Meaning of Seed Habit:
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.
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.
Origin of Seed Habit:
Stewart and Rothwell (1993) proposed six evolutionary events that took place in ovule evolution after the onset of heterospory:
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:
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 (Fig. 7.144).
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 most acceptable hypothesis is the telome concept by Zimmermann in 1952. According to this hypothesis, a dichotomously branched axial system bearing terminal sporangia was the starting point (Fig. 7.145A).
There was a gradual reduction of some of the axis and one of the sporangia becomes surrounded by an aggregation of sterile telomes (Fig. 7.145B, C). The fusion of the telomes resulted in the formation of the integument (Fig. 7.145D). Thus the primitive seed (preovule) consisted of a naked megasporangium surrounded by a ring of vascularised integumentary lobes.
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 (Fig. 7.146). The cupules are planted dichotomously branched axes consisting of sterile telome trusses that are webbed in the proximal part.
Each cupule contains two short pedicels, each bearing a small orthotropous pre-ovule. The integument is serrated at the apex of each pre-ovule into a number of lobes that form a rudimentary micropyle.
Several other pre-ovules have been described from Upper Devonian. Elkinsia and Moresnetia are two other important pre-ovules.
Elkinsia consists of loose tufts of cupules produced on a cruciately forked branching system. Cupules are produced either singly or in pairs. Each cupule consists of 16 sterile branch tips surrounding a total of four orthotropous ovules. The integument is made up of four or five lobes that are fused at the basal region only and thus lacks a well-defined micropyle.
Moresnetia produced cupules much like those of Elkinsia. Moresnetia consists of 8 to 10 thin integumentary lobes that are only fused at the base and tends to flare away from the pre- ovule apex.
Evolution of ovules from pre-ovules:
The evidence in support of the fusion of telomes (integumentary lobes) of pre-ovules to form integument with its micropyle is well-documented by A. G. Long (1966). Long described a number of Lower Carboniferous pre-ovules from Scotland.
In Genomosperma kidstoni, the integument is represented by a ring of eight unfused telomes surrounding a centrally placed megasporangium (fig. 7.147A). Further fusion took place in Genomosperma latenswhere eight syntelomes are fused from the base distally for about one- third of their length (Fig. 7.147B).
In Physostoma elegans, the syntelomes are fused for one-half of their length. The more advanced degree of fusion is noted in Archaeosperma arnoldii and Eurystoma angulare where four syntelomes are fused from the base distally for about three-fourth of their length and still lacks a well-defined micropyle (Fig. 7.147).
Stamnostoma huttonense has reached the level of an ovule where the fusion of syntelomes is complete that delimited a micropyle (Fig. 7.147D).
Thus, the Lower Carboniferous ovules (seeds) demonstrate the direct relationship between the degree of fusion of integumentary lobes and concomitant formation of micropyle.