Learn about the comparison of Cycas, Pinus and Ephedra.
Comparison of Cycas, Pinus and Ephedra:
Cycas and Pinus are evergreen perennials while Ephedra is perennial with scaly leaves. Cycas and Ephedra are dioecious while Pinus is a monoecious plant. Cycas is 5-15 feet in height (C. media 20’), resembling in habit with palms on the one hand and tree ferns on the other. Pinus is about 15-45 metres in height while Ephedra is a straggling shrub with few species as lianes.
In all the three cases the plant body is distinctly differentiated into root, stem and leaves:
In Cycas it is tuberous when young but erect, columnar and un-branched. Sometimes it is branched due to injury or growth of a lateral bud. Male plants show sympodial growth (terminal cone) and female monopodial. The surface of stem is rough due to the presence of armour of persistent leaf bases and scale bases in alternate whorls.
In case of Pinus and Ephedra the growth is monopodia one. In Pinus, branches are of 2 types, i.e., long-shoots-(branches of unlimited growth) which are given out in horizontal direction; and short or dwarf shoots (branches of limited growth) present in the axils of scaly leaves on branches of unlimited growth. In Ephedra the stem is green, ribbed with distinct nodes and long internodes. The branches arise from axillary buds.
Anatomy of Stem:
All the three plants show a general pattern like that of dicot stem, i.e., vascular bundles are conjoint, collateral, open with end-arch xylem, and are arranged in a ring. All the members show a well developed secondary growth. In case of Cycas mucilage canals are present scattered in cortex and pith regions.
Each mucilage canal is made up of inner single layer of epithelial cells which secrete mucilage. Besides this girdle-shaped leaf traces are also found in the cortex. They really are the vascular supplies of the leaves given by the main vascular cylinder.
Each leaf receives four traces, two of which are direct traces and other two are given out from the opposite side of direct traces. These girdle around the main vascular cylinder through cortex and then enter the upper leaf base on the opposite side.
These leaf traces are known as girdle traces. In Pinus, cortex and pith cells have tannin and resin canals, which secrete resinous substances like turpentine. Each resin canal is internally lined up by epithelial cells surrounded by sclerotic sheath. In Ephedra, the cortex is differentiated into outer palisade parenchyma and inner spongy chlorenchyma which are provided with patches of sclerotic cells.
In case of Cycas annular rings are formed due to secondary growth by successive cambium rings which are active only for a short while. These cambium rings are formed either from pericycle or cortex. In C. pectinata as many as 14 concentric vascular rings are reported. The successive outer rings decrease in thickness. In Pinus annual rings are formed due to secondary growth differentiated into spring wood and autumn wood.
The cells of the spring wood are thin- walled, polygonal and larger in size while those of autumn wood are thick-walled, squarish and smaller in size. By counting the number of annual rings we can count the age of the plant.
This type of differentiation is clearly visible only in trees growing in temperate countries where the winters are severe, but where the winter is not severe, the secondary growth may even continue for more than one year without demarcation into annual rings. In this case they are called as growth rings instead of annual rings.
In Ephedra, secondary growth is very little and here also annual rings are formed with distinct spring wood and autumn wood. A sclerotic layer is also formed outside the secondary phloem. Ephedra is characterized by the presence of companion cells in phloem and vessels in the xylem elements while both these are absent in Cycas and Pinus.
In this case sieve tubes and companion cells are developed from the same initial. In the same way vessels are developed from pitted tracheids in Ephedra and from scalariform tracheids in angiosperms.
In Pinus and Ephedra cork cambium develops which cuts off secondary cortex on the inner side and cork on the outer side.
The structure of the secondary medullary rays is very peculiar in Pinus. In r.1.s. the height and length of secondary medullary ray are seen. The ray on both sides is surrounded by 2 rows of dead empty elongated cells known as ray tracheids having pits in the field. In phloem region it is made up of starch cells surrounded by elongated dome-shaped albuminous cells.
The bordered pits on xylem tracheids are seen in surface view and are arranged in uniseriate manner.
In t.l.s. the wood tracheids are cut longitudinally showing longisection of bordered pits. The torus is very distinct, surrounded by two dome flaps of thickening. Meduallary rays are cut transversely and show breadth and height. These are either linear (uniseriate) or fusiform (multiseriate).
In multiseriate medullary ray, a resin canal is present in the centre. In case of Pinus the wood is dense and compact as a result of which it has been termed as pycnoxylic.
The leaves in gymnosperms show a wide variation in their external morphology. In Cycas, it is pinnately compound fern-frond like structure. The size of a frond may be as small as 5 cm. as in Zamia pygmea or may be as long as 3 metres, e.g., Cycas circinalis. In young stages they show circinate vernation—a prominently pteridophytic character.
In Coniferales the leaves are linear and acicular (Pinus). In other genera of Coniferales the shape of the leaves is varying. In Gnetum, however, the leaves are broader whereas in Welwitschia there are only two leaves throughout the life of the plant. In Ephedra, only rudimentary leaves are present at the nodes.
A characteristic feature of Cycas and conifers is the presence of dimorphic leaves, i.e., scale and foliage leaves. Their distribution is characteristic in different members.
In Cycas, scale leaves are arranged spirally alternating with the whorl of foliage leaves on the stem covering stem apex and developing foliage leaves. In Pinus, these are present on long and dwarf shoots. In Ephedra, scale leaves are present on the nodes of the phylloclade, forming a sheath which may be two or three toothed, rarely four toothed.
In Cycas scale leaves are persistent protective, and are covered with brown ramental hairs. In Pinus they fall off as the branches mature.
Foliage leaves are absent in Ephedra. In Cycas, leaves are large pinnately compound up to 3 metres long in C. circinalis. In Pinus they are needle like simple borne on dwarf shoot constituting with the latter the foliar spur. The leaves of Cycas are shed leaving leaf bases but in Pinus foliar spur is shed. Number of leaflets in C. revoluta is more than hundred, in C. circinalis 80-100.
In Cycas revoluta, the leaflets are lanceolate, leathery with entire curved margin and spiny apex. In C. circinalis margin is not curved. In Pinus foliar spur may be monofoliar, bifoliar, quadrifoliar or pentafoliar, depending upon the number of foliage leaves on the dwarf shoot. Sometimes the leaves are present in groups directly on the main branch or shoot, e.g., in Cedrus.
Scale leaves are dry and without vascular tissue in Pinus but in Ephedra each scale leaf is provided with a midrib which consists of two parallel vascular bundles. In Pinus, needle may be circular, semi-circular or triangular in its outline. Epidermis consists of thick-walled parenchymatous cells covered with thick cuticle.
Cycas leaf is hypostomatic and Pinus leaf is amphistomatic. Stomata are sunken. A well defined hypodermis is single layered below upper epidermis but is absent above lower epidermis in Cycas.
In midrib region, however, it is 2-4 layered on both the sides. Pinus needle has multi-layered hypodermis on all the sides. Mesophyll is differentiated into palisade tissue and spongy parenchyma in Cycas but not so in Pinus. Resin canals are found in Pinus. Transfusion tissue is present in both. Midrib consists of only one vascular bundle in Cycas while in Pinus two vascular bundles are found.
In Cycas, vascular bundle is surrounded by a bundle sheath and pericycle is one layered. Both the vascular bundles of Pinus needle are separated with ‘T’ shaped tissue of sclerenchyma, xylem is diploxylic (centripetal and centrifugal) in Cycas while in Pinus only of one type.
In Cycas leaf consists of leaflets and rachis. In T.S. rachis also shows different type of tissues. Outermost layer is epidermis covered with thick cuticle. Epidermis is followed by continuous hypodermis which may be 3 to 4 layered and sclerenchymatous. Hypodermis is followed by chlorenchymatous tissue. In the centre parenchymatous ground tissue is found.
Mucilage canals are found in the ground tissue. Numerous vascular bundles are embedded in the ground tissue arranged as omega. Each bundle is conjoint collateral surrounded by a thick-walled fibrous bundle sheath and is open.
The vascular bundles are different in structure from base to apex of the rachis. At the base of the rachis bundles are with endarch xylem. As the vascular bundles are traced upward the centrifugal xylem gradually diminishes and the centripetal xylem appears on the inner side of the protoxylem.
Throughout the greater part, the bundles contain a large mass of centripetal xylem with only traces of centrifugal xylem which is separated from the protoxylem by parenchymatous cells. The vascular bundle in this region shows pseudomesarch condition.
At the apex the centrifugal xylem completely disappears and is exarch. The gradual disappearance of centrifugal and simultaneous appearance of the centripetal xylem is characteristic of Cycas.
Cycas possesses a well developed tap root system which grows deep into the soil. Wordsdell (1909), however, believed that tap root of Cycas is later replaced by adventitious roots. Coralloid roots-coral like in structure, also develop. These are negatively geotropic roots and are dichotomously branched.
Development of coralloid roots:
These roots develop from normal roots due to some changes brought about in the growing region of the roots. During growth root apex eventually comes in contact with the bacteria. The latter enter the roots through lenticels or even epiblema and growing apex and inhabit the roots.
Due to the presence of these bacteria the cortex gets loose. Soon the elements of blue green algae also enter into it. Due to the presence of these external elements, the physiology of roots changes, they become negatively geotropic, divide dichotomously repeatedly and never show signs of secondary growth.
These roots, as are comparable to corals, are known as coralloid roots. These eventually come out of the earth and help perhaps in respiration and absorption of water.
In Pinus there is a tap root system which forms a massive root system due to strongly developed lateral roots. Root hairs are few. Ectotrophic mycorrhiza is developed and as such root hairs soon vanish.
The roots have radial vascular bundles with exarch xylem. It may be diarch, triarch or polyarch.
In Cycas there are two types of roots, i.e.:
(2) Coralloid root.
Cycas normal root resembles the dicot root internally. Outermost layer is epiblema (piliferous layer) composed of thin-walled, elongated, compactly arranged parenchymatous cells. Root hairs are present on younger roots. In Pinus root hairs disappear due to formation of ectotrophic mycorrhiza. In Ephedra, root hairs are present.
Below the epiblema is a many layered cortex composed of loosely arranged parenchyma cells filled with starch. Mucilage canals are present in Cycas. The innermost layer of cortex forms the endodermis which is well differentiated in Pinus. Tannins are absent in Ephedra.
In coralloid root of Cycas, cortex is divided into three zones, i.e., outer, inner and middle zone. The last zone consists of loosely arranged, radially elongated thin-walled cells with large intercellular spaces. These spaces and the cell lumen are occupied by blue green algae.
Algal zone contains usually blue green alga like Anabaena cycadae (Reinke 1872, 1879), Nostoc punctiforme (Hariot, 1892) or Oscillatoria, diatoms and other algae associated with nitrogen fixation, or a nitrogen fixing bacteria, e.g., Azotobacter, Pseudomonas radicicola.
It has been suggested that (a) algal zone and root act as symbionts (Schneider, 1894) (b) algal zone as aeration organ (Life 1901). According to Schaede (1944), there are no bacteria in coralloid root.
Radial vascular bundles are surrounded by many layered pericycle. Xylem is diarch to triarch in Cycas, diarch to hexarch in Pinus and diarch in Ephedra. It is exarch but Atwood (1939) reported mesarch in C. revoluta. Xylem in Pinus is ‘Y’ shaped and has resin canals. Xylem consists of tracheids only in Cycas and Pinus but tracheal tubes (vessels) are also present in Ephedra.
Phloem consists of sieve tubes and parenchyma in Pinus but only parenchyma in Cycas.
In Cycas and Pinus secondary growth is due to cambium which develops inner to phloem patches. These patches ultimately become continuous and cut secondary xylem and phloem on inner and outer sides respectively.
4. Reproductive Parts:
Sexual reproduction is the only method of reproduction in these except in Cycas, which can reproduce vegetatively by means of bulbils. These are formed in the axil of scale leaves throughout the stem. These may germinate either on the stem itself giving the false appearance of dichotomy or fall off on the soil and sprout whenever suitable conditions are available.
Sexual reproduction is basically the same in all the three genera. These are heterosporous, producing two types of spores-megaspores giving rise to female gametophyte and microspores forming male gametophyte, Cycas and Ephedra are dioecious, but Chamberlain (1935) reported a plant of C. revoluta from Australia which at first produced a female strobilus and a few years later, a male strobilus.
Similarly Ephedra inter-media and E. foliata are monoecious. Pinus is strictly monoecious and is interesting as male and female cones are borne on separate branches.
Male reproductive parts:
The microspores are produced in microsporangia which in turn are present on leaf-like microsporophylls. The microsporophylls are aggregated to form the male cone. In Cycas, the male cone is present at the top of the main axis and is borne singly. In Pinus, groups of male cones are present in the axil of the scale leaves, while in Ephedra 2-4 male cones are present at each node in the axil of the scale leaves.
In Cycas male cones attain a length of 50-60 cm. Large number of microsporophylls are present spirally arranged around the cone axis. Each microsporophyll is more or less triangular, the apical portion of which extended in a sterile disc known as apophysis. Microsporophylls show traces of pinnate characters.
On the ventral surface of each microsporophyll are present large numbers of stalked sporangia in groups of two to five.
Around each sorus are present many multicellular hairs, known as indusial hairs. In Pinus cones are only 2-10 cms. in length, and each microsporophyll bears only two sessile sporangia. In this case pollen grains are winged. In Ephedra bracts are arranged in opposite and decussate manner and all of them except the lower 1 or 2 pairs are fertile.
A stamen is present in the axil of bilabiate perianth. The anthers are 2-5 lobed and are provided with a terminal aperture.
Female reproductive parts:
Megasporophylls are the ovule bearing organs. In Pinus and Ephedra the megasporophylls are forming compact cones while in Cycas the megasporophylls are borne in succession of the leaves and are not forming any cone. It is interesting to note that in Cycas the megasporophylls are present on the main axis while in Pinus and Ephedra female cones are present on lateral branches.
This has led Meeuse to suggest that each megasporophyll in Cycas is equal to a full cone of other genera.
In certain species of Cycas, e.g., C. circinalis, C. normanbyana and C. rumphii, there is reduction in the number and size of sporophylls. The ovules are borne on the margins of the sporophylls. The largest ovules in the plant kingdom are borne by Cycas species (about 6 cm. diameter). In Ephedra all the lower bracts are sterile except the uppermost pair which bears a pair of ovules. In Pinus the female cone matures in 3 years.
Each seed-scale complex here is differentiated into two parts:
Small, dry, membranous structure attached directly to the cone axis and bearing the ovuliferous scale in its axil.
(ii) Ovuliferous scale:
Woody structure borne above the bract scale bearing at its base two ovules with micropyles directed towards the cone axis.
The morphological nature of the female cone of Pinus and in particular that of ovuliferous scale has been variously described in the past. Views have been put forward by Robert Brown (1827), Schleiden (1839), Van Tiegham (1869) and Sachs (1868). The most accepted view is, however, that of Florin (1955).
According to him, the elongated axis is a peduncle, the bract scale is a real bract and the ovuliferous scale with two ovules is a female flower of very rudimentary type. The entire cone thus represents an inflorescence or a ‘compound strobilus’ consisting of a number of rudimentary female flowers.
The ovule of Cycas is orthrotropous and a chiefly made up of central mass of parenchymatous cells known as nucellus. The nucellus is covered by the integument all around except at the top where a small pore-the micropyle is left. The integument may be differentiated into 3 layers—the outer fleshy, the middle stony and the inner fleshy layer.
In the region of micropyle the nucellus protrudes into a nucellar beak which extends on peripheral sides to form a pollen chamber.
Embedded in the nucellus there is a functional megaspore with rudiments of non-functional megaspores here and there. The ovule of Pinus is similar to that of Cycas except that no nucellar beak and pollen chamber are present in Pinus. In Ephedra, however, a very long micropyle is present which protrudes out of the bracts also.
The female flowers of Ephedra can be identified morphologically by the presence of long micropyles at the top of the cone.
5. Male Gametophyte:
Gametophytic generation starts with the formation of spores, after meiosis in the sporogenous tissue within the sporangium. Gymnosperms are heterosporous; consequently micro- and megaspores are formed inside the micro-and mega-sporangia respectively.
The microspores, also called as pollen grains in case of gymnosperms and angiosperms are the initial stages of male gametophyte. The microspores of Cycas, Pinus, and Ephedra are built on the same architectural pattern.
Their development can be studied in two stages:
(i) Development of male gametophyte before pollination, i.e., inside microsporangium,
(ii) Development of male gametophyte after pollination i.e., inside ovule.
The germination of pollen grains starts in situ. The first division, in all the three genera results in the formation of a small prothallial cell and a bigger cell. The bigger cell directly functions as antheridial initial in Cycas, while in Pinus and Ephedra it cuts off a second prothallial nucleus.
In Pinus this division of nucleus is followed by cytokinesis resulting in second prothallial cell and antheridial cell. However, in Ephedra there is no w all between second prothallial nucleus and the antheridial nucleus.
The antheridial cell divides to form generative cell and tube cell. In Cycas, pollen is at three celled stage (one prothallial cell, one generative cell and one tube cell). In Ephedra the antheridial nucleus divides to form generative necleus and nucleus.
The generative nucleus divides giving rise to body nucleus and stalk nucleus. At the time of shedding the pollen is five nucleate (two prothallia nuclei+one stalk nucleus+one body nucleus+one tube nucleus). Each pollen grain develops a thick exine and thin inline. It is winged in Pinus.
Formation of male gametes from the body cell/body nucleus takes place in pollen chamber of ovule. Pollen tube is formed by the prolongation of intine. The tube nucleus migrates into it. The pollen tube in gymnosperms is not the gamete carrier but functions as haustorium absorbing food from the nucellar tissue for the development of male gametophyte. The tube usually divides to ramify into the nucellus.
The two prothallial cells of Pinus male gametophyte are short lived and degenerate soon. Body cell in Cycas divides to form two sperm mother cells, which give rise to motile sperms. The sperm is top shaped, multi-ciliated and is visible to naked eyes (perhaps biggest sperm in plant kingdom). In Ephedra and Pinus the male gametes formed from body nucleus are only two nuclei.
6. Female Gametophyte:
Megaspore mother cell differentiates in the nucellus. Reduction division of megaspore mother cell nucleus results in the formation of a linear tetrad of megaspores, the chalazal megaspore out of them is functional and the rest 3 degenerate.
Development of female gametophyte:
The development of female gametophyte in Cycas, Pinus and Ephedra takes place almost in the same manner. The functional megaspore, before its division enlarges by absorbing some of the neighbouring cells of the nucellus tissue. The development of the female gametophyte takes place in situ. The nucleus of the megaspore divides by a free nuclear division to form a large number of nuclei as many as 2000.
The nuclei thus produced are pushed towards the periphery as a vacuole appears in the centre of this structure. The wall formation starts from periphery and proceeds to the centre. The tissue produced in this manner is called endosperm or female prothallus. The compactly arranged upper cells of this structure are smaller in size than those of the cells of lower region. The latter cells perform the function of nutrition.
Much before the gametophyte has been cellular, one to two cells thick layer endosperm jacket (the nutritive layer) develops contact with the gametophyte. The cells of the jacket, like those of the tapetum pass on the nutritive material from the cells next to it into the growing gametophyte. The cells of the gametophyte contain starch grains. Some of the cells may contain tannin also.
Although the wall formation in Pinus is also centripetal but the first wall is perpendicular to the megaspore membrane, and extends to the middle of the megaspore cavity, so that the cells are like long tubes which are called as “alveoli”. Cross walls are then formed in such a mariner that the alveoli become divided into a number of uninucleate cells.
In Ephedra the megaspore mother cell is deep seated because of prolific divisions in the overlying nucellar epidermis. The number of nuclei produced by free nuclear division in megaspore is 256 or more within 20 days.
The tissue of the mature female gametophyte is gradually differentiated into:
(i) Micropylar region or upper reproductive zone-consisting of thin-walled loosely arranged cells which function as archegonial initials in further development.
(ii) Antipodal region:
(a) Lower storage region:
Greater part of the endosperm is of compactly arranged cells which are full of food materials.
(b) Basal haustorial zone:
It consists of outer two or more layers of cells at the antipodal end. The cells of this region are also compactly arranged and perform function of absorption of food material from the surrounding cells of the nucellus.
Development of archegonium:
Each archegonium develops from a single archegonial initial situated superficially in the tissue of endosperm on the micropylar end. It enlarges in size and gets differentiated from rest of the cells of the endosperm. It divides by a transverse or periclinal division into an upper primary neck cell, and lower central cell. The primary neck cell divides into two neck cells by a vertical division.
In Pinus this primary neck cell divides by two successive vertical divisions at right angles to each other followed by a transverse division forming an eight celled neck arranged in two tiers of four cells each. In Ephedra the primary neck cell divides to provide a neck of eight or more tiers with a minimum of 32 cells.
In the beginning these divisions are regular but later on become irregular so that the cells of neck can no longer be distinguished from the surrounding gametophyte tissue. The neck of archegonium in Ephedra is the largest among Gymnosperms.
The central cell enlarges in size and its nucleus divides into ventral canal nucleus and egg nucleus. There is no wall formation between two nuclei in Cycas and Ephedra while there is a definite wall between two nuclei in Pinus giving rise to an upper small ventral canal cell and lower larger egg. The egg of Cycas and its nucleus are large in living plants.
The neck canal cells are never formed in gymnosperms. Later the ventral canal nucleus or the cell degenerates. During this development of archegonium, the endosperm tissue near micropyle grows upward and forms the archegonial chamber. The archegonial jacket also develops in Cycas and Ephedra from the surrounding tissue which helps in the transfusion of food to egg.
Swamy (1948) has, however, observed pores in the wall of the jacket facing the venter of archegonium. It has been suggested that exchange of food materials takes place through these pores.
Each archegonium consists of an egg and a neck formed by two, eight and 32 cells in Cycas, Pinus and Ephedra respectively. The neck is not very distinct during later stage in Ephedra, but well distinct in the rest two cases. A ventral canal cell is present in Pinus but in Cycas and Ephedra there is only ventral canal nucleus.
The surrounding cells of the endosperm form the archegonial jacket, around the archegonium in Cycas and Ephedra.
From evolutionary standpoint, Cycas appears to be more primitive than Pinus and Ephedra. This is illustrated by the presence of largest egg and a massive ovule with integument differentiated into 3 regions.
Like all other gymnosperms Cycas, Pinus and Ephedra are also anemophyllous. A drop of mucilaginous substance oozes out of the micropyle in which the pollen grains are entangled. Later on the pollen grains are drawn into the pollen chamber due to the drying up of pollination drop.
The act of fertilization is not immediately accompanied, but there is an intervening period between pollination and fertilization which varies in various genera. In Cycas it may be 5-6 months, in Pinus it is about 9 months whereas in Ephedra it may be as short as only 10 hours. Fertilization is siphonogamous. The pollen tube penetrates into the archegonial chamber and bursts to release the male gametes.
In Cycas the spermatozoids swim for some time and on coming in contact with the neck cell, one of them enters the archegonium and finally penetrates the egg. Here the ciliated band is left at one of the comers of egg and nucleus slips out of it and fuses with the egg nucleus forming zygote.
In Pinus pollen tube bursts near the egg releasing two male nuclei; one of them degenerates and the other fuses with egg nucleus forming zygote. In Ephedra the pollen tube bursts within the archegonium discharging the tube nucleus, stalk-cell nucleus and two sperm nuclei into the cytoplasm of egg.
One sperm nucleus fuses with the egg nucleus to form zygote. The behaviour of other nucleus is extraordinary and it fuses with ventral canal nucleus (Khan 1943).
Embryo formation starts with a prolonged period of free nuclear divisions without resting period. In Cycas 64-256 free nuclei have been reported. A large central vacuole is formed by coalescence of smaller ones.
As the real embryo of Cycas is formed only from a part of this structure it is termed proembryo. Cell formation begins at the base of the proembryo and gradually extends upwards. The proembryo becomes differentiated into three regions.
Cells present just below the central vacuole become haustorial; those immediately below this elongate to form a long, coiled and twisted suspensor while the cells situated at the tip of the suspensor remain meristematic and develop the embrro proper.
The fertilzed egg or the zygote in Pinus nucellus divides into two and then four nuclei by free nuclear divisions. These nuclei enlarge, migrate to the base of egg cell and arrange themselves in a single plane. The four nuclei now divide simultaneously to form eight free nuclei.
It is after this division that the cell wall formation starts so that the eight nuclei become arranged in two tiers of four cells each. The two tiers again divide to form four tiers giving rise to a sixteen celled proembryo.
Uppermost tier is open at the top and is believed to be nutritive. The next tier is the rosette tier; the third one is suspensor, and the fourth the embryo proper that divides further. The suspensor cells elongate very much and split into four separate suspensors each bearing at its tip a group of embryonal cells developed from a cell of the embryonal tier.
The four separated cells of the embryonal tier give rise to four separate embryos. This phenomenon is known as cleavage polyembryony. Apart from this, adventive polyembryony is also found in Pinus.
The zygote undergoes three free nuclear divisions in Ephedra forming eight nuclei all capable of giving rise to embryos. They are evenly distributed in the cytoplasm. Each of them invests itself with cytoplasm and cell wall thus becoming a proembryo.
Out of these, usually 3-6 proembryos begin to develop into embryos. The proembryonal cell divides to form a suspensor cell and an embryo initial which is pushed down into the female prothallus by the elongating suspensor.
Seeds are mature ovules with all their contents like embryo and endosperm and integuments, etc. It is to be noted that a seed has condensed within it three successive generations: sporophytic, gametophytic and sporophytic generations, beautifully packed in one. These seeds dominate the life of higher plants to such an extent that these plants are better known as seed plants.
The formation of seed takes longer duration among gymnosperms and these do not have any resting period before germination.
Development of seeds:
In Cycas, seed formation sets on after about one year since fertilization. The integuments are variously coloured and have three layers. The outermost fleshy layer starts drying up and becomes appressed to middle stony layer and forms seed coat. The inner fleshy layer also starts giving up all cellular contents.
They are used up by developing gametophyte and embryo. The embryo increases in size without much differentiation at this stage.
The nucellus is also used up leaving behind a small papery cap just below the micropyle. The tissue of female gametophyte modifies into the massive and nutritive tissue-the endosperm. The embryo at this stage has a long twisted suspensor. At the point where suspensor joins embryo proper a hard covering tissue called as coleorhiza is formed; may be that it forms a protective layer for radicle.
The embryo usually has two cotyledons but rarely one or three cotyledons may also be present.
The cotyledons are unequal in size and sometimes the larger one may show pinnate nature. Between the cotyledons there may be one or two scales and a small leaf arising at right angles to cotyledons. The development of roots is much delayed in Cycas.
The embryo may be more than one in some cases, e.g., in C. circinalis. Shedding of seeds may take place at various stages. Soon after shedding germination starts which is very similar to that in dicotyledons.
In Pinus ovules are small in size and consequently the seeds too. Of course number of seeds per cone is large. Each sporophyll bears two seeds. The integuments after fertilization constitute the seed coat. The embryo in Pinus is slightly different from that of Cycas. It has a whorl of cotyledons which may range up to 14. It has been supposed that large number of cotyledons indicate primitiveness of embryo.
The outer fleshy layer of integument dries up and forms the wings of Pinus seeds.The inner fleshy layer helps in nutrition of embryo and forms a papery layer. Remains of nucellus are represented by a small cap-like and shrunken layer of cells at apex. At the time of germination the first leaves are needle-like and spurs arise in the axil of these primary leaves.
The cotyledons may develop chlorophyll without light also. Due to ecological factors seedlings of Pinus do not develop under the canopy of parent tree.
Ephedra ovules are smaller and bitegmic. The outer integument is of doubtful status and is not considered generally as true integument. It is supposed to be made of two closely appressed valves. Moreover, the bracts which are present outside the ovule become fleshy. Perhaps a step forward towards fruit formation from protection and dispersal point of view.
The embryo has two cotyledons embedded in female gametophyte tissue and the nucellus. The nucellus results in a papery covering and gametophyte forms endosperm. The outer integument becomes very hard after some time and forms the seed-coat with two layers. The germination may even take place on the parent plant. The polyembryony may also be present in Ephedra, but is very rare.
Dispersal and germination of seeds:
Cycas seeds are very large and variously coloured attracting, birds and animals for dispersal. Seeds of Pinus are small and winged, therefore, wind currents also help in their dispersal. Seeds of Pinus are good to taste and taken away by animals too. The Ephedra seeds may start germinating on parent plant without going away from parents.
Cycas and Pinus seeds, however, germinate after detachment from the parent plants. Cycas seeds are very sensitive to injury which may result in failure of germination.
At the time of germination of Cycas seed, root initial forms the primary root and pushed out through the micropyle. Germination is hypogeal. Primary leaf starts arising at right angles to cotyledons and becomes green on coming out.
Pinus seeds show epigeal germination, cotyledons also with primary needle-like leaves coming out of the seed coat and seed coat too being pushed out of the soil.
Ephedra seeds do not require long resting periods and germinate soon after maturity. In fact germination process is alike in Pinus and Ephedra and is epigeal.