Q.4. Give an account of polyembryony in gymnoperms. (2001, 05,10)
Ans. Polyembryony in Gymnosperms: -
A single zygote may form several embryos i.e. polyembryony (formation of more than one embryo from zygote). Several proembryos develop as a result of splitting thus the phenomena is called “cleavage polyembryony”. Usually a single zygote forms four embryos but ultimately only one of them matures. The number of the embryos formed is 4x (multiplied by) the number of archegonia fertilized. Out of the several embryos (8 to 12 or more) only one develops inside a seed others perish.
The zygote or oospore is the unit of sporophytic generation. The nucleus of zygote or fertilized eggs or oospore divided twice by two succesive mitotic division to form 4 nuclei without the formation of cross wall. These nuclei move to the base of the egg. Where they become arranged in a horizontal plane. Each nucleus divides and wall come informing two tiers of 4 cell each. Of the two tiers thus formed the cell of upper tiers remain divide again and the upper cell take no further part in the formation of the embryo than the cells of the lower tier divided thus forming 4 tiers of 4 cells each. This 16 cells mass formed at the lower end of the oospore forms the proembyro. The lower tier of the embryo forms the embryonal tier nent above the suspensor tier the 3 tier is called rosette tier and the 4 tier give rise to embryos. The suspensor tier to the primary suspensor the nentone rosette tier may give rise to 4 embryos also while the 4, upper cells serve as a large food reservoir. The primary suspensor elongates very greatly and the embryonal cells are burst down in the tissue of the female prothallus which functions as nourishing tissue. It is a curious feature of embryogeny in pinus that following the formation of the above 4 tiers the 4 cells of the embryonal tier divide and interpos another tier between them and the primary suspensor cell, thus family secondary suspensor cells. The process may be repeated once or twice. The 4 cells of the lower embryonal tier now separate and secondary suspensor continues to sull an entent that they become coiled and twisted. The whole structure of each embryo therefore, comes from one cell only such a condition is termed poly embryo or more strictly cleavage embryony.
In course of time however as a result of receiving unequal amount of food the 4 primary embryo’s grow at different rates. The largest finally survives and over become abortive. Meanwhile one or more cell of the rasette tier may divide to form a rosette embryo. These ordinarily do not develop very far, ercc epions has only one embryo.
Q.5. Describe the economic importance of Gymnosperms. (2011)
Ans. Economic Imortauce of Gymnosperms: -
Gymnosperms plants are of great economic value. Some of the major uses are detailed here under:
(1) Wood: -
As a source of wood conifers are the most economically exploited plants among gymnosperms. The timber obtained from them constitutes the major forest wealth in the countries where they occur. They furnish a light coloured straight grained and light weighted wood useful where strength and durability are not essential. Wood takes a good polish and paints satisfactorily. It bears good nail taking properties and takes a good finish with sharp tools. These properties make the wood used extensively for cabinet and furniture making, interior decoration and joinery. The wood of some coniferous plants is exceedingly important and is used for making railway sleepers and carriages.
Pinus yields a light timber which is used for making cheap furniture, packing cases, agricultural implements, fencing poles, crates, doors, frames, toys, general mill work etc.
(2) Resin: -
Resins are plant exudates secreted in specialized ducts. These largely come from conifers as a result of tapping. They are insoluble in water and soluble in organic solvents and fine extensive use in varnishes, paints, medicines etc. Various kinds of resins and their uses are given below-
(i) Rosin: -
It is obtained as a residue after the distillation of pine oleo resin or turpentine. It is used in making varnishes, enamels and in prepration of plasters and ointments. Inferior grade of rosin are used in making yellow laundry soap, grease and oil, oil cloth etc.
(ii) Copal: -
It is tapped from Agathis australis as a green gum or candle gum which is used in spirit varnishes, in making linoleum, preparation of plastics, polishes etc,
(iii) Canada Balsam: -
It is obtained from Abeis balsamea. It has a high refractive index as that of glass and is used as a mounting medium for microscopic objects and as a cement for lenses in optical world.
(iv) Sandrac: -
It is obtained from various species of Callitris and is used in as metal varnish giving good luster when applied in thin coats. It is used as paper and leather varnish.
(v) Venice Turpentine: -
It is obtained from Larix deciduas. It is used for making special type of varnish and veterinary medicines.
(vi) Essential Oils: -
Various kinds of essential oils are obtained from different species of coniferous plants.‘ Japanese pine needle oil’ is used for making scented soaps. C.deodara is also used in perfumery and in medicines to cure bronchitis, tuberculosis, skin diseases and gonorrhea.
(3) Fatty Oils: -
Fleshy outer layer of seeds of Macrozamia yields an oil which has properties similar to palm oil. Oil obtained from the seeds of Gnetum ula is used for illumination.
(4) Fibres: -
Stuffing fibres are obtained from ramantal hairs removed from the leaf bases of Macrozamia.
(5) Paper: -
It is made from wood pulp of some Indian species of conifers. Various species of Pinus provide newsprint almost all over the world. High grades of paper are made from the pulp of Abeis balsamea.
(6) Food: -
The stem and the seeds of Cycas yield a starch Sago. Seeds of Gnetum and Pinus are also edible.
(7) Medicines: -
Resin obtained from Cycas rumphii is applied to ulcers. The juice of the tender leaves of Cycas is used for flatulence and vomiting and the seeds and the bark in the form of paste are used as poltice for sores and swellings. Seeds of Pinus gerardiana yield an oil which is applied as dressing to wounds and ulcers. The seed juice of Ephedra is applied to cure respiratory troubles.
(8) Ornamentals: -
Species of Cycas are extensively grown as garden plants and for decorative purposes. Ginkgo is grown as an ornamental plant in the temples of China and Japan and is worshipped. Species of Pinus and Aroucaria are also raised as ornamental plants in North India.
(8) Poisonous Properties: -
Pollen grains of Cycas circinalis possess narcotic properties. Leaves of Macrozamia are poisonous and may cause rickets if eaten continuously by animals. Eating seeds of Ginkgo in larger quantities may prove fatal. Gnetum monatanum is used as a fish poison.
Q.6. Where does cycas occur? What are its Indian species? Give fern like characters of cycas.
Ans. The genus cycas shows world wide distribution. It has nearly 20 species, occuring mostly in Austrialia but entending beyond various islands it touches India.
In. India C. circinalis, C. revoluta, C. pectinatsa, C. beddomei, C. rumphii are commonly growing species most of which are cultivated in gardens as ornnamental plants.
C. revoluta, a most common Indian species has been called as “a living fossil” on account of its primitive characterstics. It natural habit is in open, sunny, well drained situations. It is the “sagopalm” of Japan and Chaina.
Systematic Position
Group - Spermatophyta
Subgroup - Gymnospermae
Division -. Cycadophyta
Class - Cycadopsida
Order - Cycadales
Family - Cycadaceae
Genus - Cycas
Cycas belong to the group cycadophyta of gymnosperms. In the Paleozoic, period a group of gymnosperm were found which showed seed habit and also form like known as pteridosperm or cycadofilcales. This group cycadofilicales is supposed to be 9 transitional group between ferms and cycades and are probably ancestral to cycadales. So some they have some ferm like characters. Some of ferm like character found in cycas are as follows:
1. The plant body is sporophyte differentiated into root, stem and leaves. The upright plant body has a columnar unbranched stem covered with armour of leaf bases. In the past, ferns were not always herbaceous but also tree like. These are certain ferns called as tree ferns which also possess and thick columnal stems like that like of cycas.
2. The leaves of cycas are always pinnate. The fern leaves are also pinnately compound.
3. Both in cycas and ferns the leaves in young condition show circinate vernation.
4. Rementa is found on the young leaves of cycas which shows a fern ancestory.
5. There are scales in the cycas. In the fern also one often comes across scales on stem.
6. The vascular anatomy of both i.e. cycas and fern is similar. The cycas xylem has trachied and no vessels and phloem have no companion cells. This is fern like character.
7. Mostly the ferns are homosporous, but cycas is heterosporous this a point of great difference. The microsporangia are present on the adaxial side and remain arranged in short sporangia in cycas a character similar to fern.
8. Like Ophiglossaceae and Marattiaceae two families of ferns. The development of microspirangia in cycas is also eusporania types. In their internal structure and the manner of dehiscence the cycas microsporangia have, striking similarly to these of Angiopteris fern.
9. The megasporephyll of ferns the, marginal ovules of cycas are not comparable to the sort in ferns the velvetty hair on the sporophylls resemble the fern renentatio.
10. The presence of multiciliate sperms in both of cases i.e. cycas and fern is one of the strongest similarly between the groups. One above points clearly shows that cycas have mainly character comma with ferns and may be said that cycas has descended from fern like ancestors.
Q.7. Describe the anatomy of stem of Cycas.
Related Questions
Q. Write short notes on the following:
(i) Mucilaginous canal
(ii) Manoxylic wood,
(iii) Leaftrace or girdle trace (2005, 07, 09, 11)
Ans. Transverse section of stem shows irregular outline due to presence of numerous leaf bases. The outer most layer epidermis in prensence of large number of persistent leaf bases appears incomplete and ruptured. Cortex is multilayered made up of parenchymatous cells with abundant starch, mucilage canals and crystals of calcium oxilate.The cortex communicates with pith through medullary rays. The mucilage ducts are freely branched and form network in stem and petiole. Each mucilage canal has a thin layer of secretary cells surrounded by thick walled cells. Cortex shows numerous leaf traces. Endodermis and pericycle are not distinct.
Vascular Cylinder: -
It is eustelic,and made up of many small, narrow, collateral conjoint, open and endarch. Vascular bundles, arranged in a ring. Phloem consists of sievetubes and phloem parenchyma. Albuminous cells are associated with sieve tubes and phloem parenchyma. Campanion cells are absent. A thin layer of shrot lived cambium is present between phloem and xylem. Xylem is mesarch but appears endarch in young stem and has tracheids only.
Protoxylem has spiral thickenings and metaxylem with scalariform thickenings. Vessels are absent but Greguss (1958) reported them in C. revoluta.
Leaf traces: -
These are small and endarch in the stem, pseudo mesarch when enter the base of leaf, due to formation of centripetal xylem and exarch at the extreme tip of the rachis. Each leaf is normally supplied with two pairs of traces, one pair constitutes the main or girdle trace, while the other pair are direct or radial trace. The girdle trace arise from protoxylem of vascular ring, opposite to the leaf to which they supply these traces pass obliquely through cortex, then bifurcate. Each bifurcated trace, running round in oppostite direction, meet at the base of the leaf and then enter the petiole. Direct Fraces similarly arising from main stele bifurcate and finally produce anastamosing bundles.
Secondary Growth: -
It takes place inside and outside the stele. In young stem the secondary growth inside the stele takes place by fascicular cambium produced by joining of inter and intrafascicular cambia. The primary bundles are open with a strip of interfacsicular cambium. As when secondary growth intrafascisular cambium arises as in normal dicot system. This cambium cuts off secondary xylem towards the inner side and secondary phloem towards the outer side. However, the primary camibium is of short duration. Later on an accessory cambium ring is formed either in pericycle or in the cortex. This cambium ring behaves like the previous one and is again produces secondary xylem and phloem on its inner and outer sides respectively. In this way, several rings of accessory cambium arise in the cortex and corresponding to them there are several distinct rings of secondary vascular tissue. These rings are the growth rings which are quite different from annual rings of Angiosperms.
