in an irregular manner to disperse the spores. Peristome and a detachable operculum or lid are
absent in such mosses. Seta is also ill-developed or even absent in these mosses. Only a few leaves
present in the gametophyte of these mosses surround the sporophyte. In dry conditions, the entire plants
of these mosses can be blown away by winds and are also transported far by humans and animals.
In gymnostomous mosses (e.g. Pottia truncata), the operculum may or may not be present in the
capsules. They also lack peristome. Due to these characteristics, these mosses have no gradual or
regulated mechanism of dispersal of spores. Dehiscence of the capsule takes place by blowing off of
 Sporophyte of Bryophytes ♦ 215

the upper part of the capsule. The spores are dispersed by gravitational force because the capsule mouth
is directed downwards in these mosses
According to Edwards (1980), weather plays an important role in the dehiscence of the capsule and
dispersal of spores in aquatic mosses like Scouleria and Wardia. It is so because their capsules are
not directed downwards and they also lack teeth. In dry weather, their lid opens but in wet weather, it
remains intact, and spores are exposed due to wind.
Peristome is the major part to regulate the spore dispersal in the moss capsule, e.g. Funaria
hygrometrica. Two well-defined rings of peristome are present in this moss. In some other species
(e.g. Funaria fascicularis), however, the peristome is either missing or rudimentary. Similarly, two
rings of peristome are present in Encalypta streptocarpa but only one ring of peristome is present in E.
rhabdocarpa. Peristomate mosses thus fall in two categories, viz. haplolepideae (having single ring
of peristome), and diplolepideae (having two rings of peristome). Usually, a single ring of peristome
has 16 teeth. They form a close-fitting circle at the base and their apical parts taper to a point. Each
tooth of the peristome is a barred structure, and the tooth bars are actually remnants of the cell wall.
Thickenings, which are characteristic of outer teeth, are usually absent in the inner teeth. Usually,
16 teeth of the inner ring alternate with the 16 teeth of the outer ring. 16 thread-like cilia, in groups of 3
or 4, are also usually present on the same radii as that of the outer teeth. Dispersal of spores is actually
regulated by the peristome.
Regarding variations in peristomial teeth (Fig. 12.7 A-F), solid type of 4 erect peristome teeth are
present in Tetraphis (Fig. 12.7A), a ring of spirally twisted filliform teeth are present in Barbula (Fig.
12.7B), a ring of 16 barred slow-moving teeth are present in Dicranella (Fig. 12.7C) while double
peristome quite active in spore dispersal, are present in Hypnum (Fig. 12.7D) and Bryum (Fig. 12.7 E).
Peristome shows teeth curving over epiphragm in Atrichum (Fig. 12.7F).

f<

~ A ;((~\
j(.J
()
C)
L),
0
0 D
0 0
0
0

~
C

Fig. 12.7 A-F Showing variations in peristomial teeth in Tetraphis (A), Barbu/a (B), Dicranella
(C), Hypnum (D), Bryum (E) and Atrichum (F)
216 ♦ Bryophyta

The entire process of spore dispersal in mosses can be grouped in three broad categories, as under:

(a) Primitive Type In mosses like Barbula (Fig. 12.7B) and Tortula, teeth movements have very little
or no active role in spore dispersal, and this is called primitive type. The peristome in such mosses
serves only as a hygroscopic lid.

(b) Intermediate Type In Dicranella (Fig. 12.7(C) and some other mosses, the peristome has 16
forked and freely moving teeth. In these mosses, the spores accumulated under the capsule mouth are
caught in between slowly moving teeth and get dispersed.

(c) Advanced Type Gradual discharge of spores is seen in the advanced type of mosses. Teeth play
a more active role in this type of dispersal. A majority of the mosses, which fall under this category,
have two rings of peristomial teeth. Advanced type is exemplified by mosses such as Brachythecium,
Bryum (Fig. 12.7E), Hypnum (Fig. 12.7D) and Mnium. The spores are dispersed in dry conditions in
these mosses by a process in which inner peristomial teeth stand up as a central cone and the tips of
teeth of outer ring are inserted into the gaps present in between different inner structures. In moist
conditions, the process of gradual discharge of spores depends on the ability of peristomial teeth to
close the capsule mouth.

In Atrichum (Fig. 12.7(F), Oligotrichum and Polytrichum, the peristome is of complex type, and
dispersal of spores takes place by censor mechanism. As many as 32 or 64 peristomial teeth of
different structures, are present in these mosses. Each tooth consists of fibre-like cells of several
layers of thickness, and this represents a solid construction. All these teeth unite at their tips to form
a membranous structure called epiphragm. Dispersal of spores takes place through very fine holes
present between the successive teeth, and this type of dispersal of spores is called censor mechanism.
An unusual type of spore dispersal is observed in Tetraphis (fig. 12.7A). Its peristome contains four
large teeth of solid construction, but it lacks epiphragm.
Sphagnum (Fig. 12.8A-D) shows air-gun mechanism of dehiscence of capsule and dispersal
of spores. The mature sporogonium of this moss dries out, shrinks in diameter, and due to this, the
columella collapses and a high air pressure is resulted inside the capsule. Due to this pressure, the
operculum is thrown away and spores are shot away into the atmosphere. This violent process of
dispersal of spores is known as air-gun mechanism.

A[ C D

Fig. 12.8 A-D Dehiscence of capsule in Sphagnum showing air-gun mechanism of spore dispersal
 27
Origin and
Evolution of
Sporophyte in
Bryophytes

•
WHAT IS EVOLUTION OF THE SPOROPHYTE
IN BRYOPHYTES? , 27.1
•
The zygote is the first cell of the sporophytic generation. It divides and redivides to form the sporophyte,
which is never an independent body in bryophytes. It is always dependent on the gametophyte, either
completely or partially. The function of the sporophyte is the production of spores.
Two different theories have been put forward by the bryologists to explain the evolution of the
sporophyte in bryophytes. These are (i) theory of the progressive sterilization of the potentially
sporogenous tissue, and (ii) theory of progressive simplification or reduction theory.

THEORY OF PROGRESSIVE STERILIZATION

OF POTENTIALLY SPOROGENOUS TISSUE 27.2
This theory, also called the theory of sterilization, has been proposed by Bower (1908), and supported
by Cavers (1910) and Campbell (1918, 1940). According to this theory, Riccia possesses the simplest
and the most primitive sporophyte, and from such a sporophyte of Riccia have evolved more advanced
sporophytes, through the process of progressive sterilization of potentially sporogenous tissue. A
definite series of such an evolution of the sporophyte from Riccia may be traced in the sporophytes of
Sphaerocarpos, Targionia, Marchantia, Pellia, Anthoceros and Funaria. A brief discussion of all these
stages from Riccia to Funaria follows:

1. First Stage The first stage is seen in Riccia (Figs. 27 .1, 7 .10) in which the zygote divides several
times to form a multicellular diploid structure, of which the outermost layer develops into a sterile
jacket while all the central mass remains sporogenous in nature. Each cell of this sporogenous tissue
 Origin and Evolution of Sporophyte in Bryophytes ♦ 315

divides meiotically to form four haploid spores. Thus, the only sterile part of this simple sporophyte is
the sterile jacket, while the entire remaining tissue is fertile. Also, there is no differentiation of organs,
such as foot, seta and capsule, and, therefore, the sporophyte of Riccia is the simplest and the most
primitive (Bower, 1908).

• ~8:2

~ ~ o:;;<--H--¥\-- Spore
~ ~ . tetrads

~ ~·
Fig. 27.1 Sporophyte of Riccia

2. Second Stage In Riccia crystallina and Oxymitra, the sporophyte, of course, consists of a
single-layered jacket enclosing the central mass of the sporogenous tissue like that of other species of
Riccia discussed above. But some potential spore mother cells remain unable to form the spores and
form the sterile or abortive nutritive cells. Thus, the sterile tissue in the sporophyte is a little more in
amount than that of the first stage, i.e. other species of Riccia.

3. Third Stage In both Corsinia and Sphaerocarpos (Figs. 27.2, 5.2), more amount of potentially
sporogenous tissue becomes sterilised than that of Riccia crystallina and Oxymitra. In Corsinia, the
whole of the basal part of the sporophyte gets sterilised to form the foot, made up of a few cells. And in
Sphaerocarpos (Fig. 27 .2), the basal part of the sporophyte is sterilised into a bulbous foot and a narrow
two-cells broad seta. In both Corsinia and Sphaerocarpos, a single-layered sterile jacket surrounds the
sporogenous tissue in the capsule. A few sterile nurse cells are also present in both along with the
fertile spores. The nurse cells, however, lack the characteristic spiral thickenings of the elaters.

Capsule , <8 <D @·

Spore ·:[ID\~ ®
tetrad W @O /.ffi\
Nursecell @ 18), '©'

. ~ ooP-.
·-_. @.
u

Fig. 27.2 Sporophyte of Sphaerocarpos stipitatus

316 ♦ Bryophyta

4. Fourth Stage In Targionia (Fig. 27 .3), the sporophyte contains still larger amount of the sterile
region in the form of a bulbous foot, narrow seta, single-layered jacket of the capsule, and several
elaters, containing spiral thickenings. Sterile elaters present along with the fertile spores, constitute
about half of the number of the sporogenous cells of the capsule.

Spore tetrad
Elaters

Fig. 27.3 Sporophyte of Targionia hypophylla

5. Fifth Stage In Marchantia (Figs. 27.4, 7 .32), the sterile tissue in the sporophyte is slightly
more in amount than that of Targionia, and consists of a broad and bulbous foot, elongated and more
developed seta, single-layered sterile jacket of the capsule, a few sterile cells at the apex of the capsule
in the form of a small apical cap, and a large number of long elaters containing the spiral thickenings,
along with the fertile spores in the capsule.

laters

Capsule

Spores
Calyptra

Thallus

Fig. 27 .4 Sporophyte of Marchantia polymorpha

6. Sixth Stage In Pellia (Figs. 27.5, 4.16F), Riccardia and other Jungermanniales, still larger
percentage of the part of the total sporophyte is sterilised. The sterilised tissue consists of an elaborate
foot, well-developed seta, two to many-layered sterile jacket of the capsule, several elaters, and a mass
of sterile cells in the form of elaterophore. The elaterophore is either basal (Pellia) or at the apex
(Riccardia) of the capsule. Only a small percentage of the sporogenous tissue actually remains fertile
and forms spores.
 Origin and Evolution of Sporophyte in Bryophytes ♦ 317

Jacket
(multilayered)

Spores

Fig. 27.5 Sporophyte of Pellia epiphylla

7. Seventh Stage Highly reduced sporogenous tissue is present in Anthoceros (Figs. 27.6, 8.7
1-M) on account of further sterilisation. The sterile tissue of the sporophyte consists of massive foot,
small meristematic zone, 4- to 6-layered wall of the capsule, 4- to 16-celled thick columella and a large
number of pseudoelaters. Further, the sporophyte of Anthoceros shows a greater degree of independence
since its capsule wall possesses a well-defined epidermis, several stomata and chlorophyll-containing
cells.

• ~ Q 0
Columella ,, ~ - "oeo
.. ,c,
Spores
Stoma ta----''---llHf 'o

Pseudoelaters
Epidermis

Spore tetrad

Columella - ~

Archesporium ■
Meristematic
region

Fig. 27 .6 Sporophyte of Anthoceros

8. Eighth Stage The process of the progressive sterilisation of potentially sporogenous tissue
reaches at its peak in some higher members of Bryopsida, e.g. Funaria (Figs. 27.7, 10.22 B) and
Polytrichum. In Funaria, the sterile tissue of the sporophyte consists of foot, seta, entire region of the
318 ♦ Bryophyta

apophysis of capsule, many-layered capsule wall, spore sac wall, columella, peristome and operculum.
The only fertile region is in the form of two spore sacs in the theca region of the capsule. Due to the
presence of stomata, very long seta, well-developed capsule wall, and large number of chlorophyll-
containing cells in the capsule, the sporophyte in these bryophytes shows still greater degree of
independence. According to the theory of progressive sterilisation of potentially sporogenous tissue
of Bower (1908), the sporophyte of Funaria, therefore, is the most advanced and highly evolved.

Peristome

=-Columella

IW-t~~(Ull\l~r;.~a=s-- Air sac

Apophysis

Stomata

~~II§!--- Seta

Fig. 27.7 Sporophyte of Funaria hygrometrica

THEORY OF PROGRESSIVE SIMPLIFICATION 27.3

This theory, also called the reduction theory, has been proposed by Church (1919) and supported
by Kashyap (1919), Goebel (1930) and Evans (1939). According to these bryologists, the simplest
sporophyte of Riccia is not a primitive type of the sporophyte. On the other hand, they believe that
the Riccia sporophyte is a reduced type evolved by a process of descending or regressive evolution or
"progressive simplification".
According to the reduction theory of Church (1919), the ancestral sporophyte of Bryophyta was
an erect, foliose (leafy), and independent shoot like that of mosses. During the descending course of
evolution, such an erect leafy sporophyte passed through the following changes:
(i) It became attached to the gametophyte on the permanent basis, (ii) its leaves were lost due to the
prolonged isolation and desiccation, (iii) because of its greater dependence on the gametophyte, the
intercellular spaces of its photosynthetic system disappeared, (iv) in the later stages its assimilatory
tissue also became reduced, (v) the stomata of its epidermis first reduced into simple pores and became
functionless, as in Sphagnum, and then they completely disappeared, as in Marchantia and Riccia.
Church (1919), followed by Goebel (1930) and Evans (1939), hence, believed that the complex and
well-developed sporophytes of Funaria andAnthoceros, having intercellular spaces in their assimilatory
tissue and stomata in their epidermis, are primitive and nearer to the ancestral type. On the other hand,
 Origin and Evolution of Sporophyte in Bryophytes ♦ 319

the sporophytes of Pellia, Marchantia, Targionia and Sphaerocarpos are reduced and simplified, and
this series of reduction reached at its peak in the sporophyte of Riccia.

~~I TEST YOUR UNDERSTANDING

1. Give an illustrated account of evolution of sporophyte in bryophytes.
2. What is the first cell of a sporophytic generation?
3. In bryophytes, the sporophyte is never an ................ body.
4. In bryophytes, the sporophyte is always dependent on .............., completely or partially.
5. The main function of sporophyte in bryophytes is the production of ................... .
6. What are the two major theories which have been put forward to explain evolution of
sporophyte in bryophytes?
7. Explain in detail the "theory of sterilisation" given to explain origin and evolution of sporophyte
in bryophytes.
8. According to the theory of sterilisation, which of the bryophytic genus possesses simplest
and most primitive sporophyte?
9. In the undermentioned series of evolution of sporophyte, what is missing?
Riccio ➔ Sphoerocorpos ➔ Torgionio ➔ Morchontio ➔ Pellio ➔ Anthoceros ➔ ---------.
10. Draw labelled diagrams of the sporophytes of
(a) Anthoceros
(b) Riccio
(c) Pellio
(d) Funorio
11. With reference to the evolution of sporophyte in bryophytes, what is the theory of progressive
simplification?
12. Theory of progressive simplification is also called ------------.