Ans.
Pseudopodium:-
> In Sphagnum sporogonium is elevated on a short cylindrical, leafless stalk, the pseudopodium.
> The pseudopodium is gametophytic and it is formed by the post fertilization intercalary growth of the axis of the archegonial branch.
> The pseudopodium together with the basal portion of the calyptra forms a sac-like structure encloses the foot.
> The main functions of the pseudopodium are to elevate the capsule for above the perichaetial leaves, to compensate the suppression of seta and to help in the dispersal of spores.
Ans.
Gemmae:- Gemmae are green, multicellular reproductive bodies of various shapes. These are produced in gemma cups, on the surface of the leaves, on stem apex or even inside the cells. They get detached from the parent plant and after falling on a suitable substratum gemmae give rise to a new individual directly (e.g., Marchantia) or indirectly (e.g., Mosses).
Ans.
Apogamy:- When the embryo develops from the antipodal cell or the synergid, it is called apogamy. As shown in the diagram below. Example:- Onion
Apospory:- When the development of the embryo sac occurs without meiosis from any vegetative cell of ovule (integument or nucellus or challaza), it is called as apospory. It means megaspore is not produced here. Each cell of the embryo sac is diploid. An embryo is developed from any one cell of the embryo sac. Example:- Crepis
Ans.
Stele:- The central cylinder or core of vascular tissue, consisting of xylem, phloem, pericycle and sometimes medullary rays and pith, is technically called stele.
> Van Tieghem and Douliot (1886) developed the stele theory. They used the term stele in collective sense and mentioned that the stele is not only made up of xylem and phloem, but the tissue like pericycle, vascular rays and pith are also associated with it.
Types of stele:- 7 types of steles are found in the whole plant kingdom -
i. Protostele
ii. Siphonostele
iii. Solenostele
iv. Dictyostele
v. Polycyclic stele
vi. Eustele
vii. Atactostele
Ans.
Important Features of Lycopsida:-
i. The plant body is differentiated into well-defined roots, stem and leaves.
ii. The leaves are small, that is they are microphyllous.
iii. The branching is dichotomous.
iv. The sporangia are present in the axil of sporophylls.
v. The sporophylls are arranged as strobili.
vi. The sporophyte is either homosporous as in Lycopodium or heterosporous as in Selaginella.
vii. The gametophyte develops independently.
viii. Some of the examples include Selaginella and Lycopodium.
Ans.
Differences between Pteridophytes and Gymnosperms:-
i. The sporophytic plant body is by and large arborescent in gymnosperms while it is not so in pteridophytes.
ii. Roots are adventitious in pteridophytes while they arise from the radicle (tap root) in gymnosperms.
iii. Pteridophytes may be homosporous or heterospoers, while all gymnosperms are heterosporous.
iv. Most of the pteridophytes have a preference for cool, moist areas, while gymnosperms generally exhibit xeric characters.
v. Branching of the stem is dichotomous in pteridophytes while it is lateral in gymnosperms.
vi. Heterospory has never resulted in dioeciousness in the sporophyte in pteridophytes, while it is so in many gymnosperms.
vii. Generally secondary growth is absent in pteridophytes, while it is present in gymnosperms.
viii. In pteridophytes both microspores and megaspores are released from their respective sporangia, whereas in gymnosperms, megaspore is permanently retained.
ix. There is pollination in gymnosperms, while it is absent in pteridophytes.
x. Siphonogamous fertilization (pollen tube development) is seen in only gymnosperms.
xi. Male gametes are ciliate in pteridophytes, while they are not in gymnosperms (except for few like cycas).
xii. Gymnosperms are seed plants (spermatophytes), while there is no seed in pteridophytes.
Ans.
Resemblance of Bryophytes with Algae (Primitive features):-
i. Plant body simple, thalloid and gametophytic.
ii. Autotrophic.
iii. Gametophytic phase is dominant.
iv. Roots are absent.
v. Cell wall is made up of cellulose.
vi. Pigments (chlorophyll a, chlorophyll b, α and β carotene, Lutin, Violaxanthes and Xeoxanthin) are similar in chloroplast.
vii. Vascular tissue is absent.
viii. Antherozoids are motile (bi-flagellated).
ix. Flagella are whiplash type.
x. Water is essential for fertilization.
xi. A filamentous protonema is produced by Bryophytes (juvenile stage in mosses) which resembles with the filamentous green algae.
xii. In order Anthocerotales of Bryophytes, plastids are with pyrenoids which is a characteristic of Chlorophyceae (Green algae).
Resemblance of Bryophytes with Pteridophytes (Advanced features):-
i. Plants are terrestrial.
ii. Primitive simple leafless and rootless sporophytes of Pteridophytes (members of order Psilophytales) can be compared with the sporophytes of Bryophytes.
iii. Sexual reproduction is oogamous.
iv. Androcytes are enclosed by sterile jacket layer.
v. Antherozoids are flagellated.
vi. Water is essential for fertilization.
vii. Permanent retention of zygote within the archegonium.
viii. Zygote forms the embryo.
ix. Moss capsule is similar to terminal sporangium and columella of Psilophytales.
x. Both Bryophytes and Pteridophytes are characterised by heteromorphic alternation, of generation.
Ans.
Study of gametophyte (युग्मकोदभिद का अध्ययन):-
> It resembles to the Calobryum.
(यह Calobryum जैसा दिखता है।)
> The gametophores about 1 - 1.5 cm. tall.
(इसका गैमेटोफ़ोर लगभग 1 - 1.5 सेमी. लंबा होता है।)
> 3 rows of appendages are arranged spirally on main axis. These appendages are homologous to leaves, so called as phyllids. Each phyllid consist of 2 cells.
(उपांगों की 3 पंक्तियाँ मुख्य अक्ष पर सर्पिल रूप से व्यवस्थित होती हैं। ये उपांग पत्तियों के समरूप होते हैं, जिन्हें फाइलिड्स कहा जाता है। प्रत्येक फ़ाइलिड में 2 कोशिकाएँ होती हैं।)
> Phyllids are of 2 types:-
(फाइलिड्स 2 प्रकार के होते हैं:-)
i. Non - beaked phyllids (चोंच रहित फाइलिड्स):-
ii. Beaked phyllids (चोंच युक्त फाइलिड्स):- They secrete slime or mucilage which is protective in function from desiccation or dryness.
(ये श्लेष्मा स्रावित करते हैं जो शुष्कन या सूखने से सुरक्षा करने का कार्य करता है।)
> Plant body is rhizomatous just like Calobryum.
(पादप शरीर Calobryum की तरह ही प्रकंदमय होता है।)
> Both the rhizome and the leafl axis are completely devoid of rhizoids.
(प्रकंद और पर्ण अक्ष दोनों पूरी तरह से मूलाभासों से रहित होते हैं।)
> Anatomy of leaf (पर्ण की आंतरिक संरचना):-
- The leaf is 3-5 cells thick except at tip region.
(पत्ती शीर्ष क्षेत्र को छोड़कर 3-5 कोशिका मोटी होती है।)
- They gradually taper towards the apex ending in short bluntly conical cell.
(ये धीरे-धीरे शीर्ष की ओर सिकुड़ते हुए छोटी कुंद शंक्वाकार कोशिका में समाप्त होती हैं।)
- The cells are parenchymatous and contain chloroplasts.
(कोशिकाएँ मृदूतकीय होती हैं और इनमें क्लोरोप्लास्ट होते हैं।)
> Anatomy of stem (तने की आंतरिक संरचना):-
- The transvers section of Takakia stem showed that it consists of two zones, outer cortical region surrounding the inner medullary region.
(Takakia तने के अनुप्रस्थ खंड से पता चला कि इसमें दो क्षेत्र होते हैं, बाहरी वल्कुट क्षेत्र जो आंतरिक मज्जा क्षेत्र को घेरे रहता है।)
- The cortical region is 1-2 stratose thick and consists of slightly to strongly thick - walled cortical cells with brownish wall. This region is chlorophyllose.
(कॉर्टिकल क्षेत्र 1-2 कोशिका मोटा होता है और इसमें भूरे रंग की भित्ति के साथ थोड़ी से अधिक मोटी भित्ति वाली वल्कुट कोशिकाएं होती हैं। यह क्षेत्र क्लोरोप्लास्ट युक्त होता है।)
- The medullary region is made up of conducting strands.
(मज्जा क्षेत्र संवहन तंतुओं से बना होता है।)
> Takakia is a xerophyte. Due to physiological dryness its length is restricted to 1 to 1.5cm.
(Takakia एक मरुदभिद पादप है। कार्यीकीय शुष्कता के कारण इसकी लंबाई 1 से 1.5 सेमी तक सीमित होती है।)
Note (नोट):- Physiological dryness (कार्यीकीय शुष्कता):- If water is present but cannot be used by the plant due to its variable from as snow or water vapours. Eg.- Alpine zones.
(यदि जल उपस्थित है, लेकिन बर्फ या जल वाष्प के रूप में परिवर्तनशील होने के कारण पौधे द्वारा इसका उपयोग नहीं किया जा सकता है। जैसे- एल्पाइन क्षेत्र।)
Study of sporophyte (बीजाणुदभिद का अध्ययन):-
> Fertile plants bearing antheridia and sporophytes were first reported in 1993 from the Aleutian Islands, and both structures were clearly of the form found in primitive mosses. This discovery established Takakia as a genus of moss, albeit an unusual one.
(पुंधानियों व बीजाणुदभिद धारण किए हुए उर्वर पौधों को पहली बार 1993 में एल्यूटीयन द्वीप समूह से रिपोर्ट किया गया था, और दोनों संरचनाएं स्पष्ट रूप से आदिम मौस में पाए जाने वाले रूप की थीं। इस खोज ने Takakia को मौस के एक वंश के रूप में स्थापित किया, यद्यपि यह एक असामान्य वंश था।)
> The sporophyte develops a long stalk ending in an elongated spore capsule.
(स्पोरोफाइट एक लंबा डंठल विकसित करता है जो एक लंबे बीजाणु कैप्सूल में समाप्त होता है।)
> The capsule contains a central columella over and around which the spores are produced.
(कैप्सूल में एक केंद्रीय कोलुमेला होता है जिसके ऊपर और चारों ओर बीजाणु उत्पन्न होते हैं।)
> When the sporophyte is mature, the capsule ruptures along a single spiral slit to release the spores.
(जब बीजाणुदभिद परिपक्व हो जाता है, तो कैप्सूल बीजाणुओं को मुक्त करने के लिए एक सर्पिल दरार के साथ टूट जाता है।)
Ans.
Bryophytes as Pollution Indicator:-
Geobotany (Phytogeography):- The branch of biogeography that is concerned with the geographic distribution of plant species. It is the scientific study of the distribution of plants. Climate is considered the primary control on plant life, but within a particular climatic region the rock beneath soil—known as the parent material of soil—is typically the key factor influencing the vegetation growing above.
Distribution of Bryophytes:-
> Bryophytes are represented by 960 genera and 24,000 species. They are cosmopolitan in distribution and are found growing both in the temperate and tropical regions of the world at an altitude of 4000-8000 feet.
> In India, Bryophytes are quite abundant in both Nilgiri hills and Himalayas; Kullu, Manali, Shimla, Darjeeling, Dalhousie and Garhwal are some of the hilly regions which also have a luxuriant growth of Bryophytes. Eastern Himalayas have the richest in bryophytic flora. A few species of Riccia, Marchantia and Funaria occur in the plains of U.P., M.P. Rajasthan, Gujarat and South India.
> In hills they grow during the summer or rainy season. Winter is the rest period. In the plains the rest period is summer, whereas active growth takes place during the winter and the rainy season. Some Bryophytes have also been recorded from different geological eras e.g., Muscites yallourensis (Coenozoic era), Intia vermicularies, Marchantia spp. (Palaeozoic era) etc.
Ans.
Evolution of stele:-
Stele:- The central cylinder or core of vascular tissue, consisting of xylem, phloem, pericycle and sometimes medullary rays and pith, is technically called stele.
Stelar theory:-
> Van Tieghem and Douliot (1886) developed this theory.
> They used the term stele in collective sense and mentioned that the stele is not only made up of xylem and phloem, but the tissue like pericycle, vascular rays and pith are also associated with it.
> According to them the cortex and the stele are the fundamental parts of a shoot and both these parts are separated from each other by the endodermis.
> According to stellar theory, primarily there is no fundamental difference in the gross anatomy of stem and roots, because in both of them a stele is surrounded by the cortex is present.
> Foster and Gifford (1959) have mentioned that the most debated and controversial aspect of “stellar theory is the nature of the anatomical boundaries which separate the cortex from the stele”.
> According to Van Tieghem and Douliot (1886) the endodermis represents the inner boundary of the cortex. The cells of the endodermal layer have the characteristic casparian strip strips.
> But in the stems of many seed plants, the characteristic endodermal layer is not present. Some have mentioned that in such cases the pericycle serves as separating layer between the stele and the cortex.
Types of stele:- 7 types of steles are found in the whole plant kingdom -
1. Protostele
2. Siphonostele
3. Solenostele
4. Dictyostele
5. Polycyclic stele
6. Eustele
7. Atactostele
Note:- Out of these the first 5 types of steles are found only in the stem of pteridophytes. Eusteel is found in the stem of gymnosperms and dicots. Atactostele is found in the stem of monocots.
Note:- The development of stele has also happened in this sequence. Atactosteel is considered to be the most advanced type of stele.
1. Protostele:- This is the most primitive type of stele in which the pith is completely absent and the phloem surrounds the xylem. According to Jeffrey (1898), the protostele is primitive type of stele in vascular plants.
On the basis of the shape of the xylem in the cross section, it is of 4 types -
i. Haplostele:- The xylem appears circular in cross section. It has been observed in fossil genera like Rhynia, Horneophyton and living genera like Selaginella chrysocaulos, S. kraussiana, S. selaginoides, S. willdenowi, Gleichenia dichotoma, Lygodium and Cheiropleuria.
ii. Actinostele:- The xylem appears star shaped in cross section. Brebner (1902) named such a protostele as actinostele. Eg.- Asteroxylon, Psilotum, Lycopodium serratum and Sphenophyllum.
iii. Plectostele:- In cross section, the xylem appears as parallel plates. Eg.- Lycopodium clavatum and L. volubile.
iv. Mixed protostele:- In cross section, the xylem is seen scattered in the form of segments. Eg.- Lycopodium cernuum
2. Siphonostele:- In this, pith or pith cavity is found in the center around which vascular tissues are found.
Origin of siphonostele:- There are different views among scientists regarding the origin of siphonostele. But they all agreed that the siphonostele originated from protostele by the formation of a pith in the center. Here, the centrally placed xylem core is replaced with a parenchymatous pith.
Different stages of changing protostele to siphonostele can be observed in the T.S. at different levels in Gleichenia, Osmunda, and Anemia.
There are two views regarding the origin of pith in the siphonostele.
i. Jeffrey’s theory:-
- According to this theory, the pith is formed as a result of the invasion of cortical parenchymatous cells into the stele. The invasion of the pith occurs through the leaf gap or branch gap.
- Thus pith and cortex are homogeneous structures according to this theory.
- This theory is not accepted by most of the authors since in many pteridophytes there is a stele without leaf gaps but having siphonostele.
ii. Boodle’s theory:- According to the theory proposed by Boodle (1901), and Gwynne Vaughan, the siphonostele has evolved from the protostele by a transformation of the inner vascular tissue into the parenchyma.
On the basis of the location of phloem it is of two types –
i. Ectophloic siphonostele:- Phloem is found around the xylem. Eg.- Osmunda, Schizaea, etc.
ii. Amphiphloic siphonostele:- Phloem is found at both outside and inside the xylem. Eg. Rhizome of Marsilea, Adiantum, Dipteris.
3. Solenostele:- The vascular tissue in the ectophloic siphonostele forms a leaf gap when it enters the leaf as a leaf trace.
4. Dictyostele:- When the leaves are found densely, many leaf gaps come together causing the vascular tissue of the amphipholic siphonostele to break into many pieces. Each piece is called a meristele and each meristele is a haplostele. In this way many merysteles are found in the form of a circle.
5. Polycyclic stele:- When 2 or more cycles of meristeles are found.
6. Eustele:- When vascular bundles are found in a circle.
7. Atactostele:- When vascular bundles remain scattered throughout the ground tissue.
Ans.
Salvinia:-
1. Systematics:-
Division:- Pterophyta
Class:- Leptosporangiopsida
Order:- Salviniales
Family:- Salviniaceae
Genus:- Salvinia
2. Habit and Habitat:-
> The genus Salvinia has 12 species most of which are found to be distributed in the African continent.
> The genus is represented in India by two species namely S. natans and S. auriculata. S. natans is found in Kashmir.
> Of the several species of Salvinia, S. natans is the only annual.
3. Study of Sporophyte:-
> The sporophytic plant body consists of a long slender floating rhizome which grows horizontally.
> The rhizome is completely covered by whorls of leaves.
> The plant body generally exhibits radial symmetry. But according to Loyal and Grewal (1966), in S. auriculata, the stem exhibits bilateral symmetry at intermodal region, while it is radial at the nodal region.
a. Stem:-
- The stem is divided into nodes and internodes and a branch initial is always produced at every node.
- But the stem is sparsely branched because of the non-functioning of many of the branch initials.
b. Root:-
- There are no roots in the sporophyte. Absorption is mainly carried out by the general surface.
- It is suggested that the hair like structures found on the submerged filiform leaves may help in the absorption of water and nutrients.
- These hairs however are different from root hairs in being multicellular.
c. Leaves:-
- Leaves are oblong or sometimes hemispherical.
- They usually arise in groups of three.
- Of the three leaves, two lateral ones float on the surface of water while the third one is submerged.
- There is a distinct morphological differences between the submerged and floating leaves.
- The submerged leaves are highly dissected, long and filiform. They very much resemble the roots. But is it not certain whether they also function like roots.
- It is very often suggested that the function of fill form leaves is:
i. To protect the sporocarp.
ii. To act as a ballast or a stabilizer in preventing undue drift of the plant.
- The floating leaves are green and ovate or oblong. They are covered with stiff hairs that prevent wetting. Besides the hairs, papillate projections are also found.
- The members of the leaf whorl alternate at successive nodes, so that there are actually six rows of leaves though the arrangement apparently suggests three rows.
4. Reproduction:-
> The sporophyte reproduces by means of sporangia.
> As in Marselia, the sporangia are aggregated into sporocarps.
> The details of reproduction have been studied extensively in S. natans.
> Sporocarp:-
- The sporocraps appear in clusters ranging in number from 4 to 20.
- The sporocarps are usually borne on the inner segments of submerged leaves.
- In arrangement, the sporocarps are sympodial.
- The shape of the sporocarp varies from globose to ovoid, occasionally flattened also with ridged surfaces.
- The wall of the sporocarp is made up of two layers of cells. The outermost layer when young is clad with hairs.
- Externally all sporocarps are alike. The sporocarps are always bisporangiate.
- Each sporocarp contain many sori.
- The sporocarp at its base has a stout columnar receptacle which carries the vascular supply into the sporocarp.
- The receptacle is un-branched in the sporocarps of S. natans.
> Internal structure of sporocarp:-
- A sectional view of mature sporocarp shows a well-developed wall enclosing the, sporangia.
- Each sprocarp has 4 valves.
> Sporangia:-
i. Microspoangia:-
- The micro-sporangium consists of a short stalk and a generally ovoid capsule.
- Enclosed in the sporangium are 16 microspore mother cells and consequently 64 microspores are formed.
- All the spores attain maturity and survive.
- The microspores are embedded in the cytoplasmic fluid derived from the tapetum.
- As the spores mature, the tapetal cytoplasm encompassing the spores gets hardened and forms a mass called ‘massula’ or glochidium.
- The microspores are extremely small, triradiate in appearance and have two wall layers.
- There is a single nucleus in each microspore.
- Occasionally the number of microspores per microsporangia might vary. In S. auriculata, according to Loyal and Grewal (1966), there are only 8 microspore mother cells and consequently only 32 microspores are formed.
ii. Megasporangia:-
- The mega-sporangium consists of a short stalk and a generally ovoid capsule.
- The capsule is made up-of a single layer of cells. Internal to the wall of the capsule is a layer of tapetum.
- Enclosed by the tapetum are found 8 mega-sporocytes which divide meiotically to produce 32 megaspores. Of these spores, all except one degenerate.
> Gametophyte:-
- Two types of gametophytes may be expected since Salvinia is heterosporous.
- In both micro and mega gametophytes the development is endosporic.
- The gametophytes begin their development much earlier to the dehiscence of the sporangium.
i. Male gametophyte:-
- The microsporangium does not dehisce, hence the microspores develop within the microsporangium.
- At the first sign of development, the microspore gets divided into three cells. Of these, the lowest divides slightly obliquely to form a prothallial cell.
- Further divisions are restricted only to the upper two cells.
- From these two cells, by successive divisions, two spermatogenous cells and four sterile cells are formed.
- The four sterile cells constitute the jacket cells.
- The two spermatogenous cells by further divisions, form eight spermatocytes which appear in two clusters separated by means of a sterile cell.
- When the gametophytes are mature the sterile cells (Jacket cells) disintegrate, releasing the multi-flagellate spermatozoids which develop from the spermatocytes.
ii. Female gametophyte:-
- The megaspore generally floats on the surface of water in a prone position.
- Prior to the first division, the nucleus migrates to the apical portion of megaspore.
- First division of the nucleus results in forming two unequal cells.
- The upper cell is small and lenticular while the lower cell is very large and is filled with plenty of nutrition.
- Most of the gametophyte is derived from the upper lenticular cell, while in the lower large cell there are only free nuclear divisions.
- This multinucleate cell functions as a store house of reserve food material.
- The upper lenticular cell by means of several divisions forms a lobed apical cushion which comes out penetrating the spore wall and perispore.
- Archegonia appear on this apical cushion.
- Generally, two to four archegonia develop in the apical cushion.
- The archegonia are deeply sunk in the apical cushion. They have a very short neck, an egg cell, a venter canal cell and a two nucleate neck canal cell.
> Fertilization:-
- The female gametophyte at maturity exposes the archegonia.
- In the archegonium, the neck canal cell and venter canal cell disorganize to facilitate the entry of the sperms.
- A large number of sperms enter into the neck of the archegonium while only one succeeds in fusing with the egg.
- The resultant zygote develops into embryo.
> Embryogeny:-
- The first division of the zygote is longitudinal i.e., parallel to the long axis of the neck of the archegonium the next division is transverse resulting in the formation of a quadrant.
- Vertical division of the quadrant cell results in the octant stage.
- The basal cells of the octant form a foot which is haustorical in nature.
- Of the remaining upper four cells of the octant, the two anterior ones-develop into the first leaves.
- Of the remaining two cells, one gives rise to the stem apex while the other is functionless.
- There are no roots in Salvinia.
- Further divisions from the octant stage onwards are bit difficult to follow.
- From the octant cells, a cell plate is formed, which functioning like a column connects the foot with the leaf and stem apex.
- According to some workers the foot plus the column represents a vestigial root.
- The leaf segment enlarges and grows into a cordate leaf.
- The stem apex divides actively and forms the rhizome.
5. Phylogeny:-
> Phylogeny:- The history of the evolution of a species or group, especially in reference to lines of descent and relationships among broad groups of organisms.
> Of the three members of aquatic ferns Salvinia is closer to Azolla than Marselia. While Marselia may be called terrestrial or semiaquatic, Salvinia is purely aquatic.
> This aquatic environment has certainly stamped its impressions on the bodily features of Salvinia.
> The absence of roots, highly dissected filiform leaves and poor development of vasculature are all signs of a hydrophytic environment that houses the plant body.
> In reproductive structures too Salvania is closer to Azolla.
> Ontogenetic studies clearly reveal that in the group of aquatic ferns a bisporangiate sporocarp is probably more primative than a mono-sporangiate one.
Ans.
Study of sporophyte:-
> Each plant consists of an aerial portion and a rhizome, or subterranean region, but has no roots.
> The plant possesses no true leaves, what appear to be leaves are flattened stems.
> The fronds emerge directly from the fibrous root-mats which clad the trunks of mature tree ferns such as Dicksonia and Cyathea.
> Aerial part varied from 3 to 8 inches in length.
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Heterospory:- The phenomenon of the development of two types of spores (differing in size, structure and function) by the same species is known as heterospory.
> The two different sizes are smaller spores also known as microspores and the larger spores also known as megaspores.
> Rashid (1999) listed nine genera as living examples of heterosporous pteridophytes. e.g. Selaginella, Isoetes, Stylites, Marsilea, Pilularia, Regnellidium, Salvinia, Azolla and Platyzoma.
Origin of Heterospory:- The origin of heterospory can be better discussed on the basis of evidences from paleobotany, developmental and experimental studies.
1. Palaeobotanical evidences:-
- It has been suggested that heterospory arose due to degeneration of some spores in a few sporangia. As more nutrition becomes available to less number of spores, the surviving spore grow better, hence increase in their size.
- Palaeobotanical evidences show that the earlier vascular plants were all homosporous and the heterosporous condition appeared subsequently in the lowermost upper Devonian.
- A number of heterosporous genera belonging to the Lycopsida, Sphenopsida and Pteropsida were known in the late Devonian and early Carboniferous periods.
2. Evidences from Experimental Studies:-
- Experimental studies on Selaginella (Goebel, 1905) and Marsilea (Shattuck, 1910) suggest that nutritional factors mainly govern the heterospory.
- Under conditions of low light intensity, the photosynthetic activity of Selaginella was retarted and it produced microsporangia.
- By sudden lowering of the temperature, the size of the microspores in the sporocarp of Marsilea increases by six times.
3. Evidences from Developmental Studies:-
- In heterosporous pteridophytes, the development of micro and megasporangia follow the same pattern. While in megasporangia most of the spore mother cells degenerate but in microsporangia only a few mother cells are disorganized.
- The phenomenon of heterospory becomes distinct either before or after meiosis in Selaginella. Isoetes, it is distinct before meiosis.
Importance of heterospory:-
i. The differentiation in the size of the spore is directly related to the differentiation of sex of the gametophyte. Therefore, heterospory is an expression of sex determination in plants.
ii. Because of heterospory, plants become free to grow in varied environmental conditions, instead of only aquatic surroundings.
iii. It ultimately leads to seed development.
iv. The megaspore having female gametophyte derives nourishment from sporophyte and thus remains independent from external factors.
Seed habit:- Because of heterospory, the gametophytic tissue is reduced. Partial or complete retention of megasporangium and female gametophyte on the sporophyte also takes place. They show reduction in megaspores and male gamete. All these characters show a tendency towards seed habit.
Origin of seed habit:- One of the most important advantages of heterospory is the formation of seed habitat. The origin of seed habit is associated with the following:
i. Production of two types of spores (heterospory).
ii. Reduction in the number of megaspores finally to one per megasporangium.
iii. Retention and germination of the megaspores and fertilization of the egg.
iv. Continued development of the fertilized egg into the embryo while still in situ.
Seed habit in Selaginella:- Selaginella exhibits remarkable approach to the seed habit because of the following features:
i. The heterospory occurs in almost all the species of Selaginella.
ii. In most species only one functional megaspore mother cell is produced which by reduction division produces four haploid megaspores. In some species e.g. rupestris and S. monospora only one megaspore is formed in each megasporangium.
iii. Germination of megaspore starts within the megasporangium, but its stage and time of retention within megasporangium differ in different species.
iv. Fertilization and embryo development also takes place within the megasporangium.
Although certain species of Selaginella suggest early stages in the evolution of seed habit, Selaginella should not be considered as an ancestor to seed plants because:
i. Absence of integuments around the megasporangia.
ii. Absence of permanent retention of megaspores within megaspoarangia except a few species.
iii. Absence of complete histological fusion between megasporangium wall and megaspore.
iv. After the development of embryo there is lack of resting period.
Therefore, Selaginella has considerably advanced towards seed habit but it’s approach to true seed is not complete.
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Life cycle of Gleichenia:-
> Sori are round, abaxial, not marginal, exindusiate.
> Sporangia are round to pear-shaped, up to 5–15 sporangia per sorus, developing simultaneously, the annulus transverse-oblique.
> Spores are globose-tetrahedral or bilateral, trilete or monolete, 128–800 per sporangium.
> Gametophytes are large, green, surficial, with club-shaped hairs, developing endotrophic mycorrhizae.
> The mature gametophyte is cordate with wings which have a more or less irregular margin.
- As the thallus continues growth it becomes ribbon-like with a heavy midrib (10-18 cells thick) and uplifted ruffled wings.
- The base in large gametophytes is usually heavy and club-shaped.
- Old gametophytes may branch and maintain two apices producing archegonia.
- The rhizoids are stiff, abundant, usually reddish brown, and in general limited to the midrib.
- In many species two-celled glandular hairs develop on the midrib or wings in the region of the archegonia; the large tip cell may contain tannin.
- The hair arises from a special initial, a wedge-shaped cell cut off from the anterior side of a cell in the region of the archegonia. Occasionally such hairs are also found on the margin.
- The gametophytes are monoecious.
> Antheridia:-
- The antheridia vary greatly in size from relatively small to large and complex with 10-12 cells in the wall and with a spermatocyte content of several hundred.
- The cap cell is thrown off at dehiscence.
> Archegonia:-
- The archegonia have long necks directed towards the apex, ordinarily with seven to nine cells in a tier, but in Gleichenia vulcanica sometimes 12-14.
- The necks are approximately straight or bent forward towards the apex.
- The axial row usually consists of egg, ventral canal and a binucleate neck canal cell, but the presence of two canal cells in the neck is not unusual.
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