Structure and theories regarding origin of paleozoic ovule

Origin of ovule in pteridospermales is seen in paleozoic era so the ovule is called paleozoic ovule, paleozoic ovule is 3 types 1. Cardicarpels ,2. Lagenostomales 3. Trigonocarpales
In cardiocarpales and trigonocarpales the ovule is free from nucellus and the lagenostomales the ovule is fused with the nucellus
In cardiocarpales and trigonocarpales the ovule is bilateral symmetrical but in lagenostomales the ovule is redially symmetrical

1. GOVT. J. YOGANANDAM CHHATTISGARH COLLEGE RAIPUR GUIDED BY – MRS. NAMRATA DUBEY SUBMITTED BY – SHEIKH AYAN Paper – IV presentation Bryophyta , pteridophyta and gymnosperm

2. Topic – structure and theories regarding origin of paleozoic ovule

3. Synopsis - Introduction What is Paleozoic ovule Structure of paleozoic ovule Types of paleozoic ovule Theories regarding origin of paleozoic ovule Reference

4. The fundamental characteristic of all the spermatophyta is to producing seed and that seed is develop from ovule . ovule seed an ovule is a megasporangium ( nucellus) surrounded by one or more protective layer ( integument ) , with a small opening ( micropyle ) . The nucellus of an ovule contains megasore that develop into the female gametophyte . The ovule was first appeared in the devonian – carboniferous period of paleozoic era in early pteridosperms (seed fern) and other primitive gymnosperm .

5. A paleozoic ovule is the earliest known seed producing structure that appeared during the paleozoic era ( about 350 -250 million year ago ) . These ovule represent the transition from spore bearing plant like ( fern ) to seed bearing plant ( gymnosperm ).

6. Main parts of the paleozoic ovule - Nucellus ( megaspore producing tissue ) Integument ( protective layer , outside the nucellus ) Micopyle ( opening for pollen to enter ) Pollen chamber (small cavity found below the micropyle to receive pollen grain ) Megaspore ( which develop into female gametophyte )

7. Oliver ( 1903 , 1904 ) classified paleozoic ovule into two main types : Platyspermeae – bilaterally symmetrical ovules Rediospermeae – redially symmetrical ovules Seward (1917) classified them into three types : Cardiocarpales Lagenostomales Trigonocarpales

8. Cardiocarpales In this type of ovule the integument is free from the nucellus except at the base and consist of two or three distinct layer - Sarcotesta – outer usually fleshy layer Sclerotesta – middle stony layer Endotesta – inner fleshy layer Ovule is bilateraly symmetrical Beside the three layered single integument the nucellus appeared as a thin band of tissue surrounding the female gametophyte . Female gametophyte is consist of parenchyamatous mass of cell with two archegonia toward the mycropylar end . There is a distinct pollen chamber at the nucellus tip. The vascular supply in ovule occur through tracheidal disc which is present at the base of nucellus . Eg. Cardainthus williamsonii ,Rhabdospermum , Cardiocarpus magnicellularis . Fig . – L.S. of cardiocarpalean ovule

9. 2. Lagenostomales In these type of ovule the nucellus and the integument are united . The ovule is radially symmetrical . A protective structure called cupule is present and may enclose one or more ovules ( 1 in Lagenostoma and 4 in Stamnostoma ) . Cupule is found external to the integument Only the integument is vascularised . Eg. Lagenostoma lomaxi , S alpingostoma dasu , Sphaerostoma ovale and Tyliosperma orbiculum . FIG . Ovule of Lagenostoma lomaxi

10. 3. Trigonocarpales The integument is free from nucellus . They are radially symmetrical . Cupule have not so far been discovered . Both the nucellus and integument are vascularised . The nucellar apex is complex and has a distinct pollen chamber with a small projection . Eg. Pachytesta illionense , Stephanospermum elongatum Some lower carboniferous ovules do not fit into any of the above three categories eg. Genosperma kidstonii, G. latens , and Geminitheca scotica .

11. Benson (1940) proposed that a pteridosperm ovule is a synangium in which the central sprorangium retained a single megaspore whereas the surrounding ring of megasporangium bacame sterile and formed the integument . W alton (1953) supported this theory and proposed that the encircling element were sterile telome or ordinary branch tips . Andrew (1961) and Meeuse (1963) also supported this theory

12. 2. Halle (1933, 1937) was of the view that the cupule and the synangia of pteridosperm ovules were borne on the leaves and formed by the lateral concrescence of cyclically arranged telomes . The fuse telome in the case of cupule were sterile whereas those of synangia were fertile .

13. 3. Nucellar modification concept – Proposed by Andrews (1961) According to these theory the lagenostomalean integument is derivable from the nucellus or sporangium wall by its vascularisation and growth and lobing of apex . The following characteristics of the fern megasporangium attracted andrews attention – 1 . The sterile part of the megasporangium is vascularised . 2. Presence of a small opening at the tapering apex of the megasporangium 3. Most of the basal half of the sporangium is sterile and acted as a nutritional reservoir .

14. According to Andrews the origin of ovule and seed habit occurred in the following steps :

15. Fig. nucellar modification concept

16. 4. Telome concept – According to Smith (1961 ) , Long (1960) , Andrews (1961) the pteridospermic ovule is derived from a terminal fern megasporangium surrounded by sterile and fertile telome as those of Hedesia corymbosa or Stauropteris burnstislandica . The following stages explain the concept : Reduction in the number of megaspore to one. Elaboration of megasporangial apex to form pollen catching device . Surronding sterile telomes underwent modification , to form integument .

17. 4. Partial fusion of the encircling telome to form a cupule like structure that was initially free from the nucellus but later became fused with its base . 5. One or more terminal ovule become enclosed by further sterile telome to form a cupule or second integument . 6. The integument become fused with the nucellus. 7. The integument become simplified by disappearance of lobing and reduction of apical lobes . 8 . The micropyle was formed by the fusion of integument which function of catching pollen grain.

18. 9. Further reduction in the lobing of the cupule and the integument . 10. Fusion of the two integument This concept is widely accepted .

19. 5. Martens view – According to Marten (1966) the ovule of gymnosperm is originate from heterosporous pteridophyta . The megaspore in heterosporous pteridophyte is free from the megasporangial wall due to the deposition of callose around the megaspore wall . It has been discovered in the pteridophyte . According to marten that there must have been some primitive gymnosperms in which callose deposition is present and megaspore wall free from nucellus . During the course of evolution this callose secretion became less and ultimately disappeared and megaspore fused with nucellus .

20. Reference – B. R. VASHISTA Dr. A.K. SINHA Dr. ADARSH KUMAR

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