In this article we will discuss about:- 1. Habit and Habitat of Lycopodium 2. External Morphology of Lycopodium 3. Internal Structure 4. Reproduction 5. Morphological Nature of Protocorm 6. Economic Importance 7. Life Cycle Patterns.
Habit and Habitat of Lycopodium:
Lycopodium is commonly known as ‘club moss’ due to their moss like appearance and club shaped strobili. It has about 400 species, which are cosmopolitan in distribution. They are found in colder arctic region as well as in temperate, tropical and sub-tropical regions but they are abundantly found in tropical zones.
Thirty three species of Lycopodium have been reported from India. Mostly it is found growing in moist and shady places which are rich in humus and other organic matters. Some of the common species are L. clavatum, L. phlegmaria, L. cernuum, etc.
It has got 2 sub-genuses:
(i) Urostachya—branching dichotomous and roots originate from the base of the stem.
(ii) Rhopalostachya—stem prostrate with erect branching and roots arise adventitiously from all along the stem.
Mostly the tropical species are epiphytic (e.g., L. phlegmaria) and grow hanging from the tree trunks. The temperate species may be erect and shruby (e.g., L. reflexum), creeping (e.g., L clavatum) or erect form (e.g., L. cernuum) etc.
External Morphology of Lycopodium:
The herbaceous plant body is sporophytic. Usually they may have either prostrate stem with erect leafy branches or weak pendent stem (epiphytes).
The plant body is distinctly differentiated into following three regions (Fig. 1 A-C):
(ii) Roots, and
In the sub-genus Urostachya stem is erect (terrestrial) or pendent (epiphytic) and may be branched (dichotomously) or unbranched. In the sub-genus Rhopalostachya the stem is prostrate with erect branches. First the branching is dichotomous and later on becomes monopodial.
Usually small, adventitious roots are present. In the sub-genus Urostachya roots originate only from the base of the stem (not arising from the whole length of the stem). In some species e.g., L. selago etc. the roots arise endogenously from pericycle of the stem, do not penetrate the cortex of the stem but turn downward through the cortex and finally emerge only at the base of the stem.
Due to this reason a T. S. of stem usually shows roots within the cortex and are known as cortical roots (inner roots). In sub-genus Rhopalostachya also roots are adventitious and arise all along the underside of the prostrate portion of the stem.
Leaves are simple, sessile, small in size, eligulate and possess a single unbranched midrib and are known as microphylls. Usually the leaves are spirally arranged (e.g., L. clavatum) but may be arranged in whorls (e.g., L. cernuum) or pairs (e.g., L. alpinum).
In all the cases they condensely cover the surface of the stem. Leaves are usually homophyllous (isophyllous) i.e., of same size and shape but in some cases e.g., in L. complanatum the leaves are heterophyllous (anisophyllous) i.e., of different size.
Usually the leaves near the apical portion of the branches bear sporangia and are called sporophylls. Depending upon the species the sporophylls may or may not be differentiated from the ordinary leaves.
These sporophylls usually form a condense structure at the apex of the branches which are known as strobili. The numbers of strobili at the tip of branches differ in different species.
Internal Structure of Lycopodium:
A transverse section (T.S.) of the stem of Lycopodium is somewhat circular in outline and can be differentiated into following three regions:
It is the outermost covering layer comprising of single cell in thickness. The epidermis is cutinised on the outer side and interrupted at places by the presence of stomata.
Inner to the epidermis is present a wide zone of cortex which shows a great variation in its structure in different species.
Usually four types of cortex are recognized:
(i) The whole of the cortex is made up of parenchymatous cells with small or large intercellular spaces (e.g., L. selago). Such cortex is called homogeneous.
(ii) The whole of the cortex is made up of sclerenchymatous cells, without intercellular spaces.
(iii) The cortex is differentiated into outer and inner sclerenchymatous cells and middle parenchymatous cells (e.g., L. clavatum, Fig. 2 A).
(iv) The cortex is differentiated into outer and inner parenchymatous cells and middle sclerenchymatous cells (e.g., L. cernuum Fig. 2. B).
Next to the cortex is present a single layer of well-defined cells known as endodermis with conspicuous casparian strips but at maturity the endodermis may or may not be a distinct structure. Endodermis is followed by pericycle which is composed of one or more layers of compactly arranged parenchymatous cells.
It is made up of only primary xylem and primary phloem. It is a protostele i.e., pith is absent and the stele is situated in the centre. The arrangement of xylem and phloem tissues is different in different species and the stele is also named differently.
In case of L. serratum, L. phlegmaria etc. the xylem is star shaped with a protoxylem situated at the periphery (protoxylem exarch Fig. 3 A). In L. annotinum in actinostele the furrows in the xylem are much more and show stellate arrangement (Fig. 3B).
The phloem lays in the space between the xylem rays. This type of stele is known as actinostele. In case of L. clavatum. L. volubile etc. xylem appears to be in the form of separate plates arranged somewhat parallel, with phloem in between them.
This type of stele is known as plectostele (Fig. 2 A, 3 C). In case of L. cernuum, L. drummondii etc. xylem and phloem are uniformly distributed i.e. it appears as if strands of xylem are embedded in the phloem. This type of stele is known as mixed protostele (Fig. 2 B, 3 D).
The protoxylem is usually exarch in all the cases. Xylem is usually composed of tracheids and phloem of sieve tubes and phloem parenchyma. Cambium is absent hence there is no secondary growth i.e., no formation of secondary xylem and secondary phloem.
The roots are adventitious.
A transverse section (T.S.) of the aerial root of Lycopodium is somewhat circular in outline and shows the following internal structure:
It is the outermost covering layer and is only one cell thick. The cells are thin walled. Epidermis is provided with numerous root hairs present in pairs (characteristic of Lycopodium).
Just below the epidermis is present a wide zone of cortex. It is differentiated into outer sclerenchyma and inner parenchyma. The outer one gives the mechanical strength to the root.
It may be di-, tetra-, or polyarch. In prostrate species it is polyarch i.e., having 6-10 plates of xylem arranged radially (star shaped). The xylem is exarch. The phloem is present between the radiating arms of xylem. In erect or pendent species stele is diarch or tetrarch. In L.selago, L. serratum it is diarch and xylem is C, U or crescent shaped. The phloem is present between the 2 ends of xylem, only in one group.
The cortical roots are exactly similar in their internal structure to that of aerial roots, except that the epidermis and root hairs are absent.
The xylem is composed of tracheid and phloem of sieve tubes and phloem parenchyma. The endodermis and pericycle are indistinct structure at maturity. Pith and cambium are absent.
T. S. of the leaf shows epidermis, mesophyll tissue and a single median vascular bundle:
It is the outermost surrounding layer and is only one cell in thickness. The cells of epidermis are parenchymatous and cutinised on their outer side. The epidermis is also interrupted by the presence of stomata. In homophyllous (isophyllous) species the stomata are present on outer as well as inner epidermis (amphistomatic) but in heterophyllous (anisophyllous) species the stomata are mostly restricted on the lower epidermis (hypostomatic).
It occupies a wide zone between the epidermis and the vascular bundle. It is usually made up of thin walled chlorenchymatous cells which may be with or without intercellular spaces.
3. Vascular bundle:
In the centre of the leaf is situated only a single concentric vascular bundle made up of only xylem and phloem. The vascular bundle is surrounded on all sides by a sclerenchymatous sheath.
Reproduction in Lycopodium:
Lycopodium reproduces by two methods vegetatively and by spores.
1. Vegetative reproduction:
It takes place by the following methods:
(i) Gemmae or bulbils:
In a few species like L. selago, L. lucidulum etc. certain buds like structures known as gemmae or bulbils are usually produced in large number on new stem tips annually. The morphological nature of gemmae is still not fully known. The gemmae when fall on ground, develop root primodia and soon form the root.
(ii) Death and decay:
Species with creeping stem reproduces vegetatively by the death and decay of older parts of the stem up to the point of branching. This separates the branches which later on grow independently.
(iii) Resting buds:
In L. inundatum the whole of the plant body except the growing tip of rhizome is dead during winter. This tip portion of the rhizome acts as resting bud which in the coming spring resumes growth and develops into a new plant.
In several epiphytic species fragments of the plant body are capable of giving rise to new plants.
Spore Producing Organs:
Lycopodium is a sporophytic plant and reproduces sexually. The plants are homosporous i.e., produces only one type of spores (without differentiation of mega- and microspores). These spores are produced in sporangia which, in turn, are produced on fertile leaves known as sporophylls. Usually the sporophylls are grouped together to form a compact structure known as strobili (Sing. strobilus) which are terminal structures (Fig. 1 A).
Strobilus (Reproductive organ):
In the primitive species of the sub-genus Urostachya every leaf on the plant is a sporophyll or at least potentially so and the fertile and sterile zones alternate. The sporophylls are loosely arranged. In species of Rhopalostachya and in some species of Urostachya the leaves of the apical portion of the branches only bear sporangia and are called sporophylls. The rest behave as vegetative leaves.
The sporophylls may be of the same size or of different size from the foliage leaves in different species. The arrangement of sporophylls is same on the central axis as that of the vegetative leaves on the stem i.e., spiral, whorled or decussate etc.
The position of the sporangium is also different in different species. The sporangia may be axillary and protected with the help of sporophylls (e.g., L. inundatum Fig. 7 A) or foliar and protected (e.g., L. cernuum Fig. 7 B) or subfoliar and exposed (e.g., L. squarrosum, Fig. 7 C) or axillary and exposed (e.g., L. lucidulum, Fig. 7 D).
Longitudinal section (L.S.) of strobilus shows the presence of a strobilus axis in the centre. On both sides of the strobilus axis are present sporophylls (Fig. 8 A). Each sporophyll bears only one sporangium (Fig. 8 B). All the sporangia are similar in structure and are arranged acropetally in a strobilus i.e., the youngest are at the top (Fig. 8 C).
Structure of Sporangium:
Sporangia are sac-like structures but usually kidney shaped in appearance (Fig. 8 B). Sometimes they are sub-spherical in appearance. Their colour varies from orange to yellow. Each sporangium consists of a basal short massive stalk i.e., sub-sessile, with an upper globular unilocular body containing numerous spores.
The body of the sporangium consists of 3 or more layers of wall surrounding a cavity. The inner most layer of the wall of sporangium is called as tapetum (Fig. 9 F) which is nutritive in nature and persists till maturity.
In the young sporangium inside the wall is present a mass of sporogenous cells which in due course of development form spore mother cells which by meiotic divisions, produce haploid tetrad of spores. The spores at maturity separate from each other.
The wall of the sporangium is provided with a transverse strip of cells known as stomium from where the sporangium at maturity splits into 2 valves and the spores are dispersed away in the air.
The spores produced by a sporangium are all alike (homosporous). They are small, rounded or even spherical structures. The surface of the spores is usually rough due to the presence of reticulate ridges or knob like protrusions. Each spore is provided with a triradiate ridge (Fig. 8, D, E) and is somewhat yellow in colour. A small amount of chlorophyll may or may not be present in spores. Reserve food is in the form of oil in the spores.
Development of sporangium and formation of spores. Bower (1894) had studied the development of sporangium in Lycopodium. The sporangium develops from a small group of superficial cells arranged in a transverse row on the adaxial side of the sporophyll near the base.
Its development is of eusporangiate type. These superficial cells are called sporangial initials (Fig. 9A, B). These cells divide by periclinal divisions forming an outer and inner layer of cells. The outer cells divide periclinally and anticlinally forming three celled thick wall of the sporangium (Fig. 9A-F).
The inner layer or archesporial cells divide in all directions forming a group of cells known as sporogenous tissue which finally give rise to spore mother cells. During these developments the inner-most layer of wall is differentiated as a nutritive layer and is known as tapetum. It is a persistent structure and rich in reserve food material.
Each spore mother cell undergoes a process of meiosis thus producing a tetrad of spores (haploid) with tetrahedral arrangement. These spores later on separate from the tetrad, as a result of which, a large number of spores are produced inside each mature sporangium.
Dehiscence of sporangium and liberation of spores. As the sporangium approaches towards maturity, a transverse row of cells is differentiated near the apical portion from the wall of a sporangium known as stomium.
The walls of the cell of stomium thicken and differ from the walls of other cells of the sporangium. As the sporangium loses water, it creates a pressure on the wall which leads to the appearance of slit in the stomium as a result of which the wall splits opens into two halves and the spores are disseminated by air current.
The development of the gametophyte (prothallus) takes place from the haploid spores which are the unit of gametophytic generation. Each spore is unicellular, uninucleate haploid structure, 0.03 mm in diameter and surrounded by 2 layers, with a triradiate ridge at the surface (Fig. 8 D, E).
Chlorophyll may or may not be present in different species. In few species spores may germinate within a few days after liberation but in some species the spores germinate when they are 3-8 years old and the development of gametophyte upto formation of mature sex organs may take a time of 8 months to 6 or even 15 years.
The rate of the formation of photosynthetic tissue is usually proportional to the rate of growth of gametophyte. Both the male and female reproductive organs are produced on the same gametophyte. The male sex organs are produced earlier than female sex organs.
Usually at the time of germination of spore, it swells up to absorb water. First the spore divides into two unequal cells by a lenticular division, forming a very small lens shaped cell known as rhizoidal cell and a bigger cell (Fig. 10 A, B).
This rhizoidal cell takes no part in further development of gametophyte and is a colourless structure. At this two celled stage the spore will rupture at the triradiate ridge. Second division divides the bigger cell into two equal halves, the cell near the rhizoidal cell is known as basal cell and the other one is known as upper cell (Fig. 10 C).
The upper cell further divides by two successive divisions in such a way as to form an apical cell with two cutting faces (Fig. 10 D). At this stage the gametophyte is 5 celled structures and the symbiotic phycomycetous fungus (mycorrhizal fungus) attacks it.
If this fungus fails to attack at this stage, further development of gametophyte stops. This infection takes place through the basal cell. During further course of development of gametophyte the apical cell further divides to form six or morecells which later on develop into meristematic cells. These cells, by further divisions form a multicellular structure, the gametophyte (prothallus) (Fig 10 E-H).
Structure of the Mature Gametophytes:
The form and structure of the gametophytes varies greatly in different species.
In general they have been grouped under three categories:
Type I or Cernuum type:
Gametophyte is partially aerial and partly in soil. The prothalli are usually 2 to 3 millimetre in height and 1-2 millimetre in diameter. The gametophytes (prothalli) grow at the surface of the ground and consist of a colourless basal portion buried in soil and a conspicuous upright, fleshy, green aerial portion having lobes (Fig. 11 A).
The sex organs develop between the green expanding lobes. The prothallus itself is a nourishing body. The underground part contains endophytic fungus e.g., L. cernuum, L. inundatum etc.
Type II or Clavatum Type:
The gametophyte is wholly subterranean and totally saprophytic i.e., non- green structure. It is tuberous and without lobes. It may be one to two centimentre long or wide and is top shaped, conical or discoid in shape (Fig. 11 B, C). The endophytic fungus is present. Sex organs are formed on the upper surface e.g. L. annotinum, L. complanatum, L. clavatum etc.
Type III or Phlegmaria type:
The gametophyte is subterranean, saprophytic and colourless. This type of prothallus is seen in L. phlegmaria and other epiphytic species. The prothallus is about 2 millimeter in diameter and monopodially branched (Fig. 11 D). Sex organs are borne on upper surface of large branches and are interspersed with slender filaments known as paraphyses.
Besides these three forms some intermediate forms of prothalli are also observed. In L. selago the prothalli may be subterranean or epiterranean (aerial). If the spores are buried under the soil after liberation, they form subterranean prothalli and if the spores are not buried under soil after their liberation, they form epiterranean prothalli.
The internal structure of the prothallus is very simple. The outermost layer is epidermis, followed by cortical mycorrhizal region, palisade region and central storage region. It is attached with the substratum by unicellular rhizoids. The prothalli of all species are monoecious i.e., antheridia and archegonia develop on the same prothallus.
Development of sex organs:
Both the sex organs i.e., antheridia (male) and archegonia (female) develop on the same prothallus, usually in distinct patches on the upper surface. The gametophytes are protandrous i.e., antheridia develop before archegonia. Sex organs develop just on the back of the apical meristem.
Development of antheridium:
A single superficial cell situated just away from the meristematic cells gives rise to an antheridium. This superficial cell is known as antheridial initial (Fig. 12 A). This cell divides periclinally to form an outer cell known as jacket initial (primary wall cell) and an inner cell known as primary androgonial initial or cell (Fig. 12 B).
The jacket initial divides only anticlinally by several divisions resulting in the formation of single layered covering known as jacket layer. In the middle of the jacket layer a triangular cell is differentiated, which is known as opercular cell.
Simultaneously, the primary androgonial divides by various divisions, forming a mass of cells embedded in the prothallus, known as androgonial cells which finally give rise to androcytes (antherozoid mother cells, Fig. 12 C-F). The number of androcytes per antheridium varies in different species.
Each androcyte later on metamorphosis into a biflagellated antherozoid. Each antherozoid is a haploid, uninucleate, fusiform structure with broad rounded posterior end and an upper pointed biflagellated anterior end (Fig- 12 G).
The triangular opercular cell becomes mucilaginous as a result of which an opening is formed at the apex of antheridium through which water enters into it. The antherozoids absorb water and swell up as a result of which a pressure is created on the wall of antheridium which finally ruptures and the antherozoids are liberated.
Development of archegonium:
Just like antheridium, the archegonium also arises from a single superficial cell called archegonial initial, situated just away from the meristematic cells at the apex (Fig. 13 A). The archegonial initial divides by periclinal division into an upper primary cover cell and lower basal central cell (Fig. 13 B).
The primary cover cell later on divides vertically by two successive divisions at right angle to each other forming four neck initials which later on by transverse divisions form a 3-4 cells high neck. Each tier of the neck consists of 4 cells.
The central cell divides transversely forming an, upper primary canal cell and a lower primary ventral cell (Fig. 13 D). The primary canal cell by successive transverse divisions produces a variable number of neck canal cells (usually one in L. cernuum, seven in, L. selago and 14-16 in L. complanatum).
The primary ventral cell may directly behave as an egg or may divide transversely to form an upper ventral canal cell and a lower egg (Fig. 13 E-G). The egg is somewhat broader then the rest part of archegonium. The archegonial jacket is absent. The archegonium is a sunken flask shaped structure with neck projecting out of the prothallus.
At the time of fertilization the neck canal cells and the ventral canal cell disorganise and the cells of the upper-most tier of neck slightly separate apart forming a passage upto the egg (Fig. 13 H). Fertilization is brought about in the presence of water.
The biflagellate antherozoids reach the archegonium by swimming in water on the surface of prothallus. The antherozoids are perhaps attracted towards the neck of archegonium by a chemotactic movement. They enter the archegonium through neck and reach the egg.
Only the nucleus of one antherozoid fuses with the egg nucleus thus forming a diploid structure-known as oospore (2x). The act of fertilization ends the gametophytic generation and the initial stage of sporophytic generation is formed.
Embryo Development (Young Sporophyte):
The rate of development of the embryo is extremely slow. In Lycopodium embryo develops downward into the gametophytic tissue instead of developing upward i.e., towards the neck of archegonium. The first division of the oospore is always transverse, forming an upper cell (epibasal) and a lower cell (hypobasal) known as embryonic cell.
The upper cell does not divide further and behaves as suspensor. The lower cell (embryonic cell) divides by two vertical divisions at right angle to each other, followed by a transverse division, forming 8 cells (octant, Fig. 14 A-D). The 4 cells of the octant, situated near the suspensor by further division, form a multicellular foot which acts as a haustorium and helps in the absorption of food material from the gametophytic tissue.
Out of the 4 remaining cells of the octant, the 2 cells towards the meristematic region give rise to stem and the other 2 cells give rise to primary leaf and primary root (Fig. 14 D-J). The primary stem is short lived and is replaced by adventitious outgrowth which gives rise to horizontal stem. More roots develop from the stem.
The primary roots of the sporophyte are exogenous in origin while those arising later on are endogenous in origin. The embryo obtains its nourishment for a long time from the gametophyte.
In some species e.g., L. cernuum etc. the gametophyte is generally green. The oospore normally divides transversely forming suspensor and embryonic cell. The embryonic cell forms an octant. The tier which gives rise to stem, leaf and primary roots, develops into a massive spherical structure of parenchymatous cells, known as protocorm (Fig. 14 K, L).
It grows through the gametophyte, becomes green and develops rhizoids on its lower surface. The upper surface of the protocorm gives rise to a few to many erect outgrowths which are leaf like and are known as protophylls.
The protophylls are provided with stomata. At this stage the protocorm separates from the gametophyte. Now at the upper side of protocorm a region is differentiated which develops into stem. Protocorm is regarded as the intermediate phase in between normal embryo and definite leafy shoot.
Morphological Nature of Protocorm of Lycopodium:
Various views have been put forward to explain the morphological nature of protocorm of Lycopodium.
A few important ones are discussed below:
(1) Treub (1837) regarded the protocrorm as the remains of primitive undifferentiated structure originally possessed by the Pteridophytes and in majority of the present day Pteridophytes it has been replaced by a definite leafy shoot. This view is now only of historical importance.
(2) Bower (1908, 1935) regarded it as a swelling of occasional adaptation. It acts as an organ of perennation. It has no phylogenetic importance.
(3) Holloway (1910) regarded it as a specialised structure that helps the young sporophyte to perennate over dry season.
(4) Browne (1913) regarded it as a modified and a reduced stem.
(5) Wardlaw (1955) regarded it as a modified shoot.
Economic Importance of Lycopodium:
Different species of Lycopodium are differently important as for example, some species of Lycopodium (L. obscurum) are used in making Christmas wreaths. L. volubile is used for table decoration.
Extract from the plant of Lycopodium was used as kidney stimulant in the old times. The spores of Lycopodium are highly inflammable and have been used to produce stage lighting, for the theatres. The spores of L. clavatum etc. are used in pharmacy as water repellent protective dusting powder for tender skin etc.
Life Cycle Patterns in Lycopodium:
Lycopodium is a sporophyte (2x) with distinct sporophytic (2x) and gametophytic (x) generations which alternate with each other. The plant is homosporous i.e., reproduces by producing only one type of spores. The spore on germination produces gametophyte (x) which, in turn, produces both antherozoids and eggs in antheridia and archegonia respectively.
These reproductive structures later on after fertilization produce zygote (2x) which again on germination gives rise to a sporophytic plant (2x). In this way sporophytic and gametophytic generations alternate with each other and it shows a distinct alternation of generation although the sporophytic phase is dominant over gametophytic phase (Fig. 15).