Botany Notes On – Origin Of Seed Habit – For W.B.C.S. Examination.

উদ্ভিদ তত্ত্ব নোট – বীজ অভ্যাসের মূল – WBCS পরীক্ষা।

Seed is a ripened or fertilised ovule. A seed (ovule) may be defined as an integumented indehiscent megasporangium. There is further elabora­tion of the megasporangium so as to enable it to be an ideal starting point for the development of a new plant.Continue Reading Botany Notes On – Origin Of Seed Habit – For W.B.C.S. Examination.

Thus seeds provide parental care to the land plants, where megagametophyte is retained within the indehiscent megasporangium and is well-protected by the integument. Moreover, apex of the nucellus is elaborated for reception of microspore and fertilisation takes place through pollen tube or pollen tube-like structure.

1. Degeneration of three megaspores and formation of a single functional megas­pore in the megasporangium.

2. Retention of the functional megaspore in the megasporangium (nucellus) until embryo development.

3. Formation of endosporic megagametophytes within an indehiscent megaspo­rangium.

4. Elaboration of the apical part of nucellus to receive microspores or pollen grains.

5. Formation of an integument which delimited a micropyle.

6. Formation of pollen tube or pollen tube-­like structure from endosporic microgametophyte.

Due to the incomplete records of fossil plants it is difficult to predict the exact order of these six events. However, the evolution of seed habit can be evaluated on the basis of above- mentioned events.

Evolution of Integument in Seed Habit:

Hypothesis:

Several theories have been put forward about the origin of integument.

According to the synangial hypothesis by Benson (1904), the integument evolved from the sterilisation of the outer ring of sporangia in a radial synangium.

The nucellar modification theory was pro­posed by Andrews (1961) based on the structure of megasporangium of Stauropteris burntislandica, a coenopterid fern. According to this theory, a reduction of megaspore from two to one took place in one which was sunk towards the base of the sporangium. The simple seed was evolved by further proliferation of the spo­rangial wall and the subsequent division of the basal vascular bundle to extend into the newly formed integument. The occurrence of Palaeozoic seeds such as Lagenostoma and Conostoma supports this hypothesis.

The evidences in support of the origin of seed (ovule) following telome hypothesis came into existence after the discovery of several ovule-like structures from the Upper Devonian and Lower Carboniferous strata. Though these ovule-like structures fulfilled most of the seed characteristics, they lack a well-defined micropyle.

Stewart and Rothwell (1993) proposed the term ‘pre-ovule’ for such structures. A pre-ovule may be defined as an ovule-like structure con­sisting of a megasporangium which is either naked or invested by unfused or partially fused integumentary lobes and thus lacks a well- defined micropyle.

The investigations of Upper Devonian and Lower Carboniferous pre-ovules have provided important clues in documenting the transition between pre-ovules and the true ovules (seeds).

Archaeosperma arnoldii, a pre-ovule bearing organ reported by Pettitt and Beck (1968) from Upper Devonian, consists of a cupule that par­tially surrounds four pre-ovules . The cupules are planted dichotomously branched axes consisting of sterile telome trusses that are webbed in the proximal part.

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